Identification of two electron-transfer sites in ascorbate peroxidase using chemical modification, enzyme kinetics, and crystallography

Chemical and mutagenic modification combined with X-ray crystallography has been used to probe the ascorbate binding site in ascorbate peroxidase (APX ). Chemical modification of the single Cys residue in APX with Ellman's rea gent (DTNB) blocks the ability of APX to oxidize ascorbate but not other sm all aromatic phenolic substrates. DTNB-modified APX (APX-TNB) exhibits only 1.3% wildtype activity when ascorbate is used as the substrate but full ac tivity when aromatic substrate, guaiacol or pyrogallol, are used. Stopped-f low studies show that APX-TNB reacts normally with peroxide to give compoun d I but that the rates of reduction of both compounds I and II by ascorbate are dramatically slowed. Conversion of Cys32 to Ser leads to approximate t o 70% drop in ascorbate peroxidase activity with no effect on guaiacol pero xidase activity. These results indicate that uncharged aromatic substrates and the anionic ascorbate molecule interact with different sites on APX. Th e 2.0 Angstrom X-ray crystal structure of APX-TNB shows clear electron dens ity for the TNB group covalently attached to Cys32 in all four molecules of the asymmetric unit, indicating complete and specific modification. It app ears that the ascorbate site is blocked by DTNB modification which is well removed from the exposed delta-heme edge where aromatic substrates are thou ght to bind. This is the first experimental evidence indicating that ascorb ate oxidation does not occur at the exposed heme edge but at an alternate b inding site in the vicinity of Cys32 near Arg172 and the heme propionates.