title: Crystal structures of complexes of the branched-chain aminotransferase from Deinococcus radiodurans with α-ketoisocaproate and L-glutamate suggest the radiation resistance of this enzyme for catalysis.
abstract: Branched-chain aminotransferases (BCAT), which utilize pyridoxal 5'-phosphate (PLP) as a cofactor, reversibly catalyze the transfer of the α-amino groups of three of the most hydrophobic branched-chain amino acids (BCAA), leucine, isoleucine, and valine, to α-ketoglutarate to form the respective branched-chain α-keto acids and glutamate. The BCAT from Deinococcus radiodurans (DrBCAT), an extremophile, was cloned and expressed in Escherichia coli for structure and functional studies. The crystal structures of the native DrBCAT with PLP and its complexes with L-glutamate and α-ketoisocaproate (KIC), respectively, have been determined. The DrBCAT monomer, comprising 358 amino acids, contains large and small domains connected with an interdomain loop. The cofactor PLP is located at the bottom of the active site pocket between two domains and near the dimer interface. The substrate (L-glutamate or KIC) is bound with key residues through interactions of the hydrogen bond and the salt bridge near PLP inside the active site pocket. Mutations of some interaction residues, such as Tyr71, Arg145, and Lys202, result in loss of the specific activity of the enzymes. In the interdomain loop, a dynamic loop (Gly173 to Gly179) clearly exhibits open and close conformations in structures of DrBCAT without and with substrates, respectively. DrBCAT shows the highest specific activity both in nature and under ionizing radiation, but with lower thermal stability above 60 °C, than either BCAT from Escherichia coli (eBCAT) or from Thermus thermophilus (HB8BCAT). The dimeric molecular packing and the distribution of cysteine residues at the active site and the molecular surface might explain the resistance to radiation but small thermal stability of DrBCAT.
authors.affiliation: Life Science Group, Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan.
authors.forename: Chung-De
authors.initials: CD
authors.lastname: Chen
authors.forename: Chih-Hao
authors.initials: CH
authors.lastname: Lin
authors.forename: Phimonphan
authors.initials: P
authors.lastname: Chuankhayan
authors.forename: Yen-Chieh
authors.initials: YC
authors.lastname: Huang
authors.forename: Yin-Cheng
authors.initials: YC
authors.lastname: Hsieh
authors.forename: Tien-Feng
authors.initials: TF
authors.lastname: Huang
authors.forename: Hong-Hsiang
authors.initials: HH
authors.lastname: Guan
authors.forename: Ming-Yih
authors.initials: MY
authors.lastname: Liu
authors.forename: Wen-Chang
authors.initials: WC
authors.lastname: Chang
authors.forename: Chun-Jung
authors.initials: CJ
authors.lastname: Chen
mesh_terms: D000595:Amino Acid Sequence; D002384:Catalysis; D020134:Catalytic Domain; D003460:Crystallization; D018360:Crystallography, X-Ray; D034301:Deinococcus / Q000201:enzymology*; D004795:Enzyme Stability; D015964:Gene Expression Regulation, Bacterial; D015971:Gene Expression Regulation, Enzymologic; D018698:Glutamic Acid / Q000737:chemistry*; D007651:Keto Acids / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D013053:Spectrophotometry; D013379:Substrate Specificity; D000637:Transaminases / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D007651:Keto Acids; D018698:Glutamic Acid; C013082:alpha-ketoisocaproic acid; D000637:Transaminases