Glutamate mutase mechanism

Epimerization of sugar, mechanisms 778 Epimers, definition of 163 Epinephrine (adrenaline) 542,553, 553s Episomes. See plasmid Epithelial cells 29 Epitheliocytes 25 Epoxides, alkyation by 254 Epoxide hydrolases 591 EPR (electron paramagnetic resonance) spectroscopy 398, 399 of glutamate mutase 873 in study of phosphotransferases 639 EPSP (enolpyruvoylshikimate-3-phosphate) 687s... 

FIGURE 16. Concerted and kinetically coupled mechanisms for CooC bond homolysis and hydrogen atom abstraction, illustrated for the reaction catalyzed by glutamate mutase. Either mechanism could give rise to the deuterium isotope effects observed in pre-steady state stopped flow experiments. 

The reaction catalyzed by glutamate mutase stands out as the only case in which the migrating carbon is sp hybridized, and hence rearrangement through a cyclic intermediate is not possible (pyridoxal phosphate is not a cofactor). A plausible mechanism for the rearrangement of glutamyl radical... 

The first common step in AdoCbl-dependent readions is homolytic cleavage of the cobalt-carbon bond to generate a radical pair, cob(ii)alamin and the carbon-centered dAdo radical (Scheme 19.3). This reaction experiences a 10 -fold rate enhancement in B12 enzymes [14, 15] in the presence of substrate, and the mechanism for this rate acceleration has been the subject of extensive scrutiny. Thus, in methylmalonyl-CoA mutase and in glutamate mutase, little if any destabilization of the cobalt-carbon bond is observed in the reactant state, as revealed by resonance Raman spectroscopy [16, 17], and the intrinsic substrate binding is utilized to labilize the bond. In contrast, approximately half of the destabilization of the cobalt-carbon bond in diol dehydratase is expressed in the reactant state. This re-... 

In glutamate mutase [43], the forward and reverse steady-state deuterium (kn/ko of 3.9 forward and 6.3 reverse) and tritium kn/kj of 21 forward and 19 reverse) kinetic isotope effects are both suppressed. However large deuterium isotope effects of 28 and 35 in the forward and reverse directions respectively have been observed for cob(ii)alamin formation under pre-steady-state conditions. These large kinetic isotope effects suggest that quantum mechanical tunneling also dominates this enzyme reaction. 

Fig. 18 Glutamate mutase (GM) interconverts (S)-glutamate and (2S,3S)-3-methylaspartate. Proposed reaction mechanism of the carhon skeleton rearrangement, catalyzed by GM involving H-atom abstraction (step a), radical rearrangement (step b) and back transfer of H-atom (step c). (The experimentally supported substrate triggered formation of the 5 -deoxy-5 -adenos)d radical and of cob(ll)alamin (23, Bi2r) by homolysis of protein bound AdoCbl (2) is omitted here, see Fig. 16 [173,175,176])...

Fig. 14. Proposed mechanism of glutamate mutase involving H-atom abstraction (step a), radical rearrangement (by a fragmentation—addition path) (step b), and H-atom back transfer (step c) (62).

For systems such as glutamate mutase, 2-methyleneglutarate mutase and methylmalonyl-CoA, Golding and co-workers have proposed the rather different radical rearrangement mechanism illustrate in Scheme 8.5 for 2-methyleneglutarate mutase. 

The rearrangement described above by Barker using the coenzyme B12-dependent enzyme glutamate mutase is a most remarkable reaction. Until very recently, no analogous chemical reaction was known. In fact, elucidation of the structure of the coenzyme form of vitamin B12 did not clarify its mechanism. Beside this transformation, nine distinct enzymatic reactions requiring coenzyme B12 as cofactor are known. Most of which are without precedent in terms of organic reactions. They are listed in Fig. 6.11. In choosing vitamin B12 derivatives as coenzymes, enzymes appear to have reached a peak of chemical sophistication which would be difficult to mimic by the chemist. 

FIGURE 21. Mechanisms for the rearrangement of substrate radicals in the reactions catalyzed by carbon skeleton mutases. For 2-methyleneglutarate mutase and the acyl-CoA mutases both associative (upper pathway) and dissociative (lower pathway) mechanisms have been proposed (Halpem, 1985 Bucket Golding, 1996), whereas for glutamate only a dissociative mechanism appears feasible. 

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