Pre-steady-state kinetic investigation of intermediates in the reaction catalyzed by adenosylcobalamin-dependent glutamate mutase.

Abstract

Glutamate mutase catalyzes the reversible isomerization of L-glutamate to L-threo-3-methylaspartate. Rapid quench experiments have been performed to measure apparent rate constants for several chemical steps in the reaction. The formation of substrate radicals when the enzyme was reacted with either glutamate or methylaspartate was examined by measuring the rate at which 5'-deoxyadenosine was formed, and shown to be sufficiently fast for this step to be kinetically competent. Furthermore, the apparent rate constant for 5'-deoxyadenosine formation was very similar to that measured previously for cleavage of the cobalt-carbon bond of adenosylcobalamin by the enzyme, providing further support for a mechanism in which homolysis of the coenzyme is coupled to hydrogen abstraction from the substrate. The pre-steady-state rates of methylaspartate and glutamate formation were also investigated. No burst phase was observed with either substrate, indicating that product release does not limit the rate of catalysis in either direction. For the conversion of glutamate to methylaspartate, a single chemical step appeared to dominate the overall rate, whereas in the reverse direction a lag phase was observed, suggesting the accumulation of an intermediate, tentatively ascribed to glycyl radical and acrylate. The rates of formation and decay of this intermediate were also sufficiently rapid for it to be kinetically competent. When combined with information from previous mechanistic studies, these results allow a qualitative free energy profile to constructed for the reaction catalyzed by glutamate mutase.