Characterization of the two-component, FAD-dependent monooxygenase SgcC that requires carrier protein-tethered substrates for the biosynthesis of the enediyne antitumor antibiotic C-1027

Article abstract of DOI:10.1021/ja710601d

C-1027 is a potent antitumor antibiotic composed of an apoprotein (CagA) and a reactive enediyne chromophore. The chromophore has four distinct chemical moieties, including an (S)-3-chloro-5-hydroxy-β-tyrosine moiety, the biosynthesis of which from L-α-tyrosine requires five proteins: SgcC, SgcC1, SgcC2, SgcC3, and SgcC4; a sixth protein, SgcC5, catalyzes the incorporation of this β-amino acid moiety into C-1027. Biochemical characterization of SgcC has now revealed that (i) SgcC is a two-component, flavin adenine dinucleotide (FAD)-dependent monooxygenase, (ii) SgcC is only active with SgcC2 (peptidyl carrier protein)-tethered substrates, (iii) SgcC-catalyzed hydroxylation requires O₂ and FADH₂, the latter supplied by the C-1027 pathway-specific flavin reductase SgcE6 or Escherichia coli flavin reductase Fre, and (iv) SgcC efficiently catalyzes regioselective hydroxylation of 3-substituted β-tyrosyl-S-SgcC2 analogues, including the chloro-, bromo-, iodo-, fluoro-, and methyl-substituted analogues, but does not accept 3-hydroxy-β-tyrosyl-S-SgcC2 as a substrate. Together with the in vitro data for SgcC4, SgcC1, and SgcC3, the results establish that SgcC catalyzes the hydroxylation of (S)-3-chloro-β-tyrosyl-S-SgcC2 as the final step in the biosynthesis of the (S)-3-chloro-5-hydroxy-β-tyrosine moiety prior to incorporation into C-1027. SgcC now represents the first biochemically characterized two-component, FAD-dependent monooxygenase that acts on a carrier-protein-tethered aromatic substrate.

Full text of DOI:10.1021/ja710601d

Published on Web 04/22/2008
Characterization of the Two-Component, FAD-Dependent
Monooxygenase SgcC That Requires Carrier Protein-Tethered
Substrates for the Biosynthesis of the Enediyne Antitumor
Antibiotic C-1027
Shuangjun Lin,† Steven G. Van Lanen,† and Ben Shen*,†,‡,§
DiVision of Pharmaceutical Sciences, UniVersity of Wisconsin National CooperatiVe Drug
DiscoVery Group, Department of Chemistry, UniVersity of Wisconsin-Madison,
Madison, Wisconsin 53705-2222
Received November 26, 2007; E-mail: bshen@pharmacy.wisc.edu
Abstract: C-1027 is a potent antitumor antibiotic composed of an apoprotein (CagA) and a reactive enediyne
chromophore. The chromophore has four distinct chemical moieties, including an (S)-3-chloro-5-hydroxy-
ꢀ-tyrosine moiety, the biosynthesis of which from L-R-tyrosine requires five proteins: SgcC, SgcC1, SgcC2,
SgcC3, and SgcC4; a sixth protein, SgcC5, catalyzes the incorporation of this ꢀ-amino acid moiety into
C-1027. Biochemical characterization of SgcC has now revealed that (i) SgcC is a two-component, flavin
adenine dinucleotide (FAD)-dependent monooxygenase, (ii) SgcC is only active with SgcC2 (peptidyl carrier
protein)-tethered substrates, (iii) SgcC-catalyzed hydroxylation requires O2 and FADH2, the latter supplied
by the C-1027 pathway-specific flavin reductase SgcE6 or Escherichia coli flavin reductase Fre, and (iv)
SgcC efficiently catalyzes regioselective hydroxylation of 3-substituted ꢀ-tyrosyl-S-SgcC2 analogues,
including the chloro-, bromo-, iodo-, fluoro-, and methyl-substituted analogues, but does not accept
3-hydroxy-ꢀ-tyrosyl-S-SgcC2 as a substrate. Together with the in vitro data for SgcC4, SgcC1, and SgcC3,
the results establish that SgcC catalyzes the hydroxylation of (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2 as the final
step in the biosynthesis of the (S)-3-chloro-5-hydroxy-ꢀ-tyrosine moiety prior to incorporation into C-1027.
SgcC now represents the first biochemically characterized two-component, FAD-dependent monooxygenase
that acts on a carrier-protein-tethered aromatic substrate.
Introduction
C-1027 is a chromoprotein antitumor antibiotic produced by
Streptomyces globisporus and is isolated as a noncovalent
complex consisting of an apoprotein (CagA) and the C-1027
chromophore (1). The structure of 1 consists of four distinct
moieties: an enediyne core, a deoxy aminosugar, a benzoxazo-
linate, and an (S)-3-chloro-5-hydroxy-ꢀ-tyrosine moiety (Figure
1).1–3 Upon release from CagA, the enediyne core of 1 readily
undergoes a Bergman cycloaromatization to generate a highly
reactive diradical intermediate that is capable of abstracting
hydrogen atoms from DNA, leading to both double-stranded
breaks (DSBs) and interstrand cross-links (ICLs), and hence
ultimately cell death.4–9 In view of its unique structure,
mechanism of action, and potent cytotoxicity, C-1027 has
attracted intense interest from chemists and biologists alike in
search of novel C-1027 analogues as potential cancer chemo-
therapeutic agents. We recently reported that engineered C-1027
analogues with a single substitution to the chromophore can
Figure 1. Structures of the enediyne chromophore of C-1027 (1) and
engineered analogues 22-deshydroxy-C-1027 (8), 20-deschloro-C-1027 (21),
and 20-deschloro-22-deshydroxy-C-1027 (22).
† Division of Pharmaceutical Sciences.
shift the mechanism of DNA damage to primary DSBs or ICLs,
suggesting that the modified enediynes might prove to be
therapeutically advantageous.8,9
The biosynthetic gene cluster for C-1027 was previously
cloned and sequenced.7 Bioinformatics analysis of the open
reading frames led to the proposal that the (S)-3-chloro-5-
hydroxy-ꢀ-tyrosine moiety originates from L-R-tyrosine (2) by
‡ University of Wisconsin National Cooperative Drug Discovery Group.
§ Department of Chemistry.
(1) Hu, J.; Xue, Y. C.; Xie, M. Y.; Zhang, R.; Otani, T.; Minami, Y.;
Yamada, Y.; Marunaka, T. J. Antibiot. 1988, 41, 1575–1579.
(2) Otani, T.; Minami, Y.; Marunaka, T.; Zhang, R.; Xie, M. Y. J. Antibiot.
1988, 41, 1580–1585.
(3) Otani, T.; Yasuhara, T.; Minami, Y.; Shimazu, T.; Zhang, R.; Xie,
M. Y. Agric. Biol. Chem. 1991, 55, 407–417.
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6616 J. AM. CHEM. SOC. 2008, 130, 6616–6623
10.1021/ja710601d CCC: $40.75
2008 American Chemical Society

SgcC-Catalyzed Hydroxylation of (S)-3-Chloro-ꢀ-tyrosyl-S-SgcC2
A R T I C L E S
Figure 2. Biosynthetic pathway for the (S)-3-chloro-5-hydroxy-ꢀ-tyrosine moiety of the C-1027 enediyne chromophore from L-tyrosine (2).
virtue of five proteins: SgcC, SgcC1, SgcC2, SgcC3, and SgcC4.
Once produced, the (S)-3-chloro-5-hydroxy-ꢀ-tyrosine moiety
is incorporated into C-1027 by the condensation enzyme SgcC5
via a ꢀ-aminoacyl-S-peptidyl carrier protein (SgcC2) intermedi-
ate (3) (Figure 2). Biochemical characterization using recom-
binant enzymes has already confirmed that (i) SgcC4 catalyzes
the conversion of L-R-tyrosine to (S)-ꢀ-tyrosine (4) as the first
step,10–13 (ii) SgcC1 activates (S)-ꢀ-tyrosine as the (S)-ꢀ-tyrosyl
adenylate (5) and subsequently loads (S)-ꢀ-tyrosine onto holo-
SgcC2, a type II peptidyl carrier protein (PCP), to yield (S)-ꢀ-
tyrosyl-S-SgcC2 (6) as the second step,13,14 and (iii) SgcC3 is
a FAD-dependent halogenase catalyzing regioselective chlorina-
tion of (S)-ꢀ-tyrosyl-S-SgcC2 to form (S)-3-chloro-ꢀ-tyrosyl-
S-SgcC2 (7).15 Furthermore, in ViVo data have suggested that
the last transformation, C-5 hydroxylation of (S)-3-chloro-ꢀ-
tyrosyl-S-SgcC2, is catalyzed by SgcC since 22-deshydroxy-
C-1027 (8) was isolated from the fermentation of a ∆sgcC
inactivation mutant.7,13
different classes of enzymes that have no structural or sequence
similarities.17–19 The first class, or one-component monooxy-
genases, are single polypeptides that utilize FAD or FMN as a
cofactor and require NADH or NADPH to initiate oxidation of
substrates; thus, these monooxygenases have both flavin reduc-
tase and monooxygnease activity. The second class, or two-
component monooxygenases, utilize reduced FAD or FMN
directly as a cosubstrate and therefore require a separate
NAD(P)H:flavin reductase to supply the reduced flavin. Thus,
the two-component systems consist of two different proteins,
one serving as a reductase and the other as a monooxygenase.
Although the flavin-dependent monooxygenases typically cata-
lyze hydroxylation using small-molecule substrates, such as
p-hydroxybenzoate (by a one-component monooxygenase)20–22
and p-hydroxyphenylacetate (by a two-component monoxyge-
nase),23–26 a single example has been reported recently wherein
BtrO, a two-component FMN-dependent monooxygenase in-
volved in the biosynthesis of butirosin, requires a carrier protein-
tethered aliphalic substrate.27
A variety of enzyme families catalyze hydroxylation of
aromatic compounds, including cytochrome P-450-dependent
monooxygenases, non-heme Fe3+-dependent monooxygenases,
and flavin-dependent monooxygenases, among others.16 The last
family, the flavin-dependent monooxygenases, consists of two
The gene product of sgcC has high sequence similarity to
two-component p-hydroxyphenylacetate monooxygenases such
as HpaA from Pseudomonas aeruginosa (accession no.
NP_252780, 50% identity/62% similarity)23 and HpaB from
Escherichia coli (accession no. CAA82321, 49% identity/61%
similarity),24 and, as previously noted, gene inactivation of sgcC
led to the isolation of 22-deshydroxy-C-1027, supporting the
(4) Sugimoto, Y.; Otani, T.; Oie, S.; Wierzba, K.; Yamada, Y. J. Antibiot.
1990, 43, 417–421.
(5) Sugiura, Y.; Matsumoto, T. Biochemistry 1993, 32, 5548–5553.
(6) Xu, Y.; Zhen, X.; Zhen, Y.; Goldberg, I. H. Biochemistry 1995, 34,
12451–12460.
(7) Liu, W.; Christenson, S. D.; Standage, S.; Shen, B. Science 2002, 297,
1170–1173.
(17) Ballou, D. P.; Entsch, B.; Cole, L. J. Biochem. Biophys. Res. Comm.
2005, 338, 590–598.
(8) Kennedy, D. R.; Gawron, L. S.; Ju, J.; Liu, W.; Shen, B.; Beerman,
T. A. Cancer Res. 2007, 67, 773–781.
(18) van Berkel, W. J. H.; Kamerbeek, N. M.; Fraaije, M. W. J. Biotechnol.
2006, 124, 670–689.
(9) Kennedy, D. R.; Ju, J.; Shen, B.; Beerman, T. A. Proc. Natl. Acad.
Sci. U.S.A. 2007, 104, 17632–17637.
(19) Ziegler, D. M. Drug Metab. ReV. 2002, 34, 503–511.
(20) Gatti, D. L.; Palfey, B. A.; Lah, M. S.; Entsch, B.; Massey, V.; Ballou,
D. P.; Ludwig, M. L. Science 1994, 266, 110–114.
(21) Schreuder, H. A.; Mattevi, A.; Obmolova, G.; Kalk, K. H.; Hol, W. G.;
van der Bolt, F. J.; van Berkel, W. J. Biochemistry 1994, 33, 10161–
10170.
(10) Christenson, S. D.; Liu, W.; Toney, M. D.; Shen, B. J. Am. Chem.
Soc. 2003, 125, 6062–6063.
(11) Christenson, S. D.; Wu, W.; Spies, M. A.; Shen, B.; Toney, M. D.
Biochemistry 2003, 42, 12708–12718.
(12) Christianson, C. V.; Montaavon, T. J.; Van Lanen, S. G; Shen, B.;
Bruner, S. D Biochemistry 2007, 42, 7205–7214.
(13) Van Lanen, S. G.; Dorrestein, P. C.; Christenson, S. D.; Liu, W.; Ju,
J.; Kelleher, N. L.; Shen, B. J. Am. Chem. Soc. 2005, 127, 11594–
11595.
(22) Wang, J.; Ortiz-Maldonado, M.; Entsch, B.; Massey, V.; Ballou, D.;
Gatti, D. L. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 608–613.
(23) Cuskey, S. M.; Olsen, R. H. J. Bacteriol. 1988, 176, 393–399.
(24) Prieto, M. A.; Garcia, J. L. J. Biol. Chem. 1994, 269, 22823–22829.
(25) Galan, B.; Diaz, E.; Prieto, M. A.; Garcia, J. L. J. Bacteriol. 2000,
182, 627–636.
(14) Van Lanen, S. G.; Lin, S.; Dorrestein, P. C.; Kelleher, N. L.; Shen,
B. J. Biol. Chem. 2006, 281, 29633–29640.
(26) Xun, L. Y.; Snadvik, E. R. Appl. EnViron. Microbiol. 2000, 66, 481–
486.
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12432–12438.
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9
J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6617

A R T I C L E S
Lin et al.
functional assignment based on sequence analysis.7 To continue
our investigations on the biosynthesis of C-1027, particularly
to delineate the events leading to the (S)-3-chloro-5-hydroxy-
ꢀ-tyrosine moiety of 1, we sought to characterize the enzymatic
activity of SgcC in Vitro to provide insight into the substrate
specificity, timing of the hydroxylation step in biosynthesis of
the (S)-3-chloro-5-hydroxy-ꢀ-tyrosine moiety of 1, and pre-
liminary mechanistic details of this enzyme. In this report, we
now establish that SgcC is a two-component, FAD-dependent
monooxygenase responsible for the regioselective hydroxylation
of (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2, requiring O2 and reduced
FAD (FADH2) provided by the pathway-specific flavin reductase
SgcE6 or E. coli flavin reductase Fre, for the fourth and final
enzymatic step of the biosynthesis of 3 from L-R-tyrosine (Figure
2). Similar to BtrO, SgcC is a two-component monoxygenase
that is dependent upon a carrier-protein-tethered substrate. But
in contrast, SgcC utilizes reduced FAD to hydroxylate an
aromatic substrate instead of reduced FMN as for BtrO.27 We
also investigated the substrate specificity of SgcC and demon-
strated that SgcC is capable of hydroxylating other 3-substituted
7 analogues, including the fluoro-, bromo-, iodo-, and methyl-
substituted analogues. The functional assignment and prelimi-
nary characterization of SgcC lay the foundation for future
biochemical studies on this mechanistically intriguing group of
carrier protein-dependent monooxygenases, and the results
provided here have clear ramifications with respect to engineer-
ing novel 1 analogues by combinatorial biosynthesis.
Synthesis of 3-Fluoro-ꢀ-tyrosine (13), 3-Iodo-ꢀ-tyrosine
(14), and 3-Methyl-ꢀ-tyrosine (15). Compounds 13, 14, and 15
were prepared following the method reported by Weaver29 (see
Supporting Information for details).
Overproduction and Purification of SgcC. The sgcC gene was
amplified with cosmid pBS100530 as a template and platinum Pfx
DNA polymerase from Invitrogen (Carlsbad, CA) using the
following primers: forward 5′-GGT ATT GAG GGT CGC ATG
CCC CAC G GT GCA GAG C-3′ and reverse 5′-AGA GGA GAG
TTA GAG CTA CAG CCC TCC GAG AAG G-3′ [the start (ATG)
and stop (CTA) codons are underlined]. Purified PCR product was
cloned into pET-30Xa/LIC vector following the ligation-indepen-
dent cloning procedure as described by Novagen (Madison, WI)
to give pBS1092, and the identity of sgcC in pBS1092 was
confirmed by DNA sequencing.
pBS1092 was transformed into E. coli BL21 (DE3) and grown
in LB media supplemented with 50 µg mL-1 kanamycin. Cells
were grown at 18 °C and induced with IPTG (final concentration
of 0.1 mM) when OD600 reached ∼0.5. They were subsequently
cultured at 18 °C for an additional 15 h. Cells were harvested by
centrifugation (8000 rpm for 15 min at 4 °C) and resuspended in
buffer A (100 mM sodium phosphate, pH 7.5, containing 300 mM
NaCl) supplemented with a complete protease inhibitor tablet,
EDTA-free. The cells were lysed by sonication (4 × 30 s pulsed
cycle), and the debris was removed by centrifugation (15 000 rpm
for 50 min at 4 °C). The clarified supernatant was loaded onto a
pre-equilibrated Ni-NTA agarose (Qiagen, Valencia, CA) column
with buffer B (buffer A plus 10% glycerol). The column was
washed with five column volumes of buffer B, followed by five
column volumes of buffer B containing 20 mM imidazole. The
His6-tagged SgcC protein was eluted with six column volumes of
buffer B containing 250 mM imidazole. After desalting using a
PD-10 column (GE Healthcare, Piscataway, NJ), the purified SgcC
protein was concentrated using an Amicon Ultra-4 (10K, GE
Healthcare) and stored at -25 °C as 40% glycerol stocks. The purity
of isolated SgcC was examined upon 12% SDS-PAGE analysis.
Protein concentration was determined using the Bradford protein
assay (Bio-Rad, Hercules, CA).
Determination of Cofactor Present in Purified SgcC. SgcC
was denatured by boiling for 3 min or by adding 50% methanol
(final concentration).15 After centrifugation, the supernatant was
loaded onto an Apollo C18 reverse-phase column (5 µm, 250 ×
4.6 mm, Alltech Associates Inc., Deerfield, IL) and analyzed using
a linear gradient from 0 to 60% CH3CN in H2O at a flow rate of 1
mL min-1 with UV–vis detection at 266 nm.
Preparation of SgcC2-Tethered Substrates for SgcC. A
general procedure was used to generate all potential substrates for
SgcC, including 6, 7, (S)-3-F-ꢀ-tyrosyl-S-SgcC2 (16), (S)-3-Br-ꢀ-
tyrosyl-S-SgcC2 (17), (S)-3-I-ꢀ-tyrosyl-S-SgcC2 (18), (S)-3-Me-
ꢀ-tyrosyl-S-SgcC2 (19), and (S)-3-OH-ꢀ-tyrosyl-S-SgcC2 (20).
Recombinant Svp,31 apo-SgcC2,15 and SgcC113,14 were prepared
as described. Post-translational modification of apo-SgcC2 with the
4′-phosphopantetheine moiety of CoA was achieved in a 1.8 mL
reaction mixture containing 100 mM Tris-HCl (pH 7.5), 200 µM
apo-SgcC2, 1.0 mM CoA, 12.5 mM MgCl2, 2.0 mM TCEP, and
10 µM Svp.31 After incubation at room temperature for 45 min, a
loading solution consisting of 3.5 mM ꢀ-tyrosine analogue (4, 9,
10, 11, 13, 14, or 15), 4 mM ATP, 2.0 mM TCEP, and 12.5 mM
MgCl2 (all final concentrations) was added to an equal volume of
the above solution. SgcC1 was added to a final concentration of 5
µM, and the resulting solution was incubated at room temperature
for an additional 60 min.
Experimental Procedures
Materials and Methods. Adenosine triphosphate disodium salt
(ATP), coenzyme A (CoA), flavin adenine dinucleotide disodium
salt (FAD), flavin mononucleotide sodium salt (FMN), ꢀ-nicoti-
namide adenine dinuceotide reduced disodium salt (NADH), and
tris(2-carboxyethyl)phosphine hydrochloride (TCEP) were pur-
chased from Sigma-Aldrich (St. Louis, MO). Dithiothreitol (DTT)
and isopropyl thiogalactoside (IPTG) were purchased from Research
Products International Corp. (Mt. Prospect, IL). Complete protease
inhibitor tablet, EDTA-free, was from Roche Applied Science
(Indianapolis, IN). The starting materials for the synthesis of
ꢀ-amino acid analogues, including 3-fluoro-4-hydroxybenzaldehyde,
3-methyl-4-hydroxybenzaldehyde, and 3-iodo-4-hydroxybenzalde-
hyde, were purchased from Sigma-Aldrich and used without further
purification. 3-Chloro-ꢀ-tyrosine (9), 3-bromo-ꢀ-tyrosine (10),
3-hydroxy-ꢀ-tyrosine (11), and 3-chloro-5-hydroxy-ꢀ-tyrosine (12)
were synthesized as described.13–15 (S)-3-Amino-3-(4-hydroxyphe-
nyl)propionic acid [(S)-ꢀ-tyrosine] (4) and (R)-3-amino-3-(4-
hydroxyphenyl)propionic acid [(R)-ꢀ-tyrosine] were from PepTech
Corp. (Burlington, MA). Medium components and chemicals were
from Fisher Scientific (Fairlawn, NJ). Chemically competent E. coli
DH5R and E. coli BL21(DE3) cells were prepared using standard
procedures.28 Synthetic DNA oligonucleotides were purchased from
the University of Wisconsin-Madison Biotechnology Center
(Madison, WI). PCR was performed with a PerkinElmer GeneAmp
2400 (PerkinElmer Life And Analytical Sciences, Inc., Waltham,
MA). Electrospray ionization mass spectroscopy (ESI-MS), high-
resolution electrospray ionization mass spectroscopy (HR-ESI-MS),
or LC-ESI-MS was performed with an Agilent 1100 HPLC-MSD
SL ion trap mass spectrometer (Agilent Technologies, Inc., Santa
Clara, CA). Atmospheric pressure chemical ionization mass spec-
troscopy (APCI-MS) was measured with an Agilent 1100 VL APCI
mass spectrometer. NMR data were obtained using a Varian Unity
Inova 400 MHz NMR Spectrometer (Varian, Inc., Palo Alto, CA).
Purification of SgcC2-Tethered Substrates for SgcC. Com-
pounds 6, 7, 16, 17, 18, 19, and 20 were purified using anion-
(29) Tan, C. Y. K.; Weaver, D. F. Tetrahedron 2002, 58, 7449–7461.
(30) Liu, W.; Shen, B. Antimicrob. Agents Chemother. 2000, 44, 382–
392.
(31) Sanchez, C.; Du, L.-C.; Edwards, D. J.; Toney, M. D.; Shen, B. Chem.
Biol. 2001, 8, 725–738.
(28) Sambrook, J.; Fritsch, E. F.; Maniatis, T. Molecular Cloning: A
Laboratory Manual, 3rd ed.; Cold Spring Harbor Laboratory Press:
Cold Spring Harbor, NY, 2000.
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SgcC-Catalyzed Hydroxylation of (S)-3-Chloro-ꢀ-tyrosyl-S-SgcC2
A R T I C L E S
exchange chromatography. A 5-mL HiTrap Q column (GE Health-
care) was pre-equilibrated with 20 mM sodium phosphate buffer
(pH 7.0), and the SgcC2-tethered substrate preparations were loaded.
The SgcC2-tethered products were eluted using a linear gradient
from 0 to 100% 1.0 M NaCl in 20 mM sodium phosphate buffer
was carried out in a final volume of 200 µL with 5 mM NADH,
10 µM FAD, 1 mM TCEP, and 50 mM phosphate buffer (pH 6.0),
containing 50 mM NaCl at 25 °C. When 7, 17, or 18 (250 µM)
was used as a substrate, 1.5 µM SgcC and 2 µM SgcE6 were used.
When 6, 16, 19, or 20 (250 µM) was used as a substrate, 6 µM
SgcC and 2 µM SgcE6 were employed. The reactions were initiated
by the addition of SgcC and SgcE6 and run in duplicate. At different
time points (Table S1, Supporting Information), the reactions were
quenched by addition of 35 µL of ice-cold 70% TCA. The samples
were treated with the same workup procedure and HPLC analysis
as described above. The use of differential wavelengths allows for
facile detection and determination of the product formation (Table
S1). Standard curves were generated with synthetic ꢀ-tyrosine
analogues in order to correlate peak area with the amount of product
formed in each reaction. The product formation was fitted to a linear
equation to obtain the initial velocity, and the specific activity was
calculated from the initial velocity divided by the concentration of
SgcC as determined using the Bradford dye-binding procedure.
Determination of pH Dependence of the SgcC Activity. Three
buffers, 50 mM sodium acetate (pH 5.0 and 5.5), 50 mM sodium
phosphate buffer (pH 6.0-8.0), and 50 mM Tris-HCl (pH 9.0),
were chosen to determine the optimal pH for the SgcC hydroxy-
lation activity in Vitro. The assay solutions, consisting of 200 µM
7, 3 µM SgcC, 2 µM SgcE6, 5 mM NADH, 10 µM FAD, and 1
mM TCEP in 50 mM buffer, varying pH from 5.0 to 9.0, containing
50 mM NaCl, were incubated at 25 °C for 30 min. The product
formation was monitored by HPLC after the reactions were
terminated and worked up as described above.
(pH 7.0) for 25 column volumes at a flow rate of 3 mL min-1
.
The purified substrates, which were eluted between 0.35 and 0.4
M NaCl, were desalted by two cycles of concentration/dilution using
an Amicon Ultra-4 concentrator device (5K, GE Healthcare) prior
to use in SgcC assays.
Determination of the Stereochemistry of (S)-3-Cl-ꢀ-tyrosyl-
S-SgcC2 Substrate for SgcC. After the HiTrap Q anion-exchange
column, the purified (S)-3-Cl-ꢀ-tyrosyl-S-SgcC2 (7) was subjected
to alkaline hydrolysis in 0.1 N KOH solution incubated at 70 °C
for 15 min. The resulting solution containing the free 3-Cl-ꢀ-
tyrosine was injected into an Apollo C18 column (5 µm, 250 ×
4.6 mm, Alltech Associates Inc.) using a 20 min linear gradient
from 0 to 25% CH3CN in 0.1% TFA-H2O at a flow rate of 1 mL
min-1 and UV–vis detection at 280 nm for purification. The peak
of 3-Cl-ꢀ-tyrosine was collected and concentrated by speed-vac.
The resultant solution was analyzed through a CrownPak CR (+)
column (5 µm, 150 × 4.0 mm, Chiral Technologies Inc., West
Chester, PA) to determine the stereochemistry. The column was
run isocratically in aqueous perchloric acid buffer (pH 2.4) at a
flow rate of 1 mL min-1 as recommended by the manufacturer,
and an authentic standard of (S)-3-Cl-ꢀ-tyrosine, made by SgcC3-
catalyzed chlorination of (S)-ꢀ-tyrosyl-S-SgcC2 as described previ-
ously,15 was used for comparison (Figure S2, Supporting Infor-
mation).
Results
Characterization of the SgcC Hydroxylation Activity in
Vitro. The typical SgcC assay solution contained 250 µM 7 or other
indicated substrates, 5 mM NADH, 10 µM FAD, 1 mM TCEP, 5
µM SgcC, and 1.5 µM SgcE6 in 50 mM sodium phosphate buffer
(pH 6.0) containing 50 mM NaCl. Reactions were initiated by
addition of SgcC and SgcE6, incubated at 25 °C for 1 h, and
terminated by the addition of 35 µL of 70% trichloroacetic acid
(TCA) to a final concentration of 10%. After incubation on ice for
15 min, the precipitate was separated by centrifugation (14 000 rpm
for 15 min at 4 °C). The resulting pellet was washed twice with
200 µL of 5% TCA and once with 200 µL of ice-cold ethanol.
After drying by speed-vac for 10 min, the protein pellet was
redissolved in 150 µL of 0.1 M KOH containing 50 mM DTT and
incubated at 50 °C for 15 min to hydrolyze all thioester bonds.
After neutralization of the alkaline hydrolysis solution, the pre-
cipitate was removed by centrifugation, and the clarified supernatant
was concentrated by speed-vac and analyzed by HPLC with a
Varian ProStar 210 HPLC system equipped with an Apollo C18
reverse-phase column (5 µm, 250 × 4.6 mm, Alltech Associates
Inc.), using a 24 min linear gradient from 0 to 25% CH3CN in
0.1% TFA-H2O at a flow rate of 1 mL min-1 and UV–vis detection
at 280 nm. The identity of the peaks was determined using synthetic
standards, and the product peaks were collected and subjected to
LC-ESI-MS analysis using an Agilent 1100 HPLC-MSD SL ion
trap mass spectrometer. Control reactions were carried out under
identical conditions, except with boiled SgcC.
To determine kinetic parameters for SgcC, 7 (60-700 µM) was
incubated in a final volume of 200 µL with 3 µM SgcC, 2 µM
SgcE6, 5 mM NADH, 10 µM FAD, 1 mM TCEP, and 50 mM
phosphate buffer (pH 6.0) containing 50 mM NaCl at 25 °C.
Reactions were initiated by the addition of SgcC and SgcE6,
quenched by addition of 35 µL of 70% cold TCA after a 10 min
incubation, and carried out in duplicate. The mixture was treated
with the same workup procedure and HPLC analysis as described
above to determine the product formation using a standard curve
generated with synthetic 12.12,13 Plots of product formation were
fitted to the Michaelis–Menten equation to extract the Km and kcat
parameters.
Production, Purification, and Properties of SgcC. The sgcC
gene was amplified from the cosmid pBS10057,30 and directly
cloned into the pET-30Xa/LIC vector to generate expression
plasmid pBS1092, in which SgcC would be overproduced as
an N-terminal His6-tagged fusion protein. Standard conditions
were used for expression,28 and SgcC was purified with a yield
of about 9 mg/L following affinity chromatography. SDS-PAGE
analysis revealed that SgcC was purified to near homogeneity
with the expected molecular weight of 63.2 kD (Figure S1,
Supporting Information). The solution containing the purified
protein was colorless, and HPLC analysis showed that no flavin
was copurified with SgcC, consistent with the lack of flavin-
binding domain based on sequence analysis.
Substrate Preparation, Determination of Stereochemistry
of Substrate, and Activity of SgcC. Prior to testing the activity
of SgcC, Fre15 and SgcE615 were overproduced in E. coli, the
recombinant proteins were purified, and the reductase activity
was confirmed spectroscopically by monitoring the consumption
of NADH. The recombinant proteins SgcC1,13,14 a promiscuous
ꢀ-tyrosine-activating adenylation enzyme, and SgcC2,15 the
PCP, and Svp,31 a promiscuous 4′-phosphopantetheinyl trans-
ferase involved in bleomycin biosynthesis, were prepared as
previously described. The ꢀ-tyrosine substrate 9 was synthesized
starting from the appropriate benzaldehyde precursor as previ-
ously described.15 To generate the PCP-tethered substrate, holo-
SgcC2 was first enzymatically prepared using Svp, followed
by loading of holo-SgcC2 with 9 catalyzed by SgcC1. The
resulting product 7 was purified by anion exchange to eliminate
excessive substrates or products for Svp and SgcC1, as well as
other proteins, and was desalted prior to use in enzyme assays.
Remarkably, only the (S)-3-Cl-ꢀ-tyrosine was specifically
activated and loaded onto the holo-SgcC2 by SgcC1, although
racemic 3-Cl-ꢀ-tyrosine was employed as a substrate in the
SgcC1-catalyzed loading reaction, as determined by chiral HPLC
analysis in comparison with authentic (S)-3-Cl-ꢀ-tyrosine made
To determine the specific activity of SgcC with respect to the
other putative substrates, the SgcC-catalyzed hydroxylation reaction
9
J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6619

A R T I C L E S
Lin et al.
Table 1. Cofactor and Cosubstrate Requirements for
SgcC-Catalyzed Hydroxylation of (S)-3-Chloro-ꢀ-tyrosinyl-S-SgcC2
in Vitro
flavin
reductase
conversion
(%)a
relative
activity
entry
cofactors or cosubstrates
1
2
3
4
5
6
7
NADH, FAD, and O2
NADH, FAD, and O2
NADH and FAD
FAD and O2
NADH and O2
SgcE6
no
SgcE6
SgcE6
SgcE6
Fre
16 ( 4.5
100
0
0
0
0
0
0
0
0
NADH, FAD, and O2
NADH, FMN, and O2
19 ( 2.0
0
119
0
Fre
a Reactions were performed under initial velocity conditions, and
product formation was quantitated using HPLC peak area with
comparisons to an authentic standard.
concentration was between 1 and 10 µM. Subsequently, all
assays were performed in the presence of 10 µM FAD.
Using the optimized conditions and the HPLC-based assay,
a time course analysis of the SgcC-catalyzed hydroxylation
showed an increase in 12 with the concomitant loss of 9, and
under these conditions product formation was linear with respect
to time until approximately 20 min (Figure 3B). Finally,
preliminary kinetic analysis revealed that SgcC displayed
Michaelis–Menten kinetics with respect to 7, yielding a Km of
742 ( 69 µM and kcat of 1.4 ( 0.2 min-1 (Figure 3C).
Cofactor and Cosubstrate Requirements for SgcC
Activity. The requirement of cofactors and cosubstrates for SgcC
activity was systematically examined using the HPLC-based
assay (summarized in Table 1). No product was observed when
SgcE6 was omitted from the assay mixture, consistent with the
requirement of FADH2 for hydroxylation (entry 2). The forma-
tion of 3 was completely abolished when cofactor FAD or
cosubstrates NADH and O2 were excluded from the reaction
(entries 3-5). Comparable amounts of 3 were produced by
replacing SgcE6 with E. coli flavin reductase Fre (entry 6).
Finally, 3 was not detected when FMN was substituted for FAD
(entry 7). Thus, these results are consistent with the functional
assignment of SgcC as a two-component, FAD-dependent
monooxygenase following the known catalytic cycle for these
enzymes as shown in Figure 4.
Substrate Specificity of SgcC. The timing of the hydroxylation
step in 1 biosynthesis was examined by exploring the substrate
specificity of SgcC. Reactions with free amino acids as
substrates, including 4, (R)-ꢀ-tyrosine, 9, and 11, revealed no
enzyme activity regardless of the reaction conditions, consistent
with hydroxylation occurring after generation of the ꢀ-aminoa-
cyl-S-SgcC2 species (Figure 2). Thus, 6 was enzymatically
prepared using SgcC1 and tested under the optimized hydroxy-
lation conditions. SgcC showed activity with 6, affording the
corresponding hydroxylated product 21, but the specific activity
was decreased 120-fold with respect to that of 7 (Figure 4 and
Table 2). In combination with the data obtained with SgcC4,10–12
SgcC1,13,14 and SgcC3,15 the results here are consistent with
SgcC catalyzing the hydroxylation of 7 to afford 3, the last
intermediate prior to attachment to the enediyne core (Figure
2).
Figure 3. In Vitro characterization of SgcC as a two-component, FAD-
dependent monooxygenase. (A) HPLC profiles of 3-chloro-ꢀ-tyrosine (9,
)) standard (I), hydrolyzed compounds following incubations of (S)-3-
chloro-ꢀ-tyrosinyl-S-SgcC2 (7) with SgcC for 10 min (II), 20 min (III),
and 60 min (IV), and synthetic 3-chloro-5-hydroxyl-ꢀ-tyrosine (12, ()
standard (V). The other peak in the chromatograms is 4,5-dihydroxy-1,2-
dithiane (b) presented in the assay. (B) Time course of SgcC-catalyzed
hydroxylation of (S)-3-chloro-ꢀ-tyrosinyl-S-SgcC2 (7) as followed by HPLC
analysis for the first 20 min. (C) Single-substrate kinetic analysis for SgcC
with varying concentration of (S)-3-chloro-ꢀ-tyrosinyl-S-SgcC2 (7).
from the SgcC3-catalyzed chlorination of (S)-ꢀ-tyrosyl-S-SgcC2
(Figure S2, Supporting Information). While unexpected, this
observation is, in fact, consistent with the previous finding that
SgcC1 shows a 25-fold preference for 4 over its (R)-enantiomer,
serving as a “gate-keeper” that permits only the (S)-ꢀ-amino
acid to be incorporated into 1 in ViVo. It is therefore concluded
that, under the same conditions, only the (S)-enantiomers were
similarly activated and tethered to SgcC2 for all other 3-sub-
stituted ꢀ-tyrosine analogues used in this study.
Initial activity assays were carried out with 250 µM 7, 5 mM
NADH, 10 µM FAD, 1 mM TCEP, 5 µM SgcC, and 1.5 µM
SgcE6 in 50 mM sodium phosphate buffer (pH 6.0), containing
50 mM NaCl under aerobic conditions at 25 °C. The reaction
was monitored by subjecting the aminoacyl-S-SgcC2 substrate
and product to alkaline hydrolysis followed by HPLC analysis.
A new peak appeared eluting before 9, and this new peak had
a retention time identical to that of authentic 12 (Figure 3A).
The identity of the new peak was confirmed by LC-ESI-MS
analysis, yielding a pair of [M + H]+ ions at m/z ) 232.1 and
234.1 with a ∼3:1 ratio, consistent with a monochlorinated
species when compared to the authentic standard 12.
Optimization of SgcC Activity and Kinetic Analysis. Prior
to kinetic analysis, the pH dependence of SgcC-catalyzed
hydroxylation was examined. The pH profile exhibited a bell-
shaped curve between pH 5.5 and 9.0 with an optimal activity
at pH 6.0 (Figure S3A, Supporting Information). As a result,
all subsequent assays were performed in 50 mM phosphate
buffer at pH 6.0. Since the activity of two-component, flavin-
dependent monooxygenases has been shown to be dependent
on flavin concentration,17,26,32 the effect of FAD concentration
on SgcC activity was next examined (Figure S3B, Supporting
Information). SgcC showed a nearly 2-fold increase in activity
when the FAD concentration was decreased from 50 µM to 10
µM. Maximum product formation occurred when the FAD
Finally, the activity of SgcC was tested using C-3-substituted
analogues including 16-20, and the results are summarized in
Figure 4 and Table 2. SgcC was found to readily hydroxylate
17 and 18, yielding the corresponding products 22 and 23, with
specific activities slightly greater than that for the native
substrate 7. While both 16 and 19 were converted to the
corresponding hydroxylated products 24 and 25, the specific
(32) Entsch, B.; Chakraborty, S.; Ortiz, M.; Ballou, D. In FlaVins and
FlaVoproteins; Nishino, T., Miura, R., Tanokura, M., Eds.; Small
World Internet Publishing: Japan, 2005.
9
6620 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

SgcC-Catalyzed Hydroxylation of (S)-3-Chloro-ꢀ-tyrosyl-S-SgcC2
A R T I C L E S
Figure 4. SgcC-catalyzed C-5 hydroxylation of (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2 (7) and its (S)-3-substituted ꢀ-tyrosyl-S-SgcC2 analogues (6, 7, 16-20)
depicting a catalytic cycle involving the flavin reductase SgcE6 or Fre.
Table 2. Substrate Specificity of SgcC-Catalyzed Hydroxylation of
modulates the reactivity of the enediyne core via π-π stacking
interactions;34 as a consequence, structural permutations of the
(S)-3-Chloro-ꢀ-tyrosinyl-S-SgcC2 in Comparison with
(S)-3-Substituted ꢀ-Tyrosyl-S-SgcC2 Analogues
moiety have been shown to have a clear impact on the
bioactivity.8,9
product formed,
specific activity (min-1 a
relative
activity
substrate
)
The biosynthetic gene cluster for 1 was previously cloned
and sequenced,7 providing the first insights into how such
complex metabolites as the enediynes are assembled. We have
since shown that the (S)-3-chloro-4,5-dihydroxy-ꢀ-phenylalanine
moiety is derived from L-R-tyrosine, and we have characterized
the first three enzymatic conversions that, together, established
(S)-3-chloro-ꢀ-tyrosyl-S-SgcC2 as the pathway intermediate
most likely to serve as a substrate for hydroxylation (Figure 2).
Based on bioinformatics analysis, the best candidate for this
transformation was SgcC, and this assignment was validated
upon inactivation of the sgcC gene that led to the accumulation
of the expected compound 22-deshydroxy-C-1027 from the
fermentation broth of the ∆sgcC mutant strain.7 However, the
substrate specificity and enzymatic properties of SgcC remained
unclear. To unravel the molecular details of this hydroxylation
event, we cloned sgcC and overproduced and purified the
recombinant protein for in Vitro characterization.
An approach similar to that utilized during the functional
characterization of SgcC3 was applied here to generate the
carrier protein-tethered substrate for SgcC.15 In short, 3-chloro-
ꢀ-tyrosine was synthetically prepared, but only (S)-3-chloro-ꢀ-
tyrosine was directly loaded onto holo-SgcC2 to afford the
desired substrate (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2, by taking
advantage of the high stereospecificity but relaxed substrate
specificity of SgcC1,13,14 thus bypassing the halogenase-
catalyzed reaction (Figure S2, Supporting Information). 3-Chloro-
5-hydroxy-ꢀ-tyrosine, the expected product of hydroxylation of
(S)-3-chloro-ꢀ-tyrosyl-S-SgcC2 by SgcC following hydrolysis
from SgcC2, was also synthesized to serve as an authentic
standard. After the optimal conditions were determined for the
hydrolytic cleavage of the SgcC2-tethered substrate 7 and
product 3 to release the free acids (S)-3-chloro-ꢀ-tyrosine and
(S)-3-chloro-5-hydroxy-ꢀ-tyrosine, chemo-enzymatically pre-
pared substrate (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2 was incubated
with SgcC using typical reaction conditions for two-component
monooxygenases. HPLC analysis of the hydrolyzed product as
(S)-3-Cl-ꢀ-tyrosyl-S-SgcC2 (7)c
(S)-3-F-ꢀ-tyrosyl-S-SgcC2 (16)
(S)-3-Br-ꢀ-tyrosyl-S-SgcC2 (17)
(S)-3-I-ꢀ-tyrosyl-S-SgcC2 (18)
(S)-3-Me-ꢀ-tyrosyl-S-SgcC2 (19)
(S)-ꢀ-tyrosyl-S-SgcC2 (6)
3, 2.4 ( 0.2
24, 0.3 ( 0.1
22, 3.1 ( 0.2
23, 5.5 ( 1.0
25, 0.5 ( 0.1
21, 0.02b
100
12
129
229
21
0.8
0
(S)-3-OH-ꢀ-tyrosyl-S-SgcC2 (20)
0
a Reactions were performed under initial velocity conditions, and
product formations were quantitated using HPLC peak area with
comparisons to authentic standards. See Figure 4 for product structures.
b The conversion is so low that specific activity toward (S)-ꢀ-
tyrosyl-S-SgcC2 (6) is estimated from product formation over incubation
at 25 °C for 20, 40, and 60 min. c Number is identical to that in Figures
2 and 4.
activities of SgcC for these substrates were significantly
decreased, by approximately 5-fold, in comparison with that
for 7. No product formation was detected when 20 was used as
a substrate in the SgcC assay. The identities of these products
were all confirmed by LC-ESI-MS analysis of their free acids,
(S)-12, (S)-26, (S)-27, (S)-28, (S)-29, and (S)-30, released from
SgcC2 upon hydrolytic cleavage (Figure 4 and Table S2,
Supporting Information).
Discussion
The enediyne antitumor antibiotics are structurally complex
metabolites that are among the most cytotoxic natural products
ever described. They all share a similar enediyne core that is
directly implicated in bioactivity, and each core is decorated
with a variety of chemical moieties that alter the properties,
including cytotoxicity, of a given enediyne.33 C-1027, a model
nine-membered enediyne, is composed of a deoxy aminosugar,
a benzoxazolinate moiety, and the moiety of interest here, a
(S)-3-chloro-5-hydroxy-ꢀ-tyrosine (Figure 1). This moiety con-
tributes critical interactions with the apoprotein CagA34,35 and
(33) Shen, B.; Liu, W.; Nonaka, K. Curr. Med. Chem. 2003, 10, 2317–
2325.
(34) Tanaka, T.; Fukuda-Ishisaka, S.; Hirama, M.; Otani, T. J. Mol. Biol.
2001, 309, 267–283.
(35) Okuno, Y.; Iwashita, T.; Sugiura, Y. J. Am. Chem. Soc. 2000, 122,
6848–6854.
9
J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6621

A R T I C L E S
Lin et al.
(S)-3-chloro-5-hydroxy-ꢀ-tyrosine unambiguously established
that SgcC is a monooxygenase catalyzing the formation of (S)-
3-chloro-4,5-dihydroxy-ꢀ-phenylalanyl-S-SgcC2 (3, Figure 3).
Detailed studies with E. coli and P. aeroginosa two-
component p-hydroxyphenylacetate monooxygenases have es-
tablished the catalytic requirements for this relatively new
enzyme family.23–27 In short, hydroxylation is dependent on two
proteins, a reductase and a monooxygenase, with the former
responsible for production of reduced FADsat the expense of
NAD(P)Hsthat diffuses to the latter protein responsible for O2
activation and aromatic hydroxylation.17,36 The data presented
here for SgcC are entirely consistent with the catalytic cycle
shown in Figure 4, since excluding SgcE6, NADH, FAD, or
O2 resulted in no product formation. Furthermore, SgcE6 is
readily substituted by E. coli Fre, thus supporting the prior
findings that the reductase and monooxygenase components do
not form a complex and that oxygenase activity is independent
of the source of FADH2.26 Finally, the SgcE6-SgcC hydroxy-
lation activity decreased in the presence of excess FAD, which
has been shown previously for other two-component monooxy-
genases to be a result of substantial autoxidation of FADH2 to
generate H2O2.26,32 Consequently, the SgcE6 (or Fre):SgcC:FAD
ratio was first optimized prior to subsequent studies to afford
the most efficient coupling possible for the SgcE6-SgcC two-
component system.
After assigning the function of SgcC, the timing of the
hydroxylation step in 1 biosynthesis and the substrate specificity
of SgcC were examined. The former was interrogated by
comparing the specific activity of SgcC with that of hypothetical
pathway intermediates. While the amount of required substrate
and unavoidable limitations of the assay precluded a more
rigorous quantification of substrate specificity, the results clearly
demonstrate (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2 is preferred over
(S)-ꢀ-tyrosyl-S-SgcC2, supporting the indirect evidence obtained
from prior reports that SgcC is the fourth enzyme in the
pathway.7 Perhaps most significant, however, is the finding that
SgcC cannot convert any free acid to the corresponding
hydroxylated product. Thus, like the halogenase SgcC3,15 SgcC
hydroxylation requires a carrier protein-tethered substrate,
reflecting the essential interactions provided by SgcC2 during
substrate recognition and catalysis. These results are intriguing,
considering that SgcC and SgcC3 have low sequence homology
(12% identity and 22% similarity) yet display remarkable
parallels with respect to substrate requirements and mechanism,
wherein the only significant difference between the catalytic
cycles is the nature of the electrophile species (FAD-OCl for
SgcC3 versus FAD-OOH for SgcC) undergoing electrophilic
aromatic substitution. Having both enzymes available now sets
the stage to explore the structural determinants that lead to
similarities and contrasts in substrate recognition and utilization
for SgcC3 and SgcC.
one carrier protein-dependent, two-component monooxygenase
identified: BtrO.27 BtrO is involved in the biosynthesis of the
unusual (2S)-4-amino-2-hydroxybutyryl side chain of the ami-
noglycoside butirosin, and it has been demonstrated that BtrO
hydroxylates an aliphatic substrate bound to a carrier protein
utilizing FMN as a cosubstrate. The absolute requirement for a
carrier protein for all these hydroxylases suggests a common
strategy for substrate recognition, and the molecular details and
protein dynamics underlying the hydroxylation strategy for SgcC
with respect to BtrO and other characterized two-component
monooxgyenase systems are subjects of ongoing research.
Finally, in keeping with our goal of using combinatorial
biosynthesis to generate new 1 analogues,8,9 a panel of (S)-3-
chloro-ꢀ-tyrosyl-S-SgcC2 analogues were prepared to inter-
rogate potential substrate flexibility for SgcC. Surprisingly, the
3-iodo- and 3-bromo-ꢀ-tyrosine analogues (18 and 17) displayed
slightly higher activity with SgcC, and furthermore SgcC turned
over both the 3-fluoro- and 3-methyl-ꢀ-tyrosine substrate
analogues (16 and 19). Therefore, both the adenylation enzyme
SgcC1 and now the hydroxylase SgcC seem to have substrate
specificity that is amenable to genetic engineering and combi-
natorial biosynthetic approaches to generate new enediyne
compounds. Furthermore, the realization that 20-deschloro-
C1027 (21),14 20-deschloro-22-deshydroxy-C-1027 (22),14 and
22-deshydroxy-C-10277 are isolated upon inactivation of sgcC3
and sgcC, respectively, suggests the condensation enzyme
SgcC5 also has relaxed substrate specificity, thus clearly setting
the stage to manipulate C-1027 biosynthesis to rationally
produce novel enediynes using precursor-directed biosynthesis.
Further mechanistic studies on SgcC3 in combination with in
Vitro characterization of SgcC5 will clearly provide additional,
valuable insights into such a strategy.
In conclusion, SgcC has been functionally characterized as a
two-component, FAD-dependent monooxygenase that regiospe-
cifically hydroxylates (S)-3-chloro-ꢀ-tyrosyl-S-SgcC2. Similarly
to the halogenase SgcC3, SgcC absolutely requires O2, a carrier-
protein-tethered substrate, and a separate flavin reductase to
provide diffusible FADH2 for catalysis. Along with the results
obtained for SgcC4, SgcC1, and SgcC3, the data support SgcC
as the fourth enzyme in the pathway leading to biosynthesis of
the (S)-3-chloro-4,5-dihydroxy-ꢀ-phenylalanine moiety of C-1027
(Figure 2). Finally, SgcC is capable of hydroxylating a variety
of ꢀ-tyrosine analogues, including all of the 3-halogenated
compounds tested (Figure 4). In lieu of the relaxed substrate
specificity of other enzymes in the pathway, application of
precursor-directed biosynthesis and combinatorial biosynthesis
methods to the C-1027 biosynthetic machinery now affords a
unique opportunity to generate unnatural C-1027 analogues,
some of which may have improved biological activity.8,9
Acknowledgment. We thank Dr. Y. Li, Institute of Medicinal
Biotechnology, Chinese Academy of Medical Sciences, Beijing,
China, for the wild-type S. globisporus strain and the Analytical
Instrumentation Center of the School of Pharmacy, University of
Wisconsin-Madison for support in obtaining MS and NMR data.
This work is supported in part by NIH grants CA78747 and
CA113297. S.V.L. is the recipient of an NIH postdoctoral fellow-
ship (CA1059845).
The phenomenon of enzyme-catalyzed hydroxylation utilizing
carrier protein-tethered substrates has been predicted to be a
common occurrence in the assembly of antibiotics containing
amino acid derivatives.37 Indeed, rather recently, two heme-
dependent monooxygenases involved in novobiocin and nikko-
mycin biosynthesis have been shown to hydroxylate the amino
acids tyrosine and histidine, respectively, only after tethering
to a carrier protein.38,39 To our knowledge, there has been only
Supporting Information Available: Full experimental details
for the synthesis of 3-fluoro-ꢀ-tyrosine, 3-iodo-ꢀ-tyrosine, and
(36) Louie, T. M.; Xie, X. S.; Xun, L. Biochemistry 2003, 42, 7509–7517.
(37) Chen, H.; Thomas, M. G.; O’Connor, S. E.; Hubbard, B. K.; Burkart,
M. D.; Walsh, C. T. Biochemistry 2001, 40, 11651–11659.
(39) Chen, H.; Hubbard, B. K.; O’Connor, S. E.; Walsh, C. T. Chem. Biol.
2002, 9, 103–112.
(38) Chen, H.; Walsh, C. T. Chem. Biol. 2001, 8, 301–312.
9
6622 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

SgcC-Catalyzed Hydroxylation of (S)-3-Chloro-ꢀ-tyrosyl-S-SgcC2
A R T I C L E S
3-methyl-ꢀ-tyrosine, SDS-PAGE of purified recombinant SgcC
(Figure S1), preparation and HPLC chromatography for deter-
mining the stereochemistry of the SgcC substrate (S)-3-Cl-ꢀ-
tyrosyl-S-SgcC2 (Figure S2), pH (Figure S3A) and FAD
concentration (Figure S3B) dependence of SgcC, time points
and detection wavelengths used to determine the SgcC substrate
specificity Table S1), and the ESI-MS data for the products of
SgcC-catalyzed hydroxylation of (S)-3-Cl-ꢀ-tyrosyl-S-SgcC2
and its analogues (Table S2). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA710601D
9
J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6623