Studies on Dipeptidases Iii , Hydrolysis of Methylated Peptides; the Role of Cobalt in the Action of Glycylglycine Dipeptidase* by Emil

The recognition that the hydrolysis of the peptides, glycylglycine and glycyl-n-leucine, is due to specific dipeptidases (l-3) has led to a search for an explanation of why the responsible enzymes should require such extremely specific substrate configurations. While a complete description is obviously not yet possible, a partial answer seems to be in the role that certain metals play in these hydrolytic reactions. It was suggested that, in the case of glycylglycine dipeptidase, the function of Co++ is to act as a bridge forming a coordination compound involving the substrate, on the one hand, and the protein on the other (1). This hypothesis arose from the observation that at pH 7.6, where the enzyme is maximally active, Co++ coordinates much more readily with glycylglycine (GG) than it does with triglycine or glycine, as judged by the relative effect of these compounds on the absorption spectrum of Co++. This suggestion has now been subjected to further test in several ways. First of all, it has been found that those substituted compounds which are not split by the enzyme react either weakly or not at all with Co++. Secondly, an additional substrate for the GG dipeptidase has been found in sarcosylglycine (SG), and this compound coordinates readily with Co++. From the closely parallel data of the specificity of the enzyme and the specificity of complex formation with Co++, some evidence is available regarding the types of linkage involved. The observations with Co++ are facilitated by the characteristic absorption bands which can easily be studied, unlike the coordination of other metals with peptides in which we have observed only a change in the ultraviolet end-absorption. On the other hand, certain difficulties appear in this work with Co++; namely, the reversible oxygen binding and subsequent irreversible slow oxidation which may occur with some of these chelate compounds, as beautifully demonstrated by Burk and his collaborators (4). Nevertheless, that other effects may be found in the absence of the enzyme should not detract from the primary matter of whether or not coordination takes place.