Van Vleet Jeremy, Kleeb Andreas, Kast Peter, Hilvert Donald, Cleland W W
Institute for Enzyme Research and Department of Biochemistry, University of Wisconsin, Madison, WI, USA.
Biochim Biophys Acta. 2010 Apr;1804(4):752-4. doi: 10.1016/j.bbapap.2009.11.018. Epub 2009 Nov 27.
The (13)C isotope effect for the conversion of prephenate to phenylpyruvate by the enzyme prephenate dehydratase from Methanocaldococcus jannaschii is 1.0334+/-0.0006. The size of this isotope effect suggests that the reaction is concerted. From the X-ray structure of a related enzyme, it appears that the only residue capable of acting as the general acid needed for removal of the hydroxyl group is threonine-172, which is contained in a conserved TRF motif. The more favorable entropy of activation for the enzyme-catalyzed process (25 eu larger than for the acid-catalyzed reaction) has been explained by a preorganized microenvironment that obviates the need for extensive solvent reorganization. This is consistent with forced planarity of the ring and side chain, which would place the leaving carboxyl and hydroxyl out of plane. Such distortion of the substrate may be a major contributor to catalysis.
詹氏甲烷球菌中预苯酸脱水酶催化预苯酸转化为苯丙酮酸的(13)C同位素效应为1.0334±0.0006。该同位素效应的大小表明该反应是协同反应。从一种相关酶的X射线结构来看，似乎唯一能够作为去除羟基所需的广义酸的残基是苏氨酸-172，它包含在一个保守的TRF基序中。酶催化过程中更有利的活化熵（比酸催化反应大25 eu）已通过一个预先组织好的微环境得到解释，该微环境无需大量的溶剂重排。这与环和侧链的强制平面性一致（这会使离去的羧基和羟基不在平面内）。底物的这种扭曲可能是催化作用的主要贡献因素。
