| Abstract
| - High-level quantum chemistry calculations have been used to examine the catalytic reactions ofadenosylcobalamin-dependent glutamate mutase (GM) with the natural substrate (S)-glutamic acid. Wehave also examined the rearrangement of (S)-2-hydroxyglutaric acid, (S)-2-thiolglutaric acid, and 2-ketoglutaric acid, all of which have previously been shown to react as substrates or inhibitors of the enzyme.Our calculations support the notion that the 100-fold difference in kcat between glutamate and 2-hydroxyglutarate is associated with the relatively high energy of the glycolyl radical intermediate compared withthe glycyl radical. More generally, calculations of radical stabilization energies for a variety of substitutedglycyl radical analogues indicate that modifications at the radical center can profoundly affect the relativestability of the resulting radical, leading to important mechanistic consequences. We find that the formationof a thioglycolyl radical, derived from (S)-2-thiolglutaric acid, is highly dependent on the protonation stateof sulfur. The neutral radical is found to be of stability similar to that of the glycolyl radical, whereas the S-form of the thioglycolyl radical is much more stable, thus providing a rationalization for the inhibition of theenzyme by the substrate analogue 2-thiolglutarate. Two possible rearrangement pathways have beenexamined for the reaction of GM with 2-ketoglutaric acid, for which previous experiments had suggestedno rearrangement took place. The fragmentation−recombination pathway is associated with a fragmentationstep that is very endothermic (by 102.2 kJ mol-1). In contrast, the addition−elimination pathway hassignificantly lower energy requirements. An alternative possibility, namely, that 2-ketoglutaric acid is boundin its hydrated form, 2,2-dihydroxyglutaric acid, also leads to a pathway with relatively low energyrequirements, suggesting that some rearrangement might be expected under such circumstances.
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