Effect of aluminum on the binding properties of alpha-chymotrypsin

The effect of aluminum ions on the binding properties of alpha-chymotrypsin has been studied. The results show that aluminum does not affect the catalytic rate constant k(cat), but it acts as an enzyme activator favoring the binding of the substrate to the catalytic site (i.e. decreasing K-m). Furthermore, aluminum binding to alpha-chymotrypsin displays about a threefold decrease in its affinity for the macromolecular inhibitor bovine pancreatic trypsin inhibitor (BPTI). Altogether, the different effect of aluminum on the binding of synthetic substrates (e.g. N-alpha-benzoyl-L-tyrosine ethyl ester, BTEE) and macromolecular inhibitors (e.g. BPTI) to alpha-chymotrypsin suggests the occurrence of an aluminum-linked conformational change in the enzyme molecule which brings about a marked structural change at the primary and secondary recognition sites for substrates and inhibitors, The modulative effect exerted by aluminum on the enzyme hydrolytic activity has been investigated also as a function of pH. The ion-linked effect appears to be dependent on the pH in a complex fashion, which suggests that aluminum binding is controlled by the protonation of at least two classes of residues on the enzyme molecule.