Kinetic Studies of a Coenzyme B12 Dependent Reaction Catalyzed by Glutamate Mutase from Clostridium cochlearium

The coenzyme B12 dependent glutamate mutase is composed of two apoenzyme proteins subunits; S and E2, which while either fused or separate assemble with coenzyme B12 to form an active holoenzyme (E2S2-B12) for catalyzing the reversible isomerization between (S)-glutamate and (2S, 3S)-3-methylas- partate. In order to assay the activity of glutamate mutase by UV spectrophotometry, this reaction is often coupled with methylaspartase which deaminates (2S, 3S)-3-methylaspartate to form mesaconate (λmax = 240 nm, Ɛ240 = 3.8 mM-1·cm-1). The activities of different reconstitutions of glutamate mutase from separate apoenzyme components S and E in varied amounts of coenzyme B12 and adenosylpeptide B12 as cofactors were measured by this assay and used to reveal the binding properties of the cofactor by the Michaelis- Menten Method. The values of Km for coenzyme B12 in due to reconstitutions of holoenzyme in 2, 7 and 14 S: E were determined as; 1.12 ± 0.04 μM, 0.7 ± 0.05 μM and 0.52 ± 0.06 μM, respectively, so as those of adenosylpeptide B12; 1.07 ± 0.04 μM and 0.35 ± 0.05 μM as obtained from respective 2 and 14 S: E compositions of holoenzyme. Analysis of these kinetics results curiously associates the increasing affinity of cofactors to apoenzyme with increased amount of component S used in reconstituting holoenzyme from separate apoenzyme components and cofactor. Moreover, in these studies a new method for assaying the activity of glutamate mutase was developed, whereby glutamate mutase activity is measured via depletion of NADH (λmax = 340 nm, Ɛ340 = 6.3 mM-1·cm-1) as determined by UV spectrophotometry after addition of (2S, 3S)-3-methylaspartate and pyruvate to a mixture of E2S2-B12 and two auxiliary holoenzymes system; pyridoxal-5-phosphate dependent glutamate-pyruvate aminotransferase and NADH dependent (R)-2-hydroxyglutarate dehydrogenase. The activity of glutamate-pyruvate aminotransferase was relatively complete recovered upon the addition of (S)-glutamate and pyruvate to the mixtures of hologlutamate-pyruvate aminotransferase and (R)-2-hydroxylglutarate dehydrogenase which were incubated with each putative inhibitor of glutamate mutase. Additionally, the new assay was used to determine the kinetic constants of (2S, 3S)-3-methylaspartate in the reaction of glutamate mutase as Km= 7 ± 0.07 mM and kcat= 0.54 ± 0.6 s-1. Application of Briggs-Haldane formula allowed the calculation of an equilibrium constant of the reversible isomerization, Keq = [(S)-glutamate] × [(2S, 3S)-3-methylaspartate]-1 = 16, where the kinetic constants of (S)-glutamate were determined by the standard methylaspartase coupled assay.