1ja8
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Kinetic Analysis of Product Inhibition in Human Manganese Superoxide Dismutase
OverviewOverview
Manganese superoxide dismutase (MnSOD) cycles between the Mn(II) and, Mn(III) states during the catalyzed disproportionation of O(2)(*-), a, catalysis that is limited at micromolar levels of superoxide by a, peroxide-inhibited complex with the metal. We have investigated the role, in catalysis and inhibition of the conserved residue Trp161 which forms a, hydrophobic side of the active site cavity of MnSOD. Crystal structures of, mutants of human MnSOD in which Trp161 was replaced with Ala or Phe showed, significant conformational changes on adjacent residues near the active, site, particularly Gln143 and Tyr34 which in wild-type MnSOD participate, in a hydrogen bond network believed to support proton transfer during, catalysis. Using pulse radiolysis and observing the UV absorbance of, superoxide, we have determined rate constants for the catalytic, dismutation of superoxide. In addition, the rates of formation and, dissociation of the product-inhibited complex of these mutants were, determined by direct observation of the characteristic visible absorption, of the oxidized and inhibited states. Catalysis by W161A and W161F MnSOD, was associated with a decrease of at least 100-fold in the catalytic rate, of reduction of superoxide, which then promotes a competing pathway, leading to product inhibition. The structural changes caused by the, mutations at position 161 led to small changes, at most a 6-fold decrease, in the rate constant for formation of the inhibited complex. Solvent, hydrogen isotope effects support a mechanism in which formation of this, complex, presumably the peroxide dianion bound to the manganese, involves, no rate-contributing proton transfer; however, the dissociation of the, complex requires proton transfer to generate HO(2)(-) or H2O2.
About this StructureAbout this Structure
1JA8 is a Single protein structure of sequence from Homo sapiens with MN and SO4 as ligands. Active as Superoxide dismutase, with EC number 1.15.1.1 Full crystallographic information is available from OCA.
ReferenceReference
Kinetic analysis of product inhibition in human manganese superoxide dismutase., Hearn AS, Stroupe ME, Cabelli DE, Lepock JR, Tainer JA, Nick HS, Silverman DN, Biochemistry. 2001 Oct 9;40(40):12051-8. PMID:11580280
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