Evidence for coupled motion and hydrogen tunneling of the reaction catalyzed by glutamate mutase

被引:18
|
作者
Cheng, Mou-Chi
Marsh, E. Neil G. [1 ]
机构
[1] Univ Michigan, Dept Chem, Ann Arbor, MI 48109 USA
[2] Univ Michigan, Dept Biol Chem, Ann Arbor, MI 48109 USA
关键词
D O I
10.1021/bi0616908
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
Glutamate mutase is one of a group of adenosylcobalamin-dependent enzymes that catalyze unusual isomerizations that proceed through organic radical intermediates generated by homolytic fission of the coenzyme's unique cobalt-carbon bond. These enzymes are part of a larger family of enzymes that catalyze radical chemistry in which a key step is the abstraction of a hydrogen atom from an otherwise inert substrate. To gain insight into the mechanism of hydrogen transfer, we previously used pre-steady-state, rapid-quench techniques to measure the alpha-secondary tritium kinetic and equilibrium isotope effects associated with the formation of 5'-deoxyadenosine when glutamate mutase was reacted with [5'-H-3]adenosylcobalamin and L-glutamate. We showed that both the kinetic and equilibrium isotope effects are large and inverse, 0.76 and 0.72, respectively. We have now repeated these measurements using glutamate deuterated in the position of hydrogen abstraction. The effect of introducing a primary deuterium kinetic isotope effect on the hydrogen transfer step is to reduce the magnitude of the secondary kinetic isotope effect to a value close to unity, 1.05 +/- 0.08, whereas the equilibrium isotope effect is unchanged. The significant reduction in the secondary kinetic isotope effect is consistent with motions of the 5'-hydrogen atoms being coupled in the transition state to the motion of the hydrogen undergoing transfer, in a reaction that involves a large degree of quantum tunneling.
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页码:883 / 889
页数:7
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