The stereospecificity of oxaloacetate decarboxylase: a stereochemical imperative?
The stereospecificity of oxaloacetate decarboxylase: a stereochemical imperative?
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草酰乙酸脱羧酶的立体特异性：立体化学必要性？
DOI：
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发表时间：
1987
期刊：
影响因子：
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通讯作者：
S. Benner
中科院分区：
文献类型：
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作者：
J. Piccirilli;J. Rozzell;S. Benner
生物有机化学家通常期望在催化类似反应的酶中具有相同的立体特异性。当在一类酶中观察到立体化学多样性时，通常认为反映了潜在的机制多样性。例如，产生滞留的P-脱羧酶和产生倒位的P-脱羧酶都是已知的。此外，立体特异性似乎与机制和底物结构相关：β-酮酸脱羧酶作用于酮是羧酸酯的底物（方案I）需要金属离子并产生保留;不作用于α-酮酸的脱羧酶不需要金属离子并产生转化（表I）表1中的相关性表明，在这类酶中观察到的立体化学和机理多样性是合理的。在达尔文进化论的背景下，这就要求自然选择对每种结构类型的底物都偏爱一种立体化学过程。我们经常注意到区分酶的功能性和非功能性行为的重要性，“这两种行为都可能反映蛋白质结构和催化机制的基本原则，并指导蛋白质工程。第一次独立的相关性测试产生了一个令人惊讶的结果。乙酰乙酸脱羧酶，预测产生转化，产生基本上是外消旋产物。这一结果是合理的特设假设，中间体烯胺的反应性是足够大，允许其质子化直接由溶剂。虽然这种特别的修饰似乎与天冬氨酸和脱羧酶的立体化学数据一致，但相关性的独立测试仍然难以捉摸。
Bioorganic chemists commonly expect the same stereospecificity in enzymes catalyzing analogous reactions.' When stereochemical diversity is observed within a class of enzymes, it is often presumed to reflect an underlying mechanistic diversity.'-3 For example, both P-decarboxylases that produce retention and P-decarboxylases that produce inversion are known. Further, stereospecificity appears to correlate with mechanism and substrate structure: P-ketoacid decarboxylases acting on substrates where the ketone is a to a carboxylate (Scheme I) require a metal ion and produce retention; decarboxylases not acting on a-keto acids do not require a metal ion and produce inversion (Table I).4 The correlation in Table I suggests that the stereochemical and mechanistic diversity observed in this class of enzyme is f~nct ional .~ In the context of Darwinian evolution, this requires that natural selection favors one stereochemical course over the other for each structural class of substrates. We have often noted the importance of distinguishing between functional and nonfunctional behaviors of enzymes," both to identify behaviors likely to reflect fundamental principles of protein structure and catalytic mechanism and to guide protein engineering.I2 Therefore, mechanistic explanations for the stereospecificity of decarboxylases need to be examined further. The first independent test of the correlation produced a surprising result. Acetoacetate decarboxylase, predicted to produce inversion, produced essentially racemic product. This result was rationalized ad hoc by presuming that the reactivity of an intermediate enamine was sufficiently great to permit its protonation directly by solvent. While this ad hoc modification appeared to be consistent with stereochemical data for aspartate &decarboxyla~e, '~ an independent test of the correlation remained elusive.