TRANSITION-STATE STABILIZATION AND ENZYMIC CATALYSIS - KINETIC AND MOLECULAR-ORBITAL STUDIES OF REARRANGEMENT OF CHORISMATE TO PREPHENATE
TRANSITION-STATE STABILIZATION AND ENZYMIC CATALYSIS - KINETIC AND MOLECULAR-ORBITAL STUDIES OF REARRANGEMENT OF CHORISMATE TO PREPHENATE
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DOI：
10.1021/bi00742a022
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发表时间：
1973-01-01
期刊：
影响因子：
2.9
通讯作者：
YOUNG, IG
中科院分区：
文献类型：
--
作者：
ANDREWS, PR;SMITH, GD;YOUNG, IG
P. R. Andrews, Geoffrey D. Smith,* and IG Young abstract: The nonenzymatic conversion of chorismate to prephenate has been studied to provide information relevant to the mechanism of the enzyme-catalyzed reaction. Kinetic studies show that between 20 and 65 chorismate is approxi-mately 80% converted to prephenate and thence phenyl-pyruvate in a consecutive reaction and 20% converted to other products (mainly 4-hydroxy benzoate). By measuring the temperature dependence of the rates of these reactions we have shown that the first-order rearrangement of chorismate to prephenate has an enthalpy of activation of 20.71±0.35 kcal/mol and an entropy of activation of—12.85±0.42 eu. Using the turnover number calculated from the maximum velocity of the enzymatic reaction with chorismate mutase-prephenate dehydrogenase from Aerobacter aerogenes (Koch, G. L. E., Shaw, D. C., and Gibson, F.(1970), Biochim. Biophys. Acta 212, 375;(1972), ibid. 258, 719) we have calculated that this enzyme enhances the rate of reaction at pH p1 vn7ymw are thought to achieve their enormous rate enhancements of biological reactions in two main ways:(i) a reduction in the entropy differencebetween the substrate (s) and the transition state which results from the greater loss of entropy by the substrate (s) on binding to the enzyme. This effect has also been described wholly or partly as a proximity, concentration, or orbital steering effect (Page and Jencks, 1971; Dafforn and Koshland, 1971; Page, 1972);(ii) a reduction in the enthalpy difference between the substrate (s) and the transition state when both are bound to the enzyme. This results from the greater affinity of the transition state for the active site of the enzyme. This effect has also been described as a strain or distortion effect (Jencks, 1969). In the case of intramolecular reactions the problem is greatly simplified because the entropy effect involves only the loss of internal rotational freedom and a study of the activity of the enzyme reduces largely to a consideration of bond formation favoring the transition state over the substrate. A particularly interesting enzymatic reaction in this class is the intramolecular rearrangement of chorismate to prephenate, which is the first specific step in the biosynthetic pathways leading to tyrosine and phenylalanine in bacteria and other organisms (see Gibson and Pittard, 1968). This reaction also proceeds spontaneously in vitro, probably by an Sn/'mechanism (see Edwards and Jackman, 1965). We havestudied this nonenzymatic conversion of chorismate to prephenate in order to provide information relevant to the mechanism of the enzyme-catalyzed reaction.
