Evidence for an essential histidine residue in the Neurospora crassa plasma membrane H +-ATPase

Abstract

The Neurospora crassa plasma membrane H +-ATPase is rapidly inactivated in the presence of diethyl pyrocarbonate (DEP). The reaction is pseudo-first-order showing time- and concentration-dependent inactivation with a second-order rate constant of 385–420 M −1 · min −1 at pH 6.9 and 25°C. The difference spectrum of the native and modified enzyme has a maximum near 240 nm, characteristics of N-carbethoxyhistidine. No change in the absorbance of the inhibited ATPase at 278 nm or in the number of modifiable sulfhydryl groups is observed, indicating that the inhibition is not due to tyrosine or cysteine modification, and the inhibition is irreversible, ruling out serine residues. Furthermore, pretreatment of the ATPase with pyridoxal phosphate/NaBH 4 under the conditions of the DEP treatment does not inhibit the ATPase and does not alter the DEP inhibition kinetics, indicating that the inactivation by DEP is not due to amino group modification. The pH dependence of the inactivation reaction indicates that the essential residue has a p K a near 7.5, and the activity lost as a result of H +-ATPase modification by DEP is partially recovered alter hydroxylamine treatment at 4°C. Taken together, these results strongly indicate that the inactivation of the H +-ATPase by DEP involves histidine modification. Analyses of the inhibition kinetics and the stoichiometry of modification indicate that among eight histidines modified per enzyme molecule, only one is essential for H +-ATPase activity. Finally, ADP protects against inactivation by DEP, indicating that the essential residue modified may be located at or near the nucleotide binding site.