| Abstract: | The two fast-reacting thiol groups of pig muscle 3-phosphoglycerate kinase can be simultaneously blocked by one mole equivalent of bifunctional reagent: either mercuric chloride (HgCl2) or 1,4-bis(bromomercuri)butane. The reactions are accompanied by an enzyme activity loss of about 50-70% and 60-80% with mercuric chloride and 1,4-bis(bromomercuri)butane respectively. Removal of either of the reagents with excess cysteine leads to the recovery of at least 70-90% of the original enzymic activity. Gel chromatographic analysis revealed no change in the molecular mass of the enzyme modified with mercuric chloride, while an increase of about 30% of the apparent molecular mass was observed after the reaction with 1,4-bis(bromomercuri)butane. Since no dimer formation could be detected by independent crosslinking, the increase of the apparent molecular mass is probably due to modification causing protein conformational change. The results strongly suggest that the fast-reacting thiols are intramolecularly connected by either of the above bifunctional reagents. In the light of the known structural data on the enzyme, it may follow that the two fast-reacting thiols belong to the two sequentially neighbouring cysteinyl residues. |
