title: Cofactor dependence of reduction potentials for [4Fe-4S]2+/1+ in lysine 2,3-aminomutase.
abstract: Lysine 2,3-aminomutase (LAM) catalyzes the interconversion of l-lysine and l-beta-lysine by a free radical mechanism. The 5'-deoxyadenosyl radical derived from the reductive cleavage of S-adenosyl-l-methionine (SAM) initiates substrate-radical formation. The [4Fe-4S](1+) cluster in LAM is the one-electron source in the reductive cleavage of SAM, which is directly ligated to the unique iron site in the cluster. We here report the midpoint reduction potentials of the [4Fe-4S](2+/1+) couple in the presence of SAM, S-adenosyl-l-homocysteine (SAH), or 5'-{N-[(3S)-3-aminocarboxypropyl]-N-methylamino}-5'-deoxyadenosine (azaSAM) as measured by spectroelectrochemistry. The reduction potentials are -430 +/- 2 mV in the presence of SAM, -460 +/- 3 mV in the presence of SAH, and -497 +/- 10 mV in the presence of azaSAM. In the absence of SAM or an analogue and the presence of dithiothreitol, dihydrolipoate, or cysteine as ligands to the unique iron, the midpoint potentials are -479 +/- 5, -516 +/- 5, and -484 +/- 3 mV, respectively. LAM is a member of the radical SAM superfamily of enzymes, in which the CxxxCxxC motif donates three thiolate ligands to iron in the [4Fe-4S] cluster and SAM donates the alpha-amino and alpha-carboxylate groups of the methionyl moiety as ligands to the fourth iron. The results show the reduction potentials in the midrange for ferredoxin-like [4Fe-4S] clusters. They show that SAM elevates the reduction potential by 86 mV relative to that of dihydrolipoate as the cluster ligand. This difference accounts for the SAM-dependent reduction of the [4Fe-4S](2+) cluster by dithionite reported earlier. Analogues of SAM have a weakened capacity to raise the potential. We conclude that the midpoint reduction potential of the cluster ligated to SAM is 1.2 V less negative than the half-wave potential for the one-electron reductive cleavage of simple alkylsulfonium ions in aqueous solution. The energetic barrier in the reductive cleavage of SAM may be overcome through the use of binding energy.
authors.affiliation: Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin, 1710 University Avenue, Madison, Wisconsin 53726, USA.
authors.forename: Glen T
authors.initials: GT
authors.lastname: Hinckley
authors.forename: Perry A
authors.initials: PA
authors.lastname: Frey
mesh_terms: D003013:Clostridium / Q000737:chemistry / Q000201:enzymology; D003067:Coenzymes / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000737:chemistry / Q000378:metabolism; D019751:Intramolecular Transferases / Q000737:chemistry*; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D012435:S-Adenosylhomocysteine / Q000031:analogs & derivatives / Q000737:chemistry; D012436:S-Adenosylmethionine / Q000031:analogs & derivatives / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry* / Q000378:metabolism
chemical_list: D003067:Coenzymes; D013455:Sulfur; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine; D007501:Iron; D019751:Intramolecular Transferases; C021518:lysine 2,3-aminomutase