Journal of the American Chemical Society
Article
Intact Mass Spectrometry of GABA-AT. Three reaction samples
were prepared as described previously with only GABA-AT, including
vigabatrin, or CPP-115.30 After incubation with or without inhibitors,
half of each reaction mixture was reduced with 15 mM sodium
cyanoborohydride for 1 h. Both reduced and nonreduced samples
were buffer exchanged into 50 mM ammonium acetate using 30 kDa
MWCO filters (Millipore). Nano-LC/MS/MS runs were done on a 75
μm ID × 10 cm Kinetex C8 (Phenomenex) column connected to an
autosampler (Dionex Ultimate 3000 RSLCnano system) and a Velos
Elite Orbitrap (ThermoFisher) mass spectrometer. Results were
deconvoluted using ProMass Deconvolution software (Thermo
Scientific).
Enzyme Crystallization. Before crystallization, GABA-AT and
CPP-115-inactivated GABA-AT were exchanged into a buffer that
contained 40 mM sodium acetate (pH 5.5). After the initial
crystallization screening and optimization, the proteins were crystal-
lized via the hanging drop method. Hanging drops were prepared by
mixing 1 μL of 12 mg/mL native GABA-AT protein solution and 1 μL
of the reservoir solution, containing 0.1 M ammonium acetate, 0.1 M
bis-Tris (pH 5.5), and 17% w/v PEG 10 000. Crystals appeared within
24 h at 20 °C and grew for 5−6 days before harvesting. Crystals with
good morphology and large sizes were transferred to cryoprotecting
conditions, which contain 20% glycerol in addition to the original
composition of the reservoir solution, before being frozen in liquid
nitrogen. The same method of crystallization was applied for CPP-
115-inactivated GABA-AT; however, before freezing in liquid nitrogen,
the inactivated crystals were transferred to a cryoprotecting solution
that contained 20% glycerol and 2 mM CPP-115, in addition to the
compounds of the reservoir solution. Crystallographic data were
collected on beamlines 23ID-B and 23ID-D of GM/CA@APS of the
Advanced Photon Source (APS) using X-rays of 0.99 Å wavelength
and Rayonix (formerly MAR-USA) 4 × 4 tiled CCD detector with a
300 mm2 sensitive area. All data were indexed, integrated, and scaled
with HKL2000. Data collection and processing statistics are given in
Table S1 (Supporting Information).
Phasing, Model Building, and Refinement. Molecular replace-
ment for the native GABA-AT was carried out using the program
Phaser31 from CCP432 software suite, using the previously reported
coordinates of GABA-AT from pig liver (PDB ID: IOHV) as the
starting search model. The initial Rfree and R factor of the correct
solution were 0.1904 and 0.1934, respectively. The rigid body
refinement was followed by restrained refinement using Refmac5.33
Manual adjustment and modification of the structure based on
electron density maps were performed using the program Coot.34 The
Rfree and R factor values of the final model were 0.1513 and 0.1766,
respectively.
The data sets from the inactivated GABA-AT crystals were
isomorphous to those of native GABA-AT. Therefore, rigid body
refinement could be used directly, placing the previously refined native
GABA-AT model into the asymmetric unit of inactivated GABA-AT
crystals. Model building and refinements of the inactivated structures
were carried out following the same protocol as the native structure.
There were no ligand coordinates included in the refinement until the
refinement converged. Without ligands built in, the Fo−Fc map shows
a well-defined electron density supporting the existence of the bound
ligand.
started with energy minimization, continued with molecular dynamics
(4 ns), and had a final energy minimization step. The final structures
were used for evaluation.
ASSOCIATED CONTENT
* Supporting Information
■
S
Reactivation results of GABA-AT after inactivation; fluoride ion
release results; cofactor release results; high resolution mass
spectra; mass fragmentation data of metabolites 20−22; middle
down peptide proteomics results, omit map for inactivated
enzyme, crystallography data collection, processing statistics.
This material is available free of charge via the Internet at
AUTHOR INFORMATION
Corresponding Author
■
Present Addresses
⊥Molecular Recognition Research Center, Korea Institute of
Science and Technology (KIST), Seoul, Korea.
¶Department of Chemistry, Salisbury University, Salisbury, MD
21801.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the National Institutes of Health for
financial support (grants GM066132 and DA030604 to R.B.S.;
GM067725 to N.L.K.; contracts ACB-12002 and AGM-12006
to R.S. This research used resources of the Advanced Photon
Source, a U.S. Department of Energy (DOE) Office of Science
User Facility operated for the DOE Office of Science by
Argonne National Laboratory under Contract No. DE-AC02-
06CH11357.).
REFERENCES
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The structure of the inactivator was made in the program
ChemDraw generating a “Mol” file as the output. This output ligand
structure was then regularized and its chemical restraints were
generated in the program JLigand.35 Using Coot, the inactivator was
manually fit to the residual electron density in the difference (Fo−Fc)
map. The complex structure with the ligand bound was further refined
in Refmac5. The Rfree and R factor for the inactivated structure were
0.1575 and 0.1872, respectively.
Molecular Modeling. All renderings were performed in PyMol.36
Computer simulations were carried out as previously described.37 In
essence, the ligands (including the cofactor) were docked into the
active site of the prepared protein using Autodock 4.2,38 with Lys329
being flexible. The best docked structures were then refined by
molecular mechanics, using GROMACS 4.5.39 The sequence utilized
L
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX