Highly selective inhibitors of class II microbial fructose bis-phosphate aldolases

Article abstract of DOI:10.1021/ml100017c

We hereby describe the rationale synthesis and biochemical evaluation of the most powerful and selective inhibitors of class II fructose bis-phosphate aldolases so far reported. These inhibitors are of potential therapeutic interest, since the class II en

Full text of DOI:10.1021/ml100017c

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Highly Selective Inhibitors of Class II Microbial Fructose
Bis-phosphate Aldolases
Racha Daher and Michel Therisod*
LCBB - ICMMO, UMR 8182 Univ Paris-Sud F-91405, Orsay, France
ABSTRACT We hereby describe the rationale synthesis and biochemical evalua-
tion of the most powerful and selective inhibitors of class II fructose bis-phosphate
aldolases so far reported. These inhibitors are of potential therapeutic interest,
since the class II enzyme is present exclusively in microorganisms (among which
many pathogenic species) and is absent from man, plants, and animals.
KEYWORDS Selective inhibitors, antibiotics, microbial enzymes, metallo-enzymes,
fructose bis-phosphate aldolase, glycolysis
acterial and fungal infections due to multidrug-
resistant microorganisms represent a health problem
B
of major concern.1,2 Old known diseases of terrifying
reputation like tuberculosis or plague are re-emerging (one-
third of the entire world population is believed to be infected
by a latent form of tuberculosis).3 Recent and so far limited
epidemics of plague have been reported.4 Together with an
alarming increase in nosocomial diseases observed in hos-
pitals,5 the present situation clearly shows that there is an
urgent need for a rapid elaboration of new antibiotics. This
needs the identification of new selective microbial targets.
Class II fructose bis-phosphate aldolase (FBA)could be one
of these new therapeutic targets, since it is present exclu-
sively in microorganisms and absent in mammalians (man)
who utilize the class I enzyme. This assumption is further
supported by the inability to knock out the class II FBA genes
of Streptomyces and Escherichia coli, an indication that even
when both classes are present (like in E. coli), class II FBA is
essential.6,7 This class of enzymes is present in many
pathogenic species among which include bacteria, like
Mycobacterium tuberculosis (agent of tuberculosis), Yersinia
pestis (plague), Mycobacterium leprae (leprosy), Helicobacter
pylori (an agent of stomach ulcer and the only known
bacteria that causes cancers), and Pseudomona aeruginosa
(nosocomial severe respiratory diseases); yeasts, like Candida
albicans (an opportunistic yeast responsible for candidiases);
parasites, like Giardia lamblia; and many others including
agents of potential use in bioterrorism.
Figure 1. Mechanisms of class I (human) and class II (microbial)
FBP aldolases.
aqueous solutions, releasing (toxic) hydroxylamine, which
handicaps its possible use in vivo.
Since then, the idea of preparing inhibitors of class II FBA as
potential new antibiotics has received very limited attention.
Surprisingly, in the same time, the preparation and evaluation of
dozens of nonselective inhibitors of FBA have been reported.10
We reasoned that an “ideal” selective inhibitor should
have the following characteristics:
- Two phosphoryl groups at a proper distance from each other.
This should be longer than in the substrate FBP, to mimic the
supposed transition state of the reaction (TS in Figure 2).
- One chemical function able to strongly bind the zinc ion
present in the active site of class II FBA (and absent in class I).
- Two hydroxyl groups in charge of mimicking the alcohol functions
present at C-3 and C-4 (threo) or at C-4 and C-5 (erythro) of FBP.
Because the two classes have very different structures and
mechanisms (Figure 1), it may seem easy to prepare strong
inhibitors of the (microbial) class II FBA having only a weak
activity on (human) class I. Indeed, this had been suggested
a long time ago by Lewis and Lowe8 who reported the
synthesis of phosphoglycolo-hydroxamic acid (PGH), a first
strong inhibitor of FBA, although missing a sufficient selec-
tivity for the class II enzyme, and even for FBA.8,9 In fact,
PGH is probably a strong inhibitor of any enzyme using
DHAP as a substrate. Moreover, PGH is relatively unstable in
This logically led us to prepare the two compounds 1 and 2
represented in Figure 2. As compared to PGH, the two
Received Date: January 22, 2010
Accepted Date: February 28, 2010
Published on Web Date: March 11, 2010
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Scheme 2. Synthesis of (rac)-inhibitor 2a
Figure 2. Fischer representations of fructose bis-phosphate (FBP;
substrate of FBA), of the supposed transition state of the reaction
catalyzed by a class II FBA (TS) and of the designed inhibitors 1
and 2.
Scheme 1. Synthesis of (rac)-Inhibitor 1a
a (a) MsCl, 0.15 equiv, NEt3, dioxanne, 0 °C. (b) BnONH2, 2 equiv, MeOH,
65 °C. (c) ClCOCH2OAc, MeOH, NET3, 0 °C. (d) MeOH/NET3/H2O, room
temperature. (e) Pd (Lindlar) 10%, EtOH, room temperature. (f)
mCPBA, 2 equiv, CH2Cl2, solid Na2HPO4, 0 °C. (g) P(OBn)2N(iPr)2,
4 equiv, imidazole, triazole, AcCN, room temperature, and then
tBuOOH. (h) H2/Pd/C, MeOH, NET3, 2 equiv. (h) H2O.
inhibitors. Results of the biochemical assays are given in
Table 1.
Km/Ki values >12000 and up to 220000 are consistent
with the two compounds behaving as transition state analo-
gue inhibitors of class II FBA.11 Alternatively, one can con-
sider 1 and 2 as efficient bisubstrate type inhibitors; the main
factor responsible for the inhibitory power is the presence of
the hydroxamate function. Indeed, the moderate improve-
ment in the inhibitory properties of 1 and 2 (40-fold) as
compared to our previous product 4 (Table 2) can be
attributed to a better adjustment of the “glyceraldehyde-
phosphate” part of the inhibitor. The two compounds were
found to give pure competitive inhibition patterns. At the
same time, they are only weak substrate analogue inhibitors
of the class I enzyme RAMA (Km/Ki < 0.31). The selectivity
for class II FBA, arbitrarily but logically calculated as Km/
Ki class II/Km/Ki class I, reaches unprecedented values of up to
7.1 Â 105, which validates a posteriori the design of these
inhibitors. The relative stereochemistries of 1 and 2 seem of
importance only in the case of the C. albicans enzyme.
Differences in Ki and selectivity values were found between
enzymes from various origins, in spite of the high analogy
observed between class II FBAs of known structure.12-15
This is probably due to subtle differences in accommoda-
tion of the substrate and inhibitors within the active site,
which will be highlighted only after crystallization of the
enzyme-inhibitor complexes. This work is currently under
progress.
a (a) DIBALH, 2 equiv, toluene, 0 °C. (b) BnONH2, 2 equiv, MeOH, 65 °C.
(c) ClCOCH2OAc, MeOH, NET3, 0 °C. (d) MeOH/NET3/H2O, room
temperature. (e) mCPBA, 2 equiv, CH2Cl2, solid Na2HPO4, 0 °C.
(f) P(OBn)2N(iPr)2, 4 equiv, imidazole, triazole, AcCN, room tempera-
ture, and then tBuOOH. (g) H2/Pd/C, MeOH, NET3, 2 equiv. (h) H2O.
compounds bearing an N-alkylated hydoxamic acid are also
much more stable in water. The syntheses of the two
inhibitors, summarized in Schemes 1 and 2, are based on
classical organic reactions and are fully reported in the
Supporting Information.
Compounds 1 and 2 were characterized as single pro-
ducts, showing that the key steps, epoxidation and hydro-
lysis of the intermediate epoxides, were stereospecific
reactions. The conditions for the final debenzylation in
the presence of two molar equivalents of triethylamine
were finally selected after several trials. In the absence
of base or in the presence of 4 equiv or more of a base,
mixtures of products were obtained, probably as a conse-
quence of some rearrangement. We were somewhat sur-
prised by the spontaneous opening of the intermediate
epoxides in pure water, leading to 1 and 2 without the
addition of any acidic catalyst. Most probably, a participation
of one of the phosphate groups in an intramolecular acid
catalysis occurs.
We next took advantage of the availability of recombinant
class II FBAs derived from four pathogenic microorganisms
to test our products in vitro on these enzymes. By compar-
ison, commercial aldolase from rabbit muscle (RMA), very
close to the human enzyme (98% identity), was taken as
representative of class I FBA to check the selectivity of the
The most relevant structural characteristics for a com-
pound to be a good inhibitor of class II FBA can be deduced
from a survey of the inhibitory properties of our previously
prepared products,12,16 as compared to 1 (Table 2): As
expected, the presence of the hydroxamate function and
of two phosphoryl groups like in FBP is crucial. The distance
between the two phosphate groups in compound 4 is
obviously in good accordance with the one found in sub-
strate FBP. By increasing this distance and adding two
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Table 1. Biochemical Estimation of Compounds 1 and 2 on Class I and Class II FBA from Various Origins
compd
FBAa
Km or Ki (selectivity  103) b
RM
H.p
Y.p
C.a
M.tb
FBP
1
17 ( 1 μM
55 ( 3.3 μM
100 ( 6 μM
37 ( 2.2 μM
55 ( 3.3 μM
100 ( 6 μM
11 ( 0.7 μM
18 ( 1.1 nM (6.6)
20 ( 1.2 nM (10.9)
4.5 ( 0.3 nM (39.4)
3.5 ( 0.2 nM (92.5)
0.45 ( 0.03 nM (717)
10 ( 0.6 nM (58.8)
0.35 ( 0.02 nM (101.3)
0.47 ( 0.03 nM (137.7)
2
a RM, rabbit muscle; H.p, H. pylori; Y.p, Y. pestis; C.a, C. albicans; and M.tb, M. tuberculosis. b Expressed as Km/Ki class II/Km/Ki class I
.
Table 2. Inhibitory Properties of Various Inhibitors against
M. tuberculosis FBA
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SUPPORTING INFORMATION AVAILABLE Detailed syn-
theses of compounds 1 and 2, NMR spectra, and enzymatic kinetics
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author: *To whom correspondence should be
addressed. E-mail: therisod@icmo.u-psud.fr.
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class II fructose bisphosphate aldolases: Towards new synthetic
antibiotics. Chem.;Eur. J. 2008, 14, 8521–8529.
Funding Sources: This work was supported by a scholarship to
R.D. from the Region Ile de France.
ACKNOWLEDGMENT We are grateful to Prof. Jurgen Sygusch
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Dr. Mary Jackson (Colorado State University) for the kind supply
of M. tb and Y. pestis FBAs, and Dr. Jean-Michel Bruneau (Novexel,
Romainville, France) for the kind supply of C. albicans FBA.
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