Spectrophotometric studies on the interaction between triose phosphate isomerase and inhibitors.

The binding of ligands to chicken muscle triose phosphate isomerase was studied. Changes in u.v. absorbance of the enzyme were used to measure binding, and the dissociation constant was determined over a range of pH values. The ligands were 2-phosphoglycollate and rac-glycerol 3-phosphate (only the D-isomer, sn-glycerol 1-phosphate, binds appreciably). Non-linear regression was used to fit calculated curves to the experimental points and hence to compare different models. Both active sites in the dimeric enzyme probably bound 2-phosphoglycollate, without any interaction between the sites. The results of crystallographic analysis [phillips, Rivers, Sternberg, Thornton & Wilson (1977) Biochem. Soc Trans. 5, 642--647], and experiments on the 1H, 13C and 31P n.m.r. of enzyme or 2-phosphoglycollate were combined with the present results to provide the basis for a model in which binding depends on glutamic acid-165 being protonated and on the ligant being fully ionized; additionally, binding affects the ionization of one histidine residue (probably histidine-100). The binding of the glycerol 3-phosphate, on the other hand, was independent of pH over the range pH 6.5--8.5 but decreased at lower pH values. This is explained on a model in which the binding of the monoanion of the ligand is markedly affected by the protonation of a residue in the enzyme, but the binding of the dianion is only slightly affected by this ionization.