Department of Biological Chemistry and the Life Sciences Institute, University of Michigan Medical Center, Ann Arbor, MI 48109-5606, USA.
Proc Natl Acad Sci U S A. 2010 Dec 7;107(49):20958-63. doi: 10.1073/pnas.1011448107. Epub 2010 Nov 16.
The catalytic potential for H(2)S biogenesis and homocysteine clearance converge at the active site of cystathionine β-synthase (CBS), a pyridoxal phosphate-dependent enzyme. CBS catalyzes β-replacement reactions of either serine or cysteine by homocysteine to give cystathionine and water or H(2)S, respectively. In this study, high-resolution structures of the full-length enzyme from Drosophila in which a carbanion (1.70 Å) and an aminoacrylate intermediate (1.55 Å) have been captured are reported. Electrostatic stabilization of the zwitterionic carbanion intermediate is afforded by the close positioning of an active site lysine residue that is initially used for Schiff base formation in the internal aldimine and later as a general base. Additional stabilizing interactions between active site residues and the catalytic intermediates are observed. Furthermore, the structure of the regulatory "energy-sensing" CBS domains, named after this protein, suggests a mechanism for allosteric activation by S-adenosylmethionine.
硫化氢生物合成和高半胱氨酸清除的催化潜能在胱硫醚β-合酶 (CBS) 的活性部位汇聚，CBS 是一种依赖吡哆醛磷酸的酶。CBS 通过高半胱氨酸催化丝氨酸或半胱氨酸的β-取代反应，分别生成胱硫醚和水或 H₂S。在这项研究中，报道了来自果蝇全长酶的高分辨率结构，其中捕获了碳负离子 (1.70 Å) 和氨基丙烯酸酯中间体 (1.55 Å)。活性位点赖氨酸残基的紧密定位提供了对两性离子碳负离子中间体的静电稳定作用，该赖氨酸残基最初用于内部希夫碱形成，后来用作广义碱。观察到活性位点残基与催化中间体之间的其他稳定相互作用。此外，命名为该蛋白的调节“能量感应”CBS 结构域的结构表明了通过 S-腺苷甲硫氨酸进行别构激活的机制。
