| Abstract
| - We have investigated the reaction of glutamate mutase with the glutamate analogue,2-thiolglutarate. In the standard assay, 2-thiolglutarate behaves as a competitive inhibitor with a Ki of0.05 mM. However, rather than simply binding inertly at the active site, 2-thiolglutarate elicits cobalt−carbon bond homolysis and the formation of 5‘-deoxyadenosine. The enzyme exhibits a complicated EPRspectrum in the presence of 2-thiolglutarate that is markedly different from any previously observed withthe enzyme. The spectrum was simulated well by assuming that it arises from electron−electron spincoupling between a thioglycolyl radical and low-spin Co2+ in cob(II)alamin. Analysis of the zero-fieldsplitting parameters obtained from the simulations places the organic radical ∼10 Å from the cobalt andat a tilt angle of ∼70° to the normal of the corrin ring. This orientation is in good agreement with thatexpected from the crystal structure of glutamate mutase complexed with the substrate. 2-Thiolglutarateappears to react in a manner analogous to that of glutamate by first forming a thiolglutaryl radical at C-4that then undergoes fragmentation to produce acrylate and the sulfur-stabilized thioglycolyl radical. Thethioglycolyl radical accumulates on the enzyme, suggesting it is too stable to undergo further steps in themechanism at a detectable rate.
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