title: The conformational stability of the redox states of lipoamide dehydrogenase from Azotobacter vinelandii.
abstract: The conformational stability of holo-lipoamide and apo-lipoamide dehydrogenase from Azotobacter vinelandii was studied by thermoinactivation, unfolding and limited proteolysis. The oxidized holoenzyme is thermostable, showing a melting temperature, tm = 80 degrees C. The thermal stability of the holoenzyme drastically decreases upon reduction. Unlike the oxidized and lipoamide two-electron reduced enzyme species, the NADH four-electron reduced enzyme is highly sensitive to unfolding by urea. Loss of energy transfer from Trp199 to flavin reflects the unfolding of the oxidized holoenzyme by guanidine hydrochloride. Unfolding of the monomeric apoenzyme is a rapid fully reversible process, following a simple two-state mechanism. The oxidized and two-electron reduced holoenzyme are resistant to limited proteolysis by trypsin and endoproteinase Glu-C. Upon cleavage of the apoenzyme or four-electron reduced holoenzyme by both proteases, large peptide fragments (molecular mass greater than 40 kDa) are transiently produced. Sequence studies show that limited trypsinolysis of the NADH-reduced enzyme starts mainly at the C-terminus of Arg391. In the apoenzyme, limited proteolysis by endoproteinase Glu-C starts from the C-terminus at the carboxyl ends of Glu459 and/or Glu435. From crystallographic data it is deduced that the susceptible amino acid peptide bonds are situated near the subunit interface. Thus, these bonds are inaccessible to the proteases in the dimeric enzyme and become accessible after monomerization. It is concluded that reduction of lipoamide dehydrogenase to the four-electron reduced state(s) is accompanied by conformational changes promoting subunit dissociation.
authors.affiliation: Department of Biochemistry, Agricultural University, Wageningen, The Netherlands.
authors.forename: W J
authors.initials: WJ
authors.lastname: van Berkel
authors.forename: A G
authors.initials: AG
authors.lastname: Regelink
authors.forename: J J
authors.initials: JJ
authors.lastname: Beintema
authors.forename: A
authors.initials: A
authors.lastname: de Kok
mesh_terms: D000595:Amino Acid Sequence; D001051:Apoenzymes / Q000737:chemistry / Q000378:metabolism; D016948:Azotobacter vinelandii / Q000201:enzymology*; D002851:Chromatography, High Pressure Liquid; D003001:Cloning, Molecular; D008058:Dihydrolipoamide Dehydrogenase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004795:Enzyme Stability; D004926:Escherichia coli / Q000235:genetics; D007700:Kinetics; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000302:isolation & purification; D010449:Peptide Mapping; D011487:Protein Conformation; D011489:Protein Denaturation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D001051:Apoenzymes; D010446:Peptide Fragments; D011994:Recombinant Proteins; D008058:Dihydrolipoamide Dehydrogenase