CHANGES IN PROTONATION ASSOCIATED WITH SUBSTRATE-BINDING AND COB(I)ALAMIN FORMATION IN COBALAMIN-DEPENDENT METHIONINE SYNTHASE

Methionine synthase catalyzes the transfer of a methyl group from meth ylcobalamin enzyme to homocysteine, generating methionine and cob(I)al amin enzyme, and then from methyltetrahydrofolate to cob(I)alamin enzy me, generating tetrahydrofolate and regenerating the methylcobalamin e nzyme. The reactions catalyzed by methionine synthase require deproton ation of the substrate, homocysteine, and protonation of the product t etrahydrofolate, with no net change in proton stoichiometry for a comp lete turnover cycle. In addition, formation of the intermediate cob(I) alamin enzyme requires a change in the cobalt ligand geometry from 6-c oordinate to 4-coordinate, and this rearrangement may require the tran sient protonation of protein residues to stabilize the cob(I)alamin en zyme. In the E. coli enzyme, the lower face of the methylcobalamin cof actor is coordinated by histidine 759, which is hydrogen bonded to asp artate 757 and then to serine 810, forming a ''ligand triad''. It has previously been shown that reduction of cob(II)alamin enzyme to cob(I) alamin is associated with the uptake of a proton from solution, and it has been postulated that this proton resides within the His759-Asp757 pair. Cob(I)alamin can also be generated by demethylation of methylco balamin enzyme by homocysteine; it was not known whether this mode of cob(I)alamin formation was associated with proton uptake. In this pape r, we use equilibrium titrations and kinetic analyses in the presence of the pH indicator dye phenol red, along with studies of the pH depen dence of oxidation/reduction equilibria, to identify and characterize mechanistic steps associated with proton uptake and release in both th e turnover and reactivation of the enzyme. We confirm that cob(I)alami n formation by reduction of cob(II)alamin enzyme is associated with pr oton uptake and show that mutation of Asp757 to Glu abolishes the pH d ependence of this reduction. Demethylation of methylcobalamin enzyme a lso leads to cob(I)alamin formation and is also shown to be associated with proton uptake, By observing pre-steady-state reactions with homo cysteine and methyltetrahydrofolate in the presence of phenol red, we show that this proton uptake occurs at a rate that is equal to the rat e of formation of the cob(I)alamin enzyme. In addition, we show that b inding of homocysteine to the enzyme results in the rapid release of a proton, presumably the homocysteine thiol proton. In contrast, bindin g methyltetrahydrofolate to the enzyme does not result in proton uptak e, suggesting that the proton destined for the product tetrahydrofolat e is already present on the free methylcobalamin enzyme.