Enzymological and physiological consequences of restructuring the lipoyl domain content of the pyruvate dehydrogenase complex of Escherichia coli.
The core-forming lipoate acetyltransferase (E2p) subunits of the pyruvate dehydrogenase (PDH) complex of Escherichia coli contain three tandemly repeated lipoyl domains although one lipoyl domain is apparently sufficient for full catalytic activity in vitro. Plasmids containing IPTG-inducible aceEF-IpdA operons which express multilip-PDH complexes bearing one N-terminal lipoyl domain and up to seven unlipoylated (mutant) domains per E2p chain, were constructed. Each plasmid restored the nutritional lesion of a strain lacking the PDH complex and expressed a sedimentable PDH complex, although the catalytic activities declined significantly as the number of unlipoylated domains increased above four per E2p chain. It was concluded that the extra domains protrude from the 24-meric E2p core without affecting assembly of the E1p and E3 subunits, and that the lipoyl cofactor bound to the outermost domain can participate successfully at each of the three types of active site in the assembled complex. Physiological studies with two series of isogenic strains expressing multilip-PDH complexes from modified chromosomal pdh operons (pdhR-aceEF-IpdA) showed that three lipoyl domains per E2p chain is optimal and that only the outermost domain need be lipoylated for optimal activity. It is concluded that the reason for retaining three lipoyl domains is to extend the reach of the outermost lipoyl cofactor rather than to provide extra cofactors for catalysis.