title: Purification and properties of the lipoate protein ligase of Escherichia coli.
abstract: Lipoate is an essential component of the 2-oxoacid dehydrogenase complexes and the glycine-cleavage system of Escherichia coli. It is attached to specific lysine residues in the lipoyl domains of the E2p (lipoate acetyltransferase) subunit of the pyruvate dehydrogenase complex by a Mg(2+)- and ATP-dependent lipoate protein ligase (LPL). LPL was purified from wild-type E. coli, where its abundance is extremely low (< 10 molecules per cell) and from a genetically amplified source. The purified enzyme is a monomeric protein (M(r) 38,000) which forms irregular clusters of needle-like crystals. It is stable at -20 degrees C, but slowly oxidizes to an inactive form containing at least one intramolecular disulphide bond at 4 degrees C. The inactive form could be re-activated by reducing agents or by an as-yet unidentified component (reactivation factor) which is resolved from LPL at the final stage of purification. The pI is 5.80, and the Km values for ATP, Mg2+ and DL-lipoate were determined. Selenolipoate and 6-thio-octanoate were alternative but poorer substrates. Lipoylation was reversibly inhibited by the 6- and 8-seleno-octanoates and 8-thio-octanoate, which reacted with the six cysteine thiol groups of LPL. LPL was inactivated by Cu2+ ions in a process that involved the formation of inter- and intra-molecular disulphide bonds. Studies with lplA mutants lacking LPL activity indicated that E. coli possesses another distinct lipoylation system, although no such activity could be detected in vitro.
authors.affiliation: Krebs Institute for Biomolecular Research, Department of Molecular Biology, University of Sheffield, U.K.
authors.forename: D E
authors.initials: DE
authors.lastname: Green
authors.forename: T W
authors.initials: TW
authors.lastname: Morris
authors.forename: J
authors.initials: J
authors.lastname: Green
authors.forename: J E
authors.initials: JE
authors.lastname: Cronan
authors.forename: J R
authors.initials: JR
authors.lastname: Guest
mesh_terms: D000595:Amino Acid Sequence; D002412:Cations; D002846:Chromatography, Affinity; D002850:Chromatography, Gel; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D004795:Enzyme Stability; D004926:Escherichia coli / Q000201:enzymology*; D005784:Gene Amplification; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008969:Molecular Sequence Data; D010453:Peptide Synthases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000302:isolation & purification*; D013379:Substrate Specificity
chemical_list: D002412:Cations; D010453:Peptide Synthases; C072322:lipoate-protein ligase