S are different proteins from the corresponding subunits of the PDH complex and are encoded by the sucA and sucB genes, respectively. However, the E3 subunit is the same protein, Lpd, found in the PDH complex. In aerobically grown E. coli, this complex catalyzes a key step in the citric acid cycle and also supplies succinyl-CoA for biosynthesis of two amino acids, methionine and lysine (152). Under the appropriate conditions, E. coli strains lacking functional 2-OGDH can be supplemented with succinate or methionine plus lysine to provide metabolic bypasses of loss of this enzyme complex (152). Expression of the 2-OGDH is highly induced during aerobic growth on acetate and citric acid cycle intermediates and is severely repressed during fermentative growth where succinyl-CoA is generated by succinyl-CoA synthetase (144) although 2-OGDH is synthesized by cells gown in anaerobic media containing an electron acceptor such as nitrate or fumarate (153). Glycine SKF-96365 (hydrochloride) site cleavage system The third lipoylated protein of E. coli is the H protein of the glycine cleavage system, an enzyme widely distributed in bacteria and in the mitochondria of plants (where it is called glycine decarboxylase), fungi and mammals (154?56). The glycine cleavage system catalyzes the reversible cleavage of glycine, yielding carbon dioxide, ammonia, 5,10methylenetetrahydrofolate plus a reduced pyridine nucleotide. It consists of four component proteins termed the T, H, P and L proteins. The first three proteins are encoded by the gcvT gcvH gcvP operon while L protein is the same as Lpd, the E3 protein of the 2-oxo acid dehydrogenases as discussed above (157). P protein catalyzes the pyridoxal phosphatedependent decarboxylation of glycine and transfers the remaining methylamine moiety to one of the sulfhydryl groups of the lipoyl prosthetic group of H protein. T protein catalyzes the release of ammoniate and transfer of the one-carbon unit to tetrahydrofolate from the lipoyl residue. L protein is a lipoamide dehydrogenase that catalyzes the SKF-96365 (hydrochloride) web reoxidation of the dihydrolipoyl residue of H protein and reduction of NAD+. Thus, the lipoic acid moiety of H protein interacts with the active sites of three different enzymes in a manner analogous to that found for 2-oxoacid dehydrogenase complexes.EcoSal Plus. Author manuscript; available in PMC 2015 January 06.Author Manuscript Author Manuscript Author Manuscript Author ManuscriptCronanPageStructures of lipoylated and biotinylated proteinsIn all 2-oxoacid dehydrogenase complexes, the core of the structure is provided by the E2 subunit to which the E1 and E3 components are bound tightly but noncovalently. In the PDH and 2-OGDH complexes of Escherichia coli and other gram-negative bacteria (158, 159) plus the 2-OGDH and branched-chain 2-oxoacid dehydrogenase complexes of mammals (160, 161), the core consists of 24 copies of the E2 chain arranged with octahedral symmetry, whereas in the PDH complexes of mammals and Gram-positive bacteria (162?165), the core comprises 60 E2 chains arranged with icosahedral symmetry. In all 2-oxoacid dehydrogenase complexes, the E2 component has a multi-domain structure comprising (from the N terminus): lipoyl domain (or domains of ca. 9 kDa), a small peripheral subunitbinding domain (ca. 4 kDa) and a much larger catalytic domain (ca 28 kDa) that houses the acyltransferase activity and aggregates to form the inner core of the complexes. These domains are separated by long (25?0 residue) segments of polypeptid.S are different proteins from the corresponding subunits of the PDH complex and are encoded by the sucA and sucB genes, respectively. However, the E3 subunit is the same protein, Lpd, found in the PDH complex. In aerobically grown E. coli, this complex catalyzes a key step in the citric acid cycle and also supplies succinyl-CoA for biosynthesis of two amino acids, methionine and lysine (152). Under the appropriate conditions, E. coli strains lacking functional 2-OGDH can be supplemented with succinate or methionine plus lysine to provide metabolic bypasses of loss of this enzyme complex (152). Expression of the 2-OGDH is highly induced during aerobic growth on acetate and citric acid cycle intermediates and is severely repressed during fermentative growth where succinyl-CoA is generated by succinyl-CoA synthetase (144) although 2-OGDH is synthesized by cells gown in anaerobic media containing an electron acceptor such as nitrate or fumarate (153). Glycine cleavage system The third lipoylated protein of E. coli is the H protein of the glycine cleavage system, an enzyme widely distributed in bacteria and in the mitochondria of plants (where it is called glycine decarboxylase), fungi and mammals (154?56). The glycine cleavage system catalyzes the reversible cleavage of glycine, yielding carbon dioxide, ammonia, 5,10methylenetetrahydrofolate plus a reduced pyridine nucleotide. It consists of four component proteins termed the T, H, P and L proteins. The first three proteins are encoded by the gcvT gcvH gcvP operon while L protein is the same as Lpd, the E3 protein of the 2-oxo acid dehydrogenases as discussed above (157). P protein catalyzes the pyridoxal phosphatedependent decarboxylation of glycine and transfers the remaining methylamine moiety to one of the sulfhydryl groups of the lipoyl prosthetic group of H protein. T protein catalyzes the release of ammoniate and transfer of the one-carbon unit to tetrahydrofolate from the lipoyl residue. L protein is a lipoamide dehydrogenase that catalyzes the reoxidation of the dihydrolipoyl residue of H protein and reduction of NAD+. Thus, the lipoic acid moiety of H protein interacts with the active sites of three different enzymes in a manner analogous to that found for 2-oxoacid dehydrogenase complexes.EcoSal Plus. Author manuscript; available in PMC 2015 January 06.Author Manuscript Author Manuscript Author Manuscript Author ManuscriptCronanPageStructures of lipoylated and biotinylated proteinsIn all 2-oxoacid dehydrogenase complexes, the core of the structure is provided by the E2 subunit to which the E1 and E3 components are bound tightly but noncovalently. In the PDH and 2-OGDH complexes of Escherichia coli and other gram-negative bacteria (158, 159) plus the 2-OGDH and branched-chain 2-oxoacid dehydrogenase complexes of mammals (160, 161), the core consists of 24 copies of the E2 chain arranged with octahedral symmetry, whereas in the PDH complexes of mammals and Gram-positive bacteria (162?165), the core comprises 60 E2 chains arranged with icosahedral symmetry. In all 2-oxoacid dehydrogenase complexes, the E2 component has a multi-domain structure comprising (from the N terminus): lipoyl domain (or domains of ca. 9 kDa), a small peripheral subunitbinding domain (ca. 4 kDa) and a much larger catalytic domain (ca 28 kDa) that houses the acyltransferase activity and aggregates to form the inner core of the complexes. These domains are separated by long (25?0 residue) segments of polypeptid.
