High resolution solution structure of the 1.3S subunit of transcarboxylasefrom Propionibacterium shermanii

Transcarboxylase (TC) from Propionibacterium shermanii, a biotin-dependent enzyme, catalyzes the transfer of a carboxyl group from methylmalonyl-CoA t o pyruvate to form propionyl-Coa and oxalacetate. Within the multi-subunit enzyme complex, the 1.3S subunit functions as the carboxyl group carrier an d also binds the other two subunits to assist in the overall assembly of th e enzyme. The 1.3S subunit is a 123 amino acid polypeptide (12.6 kDa) to wh ich biotin is covalently attached at Lys 89. The three-dimensional solution structure of the full-length holo-1.3S subunit of TC has been solved by mu ltidimensional heteronuclear NMR spectroscopy. The C-terminal half of the p rotein (51-123) is folded into a compact all-beta-domain comprising of two four-stranded antiparallel beta-sheets connected by short loops and turns. The fold exhibits a high 2-fold internal symmetry and is similar to that of the biotin carboxyl carrier protein (BCCP) of acetyl-CoA carboxylase, but lacks an extension that has been termed "protruding thumb" in BCCP. The fir st 50 residues, which have been shown to be involved in intersubunit intera ctions in the intact enzyme, appear to be disordered in the isolated 1.3S s ubunit. The molecular surface of the folded domain has two distinct surface s: one side is highly charged, while the other comprises mainly hydrophobic , highly conserved residues.