Evidence for the spatial separation of the binding sites for substrate and for cytoskeletal proteins on the enzyme aldolase.
The effect of the proteolysis of aldolase on both the substrate specificity of the enzyme and binding capacity for actin have been studied. Carboxypeptidase A, trypsin, chymotrypsin and pepsin, all acted to cleave peptides from the C-terminal portion of the enzyme, resulting initially in a marked loss of activity towards fructose-1:6-bisphosphate (FBP), without impairment of activity towards fructose-1-phosphate (F1P). In some cases, however, further proteolysis caused reductions in activity with F1P as well. By correlating the size of the peptide fragments released by these enzymes with the known sequence of aldolase, evidence has been provided that cleavage of His-359 and/or Tyr-361 lead to the loss of FBP activity, while further cleavage of up to six amino acids begin to affect activity against F1P, as well. In regard to the ability of the proteolysed aldolase to bind to F-actin, it was evident from these studies that binding ability was not impaired in the initial stages of proteolysis referred to above, but was retained until the enzyme was extensively degraded. This differential behaviour of the active and binding sites on aldolase clearly establish their separate topographical localization. These results have been discussed in relation to the positioning of these separate sites on the enzyme, the nature of the interaction between aldolase and actin and the phenomenon of enzyme ambiquity in cells and tissues.