Dissection of the biotinyl subunit of transcarboxylase into regions essential for activity and assembly

Transcarboxylase, a multisubunit enzyme containing 12 S, 5 S, and 1.3 S subunits, catalyzes the transfer of a carboxyl group from methylmalonyl-CoA to pyruvate (overall reaction) via two partial reactions. In the first partial reaction, a carboxyl group from methylmalonyl-CoA bound to the 12 S subunit is transferred to the biotin of the 1.3 S subunit, and, in the second partial reaction, the carboxylated biotin transfers its carboxyl group from biotin to pyruvate, bound to the 5 S subunit. Previously we have shown that the region around the biotinyl lysine of the 1.3 S subunit is critical for catalysis, that peptides in the amino-terminal region of 1.3 S are capable of forming complexes with 12 S and 5 S, and that amino acids in the carboxyl terminus of the 1.3 S subunit form part of the recognition site for holocarboxylase synthetase. In order to further examine the role of the sequences in this subunit, we generated 8 shortened forms of the 1.3 S biotinyl subunits missing either one or both termini. Truncated 1.3 S subunits were active in both partial reactions until deletion reached amino acid 59. None of the truncated subunits was able to support stable complex formation with the 12 S and 5 S subunits or catalyze the overall reaction. The results suggest that the region between 59 and 78 is required for activity and the sequence 1-18 is required for enzyme assembly. Activity in the partial reactions correlated with intrinsic fluorescence enhancement of tryptophan residues in either the 12 S or 5 S subunit. Fluorescence enhancement was observed with the shortened 1.3 S subunits until truncation reached amino acid 59 implying either 1) that the internal sequence, 59-78, transiently associates with the other subunits to properly orient the biotin for catalysis or 2) that the sequence 59-78 contributes to the folded conformation of the 1.3 S subunit so that subunit interactions can take place.