| Abstract
| - The contribution of the active-site residue, Y89, to the trillion-fold acceleration of Co−carbonbond homolysis rate in the methylmalonyl-CoA mutase-catalyzed reaction has been evaluated by site-directed mutagenesis. Conversion of Y89 to phenylalanine or alanine results in a 103-fold diminution of kcatand suppression of the overall kinetic isotope effect. The spectrum of the enzyme under steady-stateconditions reveals the presence of AdoCbl but no cob(II)alamin. Together, these results are consistentwith homolysis becoming completely rate determining in the forward direction in the two mutants and pointsto the role of Y89 as a molecular wedge in accelerating Co−carbon bond cleavage.
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