CRYSTALLOGRAPHIC STUDY OF BRANCHED-CHAIN AMINO ACID AMINOTRANSFERASE FROM ESCHERICHIA COLI. Kengo Okada*, Ken Hirotsu*, Hidoyuki Hayashi** and Hiroyuki Kagamiyama**, Department of Chemistry, Faculty of Science, Osaka City University*, Department of Biochemistry, Osaka Medical College**

Branched-chain amino acid aminotransferase from Escherichia cold [E.C.2.6.1.42] (BCAT) is one of vitamin B₆ enzyme having pyridoxal-5'phosphate (PLP) as a co-factor. BCAT forms a hexamer (M.W. 204,000 Da) of six identical subunits each consisting of 308 residues (M.W. 34,000 Da). Crystallization of BCAT was performed by hanging-drop vapor diffusion method. Two polymorphic crystals with good quality for X-ray crystallography were obtained and diffracted to 2.2Å resolution with a conventional X-ray source. The crystals belong to the orthorhombic space group C222₁ with unit cell dimensions of a=156.0, b=101.2, c=141.3 Å, and the monoclinic space group C2 with unit cell dimensions of a=135.3, b=144.1, c=102.9 Å, [[beta]]=136.1deg., respectively. Each Matthews' Vm value is 2.7 (C222₁) and 3.4 (C2), which indicates three subunits related by a three-fold non-crystallographic axis in the aSymmetric unit. Multiple isomorphous replacement (MIR) techniques were used to determine the three-dimensional structure of BCAT. We succeeded in preparing two kinds of Hg derivatives (EMTS and CH₃CH₂HgCl), and initial phasing at 3.0 Å resolution.