Evidence for the chemical activation of essential Cys-302 upon cofactor binding to nonphosphorylating glyceraldehyde 3-phosphate dehydrogenase from Streptococcus mutans

Nonphosphorylating glyceraldehyde 3-phosphate dehydrogenase (GAPN) from Streptococcus mutans which catalyzes the irreversible oxidation of D-glyceraldehyde-3 phosphate (D-G3P) into 3-phosphoglycerate (3-PGA) in the presence of NADP belongs to the aldehyde dehydrogenase (ALDK) superfamily. Oxidation of D-G3P into 3-PGA by GAPN involves the formation of a covalent enzyme intermediate via the nucleophilic attack of the invariant Cys-302, Titration of Cys-302 in the ape-enzyme by two different kinetic probes, iodoacetamide and 2,2'-dipyridyl disulfide, shows a pK(app) of 8.5 and a chemical reactivity surprisingly low compared to a reactive and accessible thiolate, Binding of NADP causes a strong increase of the reactivity of Cys-302-which is time dependent-with a pK(app) shift from 8.5 to 6.1, Concomitant with the increase in the Cys-302 reactivity, an additional protein fluorescence quenching is observed. These data suggest that cofactor binding induces at least a local conformational rearrangement within the active site. The efficiency of the rearrangement depends on the structure of the cofactors and on the protonation of an amino acid with a pK(app) of 5.7. The rate of the rearrangement also strongly increases when temperature decreases. The data on the conformational rearrangement also reveal an amino acid with a pK(app) of 7.6 whose deprotonation increases the reactivity of the thiolate of Cys-302 by a 3-fold factor. The nature of the amino acid involved-which should be located close to Cys-302 in the hole-active form-is likely the invariant Glu-268. Changing Glu-268 into Ala or Cys-302 into Ala leads to mutants in which the rearrangement is only efficient in the presence of saturating concentrations of both NADP and G3P. The structural aspects of the conformational rearrangement occurring during the catalytic process in the wild-type GAPN should include at least reorientation of both Cys-302 and Glu-268 side chains and repositioning of the nicotinamide ring of the cofactor to permit the chemical activation of Cys-302 and the formation of an efficient ternary complex. Thus, it is likely that the conformation of the active site in the reported X-ray structures of ALDHs determined so far in the presence of cofactor, in which the side chains of Cys-302 and Glu-268 are 6.7 Angstrom apart from each other, does not represent the biological active form.