Site-directed mutagenesis of a loop at the active site of E1 (α2β2) of the pyruvate dehydrogenase complex -: A possible common sequence motif

Limited proteolysis of the pyruvate decarboxylase (E1, alpha(2) beta(2) ) component of the pyruvate dehydrogenase (PDH) multienzyme complex of Bacillus stearothermophilus has indicated the importance for catalysis of a site (Tyr281-Arg282) in the E1alpha subunit (Chauhan, H.J., Domingo, G.J., Jung, H.-I. & Perham, R.N. (2000) Eur. J. Biochem . 267 , 7158-7169). This site appears to be conserved in the alpha-subunit of heterotetrameric E1s and multiple sequence alignments suggest that there are additional conserved amino-acid residues in this region, part of a common pattern with the consensus sequence -YR-H-D-YR-DE-. This region lies about 50 amino acids on the C-terminal side of a 30-residue motif previously recognized as involved in binding thiamin diphosphate (ThDP) in all ThDP-dependent enzymes. The role of individual residues in this set of conserved amino acids in the E1alpha chain was investigated by means of site-directed mutagenesis. We propose that particular residues are involved in: (a) binding the 2-oxo acid substrate, (b) decarboxylation of the 2-oxo acid and reductive acetylation of the tethered lipoyl domain in the PDH complex, (c) an 'open-close' mechanism of the active site, and (d) phosphorylation by the E1-specific kinase (in eukaryotic PDH and branched chain 2-oxo acid dehydrogenase complexes).