title: Isolation of peptides from the carboxyl carrier subunit of transcarboxylase. Role of the non-biotinyl peptide in assembly.
abstract: Transcarboxylase is made up of a central hexameric subunit (S20,W similar 12 S), three peripheral dimeric metallo subunits (S20,W similar to 5 S), and six biotinyl carboxyl carrier subunits (S20,W similar to 1.3 S). The results presented here show that the carboxyl carrier subunit is required for assembly of the 12S and 5S subunits into the oligomer. However, only a portion of the subunit is required for assembly. On treatment of transcarboxylase briefly with trypsin at pH 6.3 extremely susceptible peptide bonds of the carboxyl carrier protein are cleaved releasing biotinyl peptides of about similar to 66 and similar to 40 residues. The resulting trypsinized transcarboxylase, though enzymatically inactive, remains essentially intact as judged by its hydrodynamic and molecular sieving properties. The modified enzyme can be dissociated at pH 8 to the central 12S subunit and peripheral 5S subunit to which the residual portion(s) of the cleaved carboxyl carrier protein is still attached. These components can then be separated by molecular sieving. The residual portion of the carboxyl carrier protein (non-biotinyl peptide) can then be isolated by dissociation of the 5S subunit complex at pH 9 and by chromatography over Bio-Gel A-1.5m. The isolated non-biotinyl peptide has been shown to contain the combining domain of the 1.3SE carboxyl carrier protein since it causes combination of the 12S and 5S subunits. Active enzyme is formed by combination of the intact carboxyl carrier protein and the 12S and 5S subunits and an inactive oligomer of similar size is formed if the non-biotinyl peptide is used in place of the carboxyl carrier protein. The similar to 66- and similar to 40-residue biotinyl peptides, which are released by the trypsin treatment, apparently occur on an exposed portion of the enzyme. This portion of the carboxyl carrier protein apparently serves to place the biotinyl group adjacent to the two substrate sites of the enzyme, one of which is on the peripheral subunit and the other on the central subunit. Thus the carboxyl carrier protein has two functions: one portion holds the 12S and 5S subunits in juxtaposition and the other portion orients the biotinyl group adjacent to the substrate sites so that it may function as a carboxyl carrier between the sites.
mesh_terms: D000123:Acetyltransferases / Q000378:metabolism; D000596:Amino Acids / Q000032:analysis; D001665:Binding Sites; D001710:Biotin; D046911:Macromolecular Substances; D008970:Molecular Weight; D010071:Oxaloacetates; D010446:Peptide Fragments / Q000032:analysis; D010455:Peptides; D011424:Propionibacterium / Q000201:enzymology*; D011485:Protein Binding; D011487:Protein Conformation; D014357:Trypsin
chemical_list: D000596:Amino Acids; D046911:Macromolecular Substances; D010071:Oxaloacetates; D010446:Peptide Fragments; D010455:Peptides; D001710:Biotin; D000123:Acetyltransferases; D014357:Trypsin