Crystallization was carried out by first dialyzing the purified enzyme against a buffer containing 10 mM NaH2PO4, 100 mM NaCl, 5 mM DTT, 1 mM EDTA, pH 7.4. The dialyzed protein was concentrated to 10 mg/ml using an Amicon concentrator (PM10, 43 mm). Crystals of gMDH were grown at 20°C by hanging drop - vapor diffusion from a mother liquor containing 100 mM sodium citrate pH 4.5, 1 mM DTT, and PEG 8000. Needle-shaped crystals of gMDH appeared in a few days in the range of 6.5–11.0 % and 9.0–13.5% (w/v) of PEG 8000, respectively. gMDH was cryo-protected by adding a 50% glycerol solution to the crystallization drop resulting in a final concentration of 33% (v/v) glycerol. The crystal was then transferred to liquid nitrogen using a nylon loop. Data sets for native gMDH were collected at the SBC Beamline 19-ID (ANL-APS) with the 3x3 CCD area detector set at 2q = 0°. Phases were found for gMDH by molecular replacement using the dimeric structure of porcine heart mitochondrial malate dehydrogenase substituted with poly-alanine as a search model using X-PLOR. One dimer was present in the asymmetric unit. The phases of D157N gMDH were also found by molecular replacement using the partially refined model of gMDH and the program EPMR. Four dimers were present in the asymmetric unit of a P21 unit cell. Both x-ray structures were refined using Crystallography and NMR System (CNS v. 0.3). The models were refined by rigid body, then simulated annealing, positional refinement, and B-factor refinement. Five percent of the data were omitted from refinement and used for Rfree calculations. Non-crystallographic symmetry was used to restrain the 4 dimers of gMDH during the early refinement, then released during subsequent rounds. The program O was used for model rebuilding after each round of crystallographic refinement This work is supported by NSF Grant MCB 0448905 to EB