Yeast aminopeptidase I. Chemical composition and catalytic properties.

An aminopeptidase (alpha-aminoacyl L-peptide hydrolase, EC 3.4.11.1) was purified to homogeneity from autolysates of brewer's yeast. The enzyme which is responsible for most of the yeast cell's aminopeptidase activity is a glycoprotein containing about 12% of conjugated carbohydrate and 0.02% Zn2+ and having a complex quaternary structure. The active species has a molecular weight of approx. 600000 and an isoelectric point of 4.7. The enzyme is remarkably stable, even in dilute solutions. All types of L-amino acid and peptide derivatives containing a free amino terminus are attacked, including amino acid amides and esters. As to its substrate specificity, the enzyme belongs to the so called leucine-aminopeptidases. It is strongly and specifically activated by Zn2+ and Cl- (or Br-) and inactivated by metal-chelating agents. The activation by Zn2+ seems to be mediated by a conformational transition which affects exclusively V and leads to a form of the enzyme which enhanced stability against heat. Halide anions, on the other hand, are acting as positive allosteric effectors, modulating both V and Km.