A Ligand-Induced Conformational Change of Yeast Dipeptidase at Physiological pH: Kinetic Consequences and Possible Regulatory Significance

Abstract

The hydrolysis of dipeptides by purified yeast dipeptidase (EC 3.4.13.?) shows marked deviations from Michaelis-Menten kinetics over a wide range of pH. Quite anomalous kinetics is observed between pH 6 and 7, indicating a drastic change in the enzyme's properties. A reasonable explanation is provided by the assumption of a conformational transition brought about either by pH shifts or, at a constant pH, by changes in the substrate concentration. The transition, which may have a half-life on the order of minutes under appropriate conditions, is a distinctly cooperative process, with a dependence on ligand concentration higher than first order. The two forms of the enzyme differ clearly from each other with respect to various properties. The magnitudes and pH dependence of the kinetic parameters as well as the type of inhibition (or activation) exerted by amino acids and other ligands are different, as are their heat stabilities and the rates of inactivation by photooxidation of proteolytic degradation. Neither the molecular weight nor the gross conformation of the enzyme changes during the transition, so it seems to be due to a local isomerization affecting mainly the geometry of the active site. The sensitivity of dipeptidase to changes in the concentrations of substrates and other ligands is most pronounced exactly at the values of pH known to prevail in the living yeast cell. Thus the observed effects, which modulate dipeptidase activities within wide and limits, according to the amounts of dipeptides and amino acids present, are likely to play a role in the regulation of the enzyme in vivo.