Cu,Zn Superoxide Dismutase from Photobacterium leiognathi Is an Hyperefficient Enzyme

Abstract

The catalytic rate constant of recombinant Photobacterium leiognathi Cu,Zn superoxide dismutase has been determined as a function of pH by pulse radiolysis. At pH 7 and low ionic strength (I = 0.02 M) the catalytic rate constant is 8.5 x 10(9) M-1 s-1, more than two times the value found for all the native eukaryotic Cu,Zn superoxide dismutases investigated to date. Similarly, Brownian dynamics simulations indicate an enzyme-substrate association rate more than two times higher than that found for bovine Cu,Zn superoxide dismutase. Titration of the paramagnetic contribution to the water proton relaxation rate of the P. leiognathi with increasing concentration of halide ions with different radii indicates that the proteic channel delimiting the active site is wider than 4.4 A. This is at variance with that found on the eukariotic enzymes, and provides a rationale for the high catalytic rate of the bacterial enzyme. Evidence for solvent exposure of the active site different from that observed in the eukaryotic enzyme is suggested from the pH dependence of the water proton relaxation rate and of the EPR spectrum line shape, which indicate the occurrence of a prototropic equilibrium at pH 9.1 and 9.0, respectively. The pH dependence of the P. leiognathi catalytic rate has a trend different from that observed in the bovine enzyme, indicating that groups differently exposed to the solvent are involved in the modulation of the enzyme-substrate encounter.

Unique Features of the sodC-encoded Superoxide Dismutase from Mycobacterium tuberculosis, a Fully Functional Copper-containing Enzyme Lacking Zinc in the Active Site

“…5A). It should be pointed out, however, that a significant pH dependence of activity has already been observed in the other prokaryotic Cu,Zn-SODs from P. leiognathi (43) and E. coli. 3 To influence the diffusion rate of the superoxide anion, both prokaryotic and eukaryotic Cu,Zn-SODs utilize electronic guidance by exploiting the electrostatic potential of positively charged residues on the rim of the active-site channel.…”

“…Catalytic Activity and Electrostatics of MtSOD-In the pH 6.5-7.5 range, the enzyme displays a catalytic rate (k ϭ 2.4 ϫ 10 9 M Ϫ1 s Ϫ1 ) comparable with that of several eukaryotic enzymes (41,42), but slightly lower than that of P. leiognathi and S. typhimurium Cu,Zn-SODs (43,19). Unlike the eukaryotic enzymes, whose activity is stable up to pH 9.0, MtSOD activity drops above pH 8.0, with an inflection point at pH ϳ8.5 (Fig.…”

Antioxidant Enzymes and Human Health

“…Calves that were fed milk supplemented with 25 ppm Cu and 100 ppm Zn showed a stronger immune response and a higher SOD activity [30] . Other recent reports involving SOD knockouts have revealed that Mn-SOD is essential for life whereas Cu/Zn-SOD is not.…”

Amsacta mooreiEntomopoxvirus Expresses an Active Superoxide Dismutase

“…Cu-Zn SODs are among the fastest enzymes known, with k cat /K m values of 2.3 ϫ 10 9 M Ϫ1 s Ϫ1 for bovine SOD (33) and 8.5 Ϯ 0.8 ϫ 10 9 M Ϫ1 s Ϫ1 for the bacterial enzyme from Photobacterium legionathi (54). Functional bovine SOD is a homodimer with an intrasubunit disulfide bond within the monomer (1).…”