CARBOHYDRATE MOIETY OF CARBOXYPEPTIDASE-Y AND PERTURBATION OF ITS BIOSYNTHESIS

Carboxypeptidase Y purified from bakers yeast, Mr 61000, contains 2.7% glucosamine and 14.1% mannose, which correspond to 8 and 53 residues/molecule respectively. By digesting the enzyme with a mixture of proteinases glycopeptides are released containing aspartic acid/asparagine, glucosamine and mannose in a ratio of 1.5:2:13. Oligosaccharides obtained after hydrazinolysis are composed of glucosamine and mannose. Four asparagine‐linked oligosaccharides of the general formula (GlcNAc)2(Man)13 are proposed to be present in carboxypeptidase Y. Similar antigenicity and immunoreactivity properties of the enzyme and of cell wall mannan indicate a close structural relationship of their sugar moieties. The enzyme synthesized in the presence of tunicamycin has a molecular weight of about 51 000, just as expected if devoid of sugar. Tunicamycin also reduces specifically the amount of the carbohydrate‐free present in the cells. Since this product is metabolically stable, the reduced amount must be due to an inhibited rate of biosynthesis. This indicates a regulatory link between the glycosylation of the protein moiety and its biosynthesis. Like the normal carboxypeptidase Y also the carbohydrate‐free form is synthesized via a larger precursor. The processing of the precursor in vivo and in vitro is not affected by the absence of its carbohydrate portion. The size of the enzyme formed in the presence of 2‐deoxyglucose is reduced too, although to a lesser extent. A glucosamine auxotroph, if starved for glucosamine, synthesizes in part a carboxypeptidase Y species analogous to that found in the cells poisoned by tunicamycin. Isolation of carboxypeptidase Y from the starved mutant cells is described. Experiments on glycosylation of such carboxypeptidase in vitro (also after its denaturation) have been negative, however. Copyright © 1978, Wiley Blackwell. All rights reserved