Conway Myra E, Yennawar Neela, Wallin Reidar, Poole Leslie B, Hutson Susan M
Department of Biochemistry, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157, USA.
Biochim Biophys Acta. 2003 Apr 11;1647(1-2):61-5. doi: 10.1016/s1570-9639(03)00051-7.
Crystal structures of the fold type IV pyridoxal phosphate (PLP)-dependent human mitochondrial branched chain aminotransferase (hBCATm) reaction intermediates have provided a structural explanation for the kinetically determined substrate specificity of hBCATm. The isoleucine side chain in the ketimine intermediate occupies a hydrophobic binding pocket that can be defined by three surfaces. Modeling of amino acids on the ketimine structure shows that the side chains of nonsubstrate amino acids such as the aromatic amino acids, alanine, or aspartate either are unable to interact through van der Waals' interactions or have steric clashes. The structural and biochemical basis for the sensitivity of the mammalian BCAT to reducing agents has also been elucidated. Two cysteine residues in hBCATm, Cys315 and Cys318 (CXXC), are part of a redox-controlled mechanism that can regulate hBCATm activity. The residues surrounding Cys315 and Cys318 show considerable sequence conservation in the prokaryotic and eukaryotic BCAT sequences, however, the CXXC motif is found only in the mammalian proteins. The results suggest that the BCAT enzymes may join the list of enzymes that can be regulated by redox status.
IV型折叠的磷酸吡哆醛（PLP）依赖性人线粒体支链氨基转移酶（hBCATm）反应中间体的晶体结构，为hBCATm动力学测定的底物特异性提供了结构解释。酮亚胺中间体中的异亮氨酸侧链占据一个可由三个表面定义的疏水结合口袋。在酮亚胺结构上对氨基酸进行建模表明，非底物氨基酸（如芳香族氨基酸、丙氨酸或天冬氨酸）的侧链要么无法通过范德华相互作用进行相互作用，要么存在空间冲突。哺乳动物BCAT对还原剂敏感性的结构和生化基础也已阐明。hBCATm中的两个半胱氨酸残基Cys315和Cys318（CXXC）是一种氧化还原控制机制的一部分，该机制可调节hBCATm的活性。Cys315和Cys318周围的残基在原核和真核BCAT序列中显示出相当程度的序列保守性，然而，CXXC基序仅在哺乳动物蛋白中发现。结果表明，BCAT酶可能会加入到可由氧化还原状态调节的酶的列表中。
