His-8 lowers the pKa of the essential Cys-12 residue of the ArsC arsenate reductase of plasmid R773.

The 141-residue ArsC arsenate reductase of plasmid R773 has an essential cysteine residue, Cys-12. The pKa of Cys-12 was determined to be 6.4, compared with a pKa of 8.3 for free cysteine. The possibility of the formation of an ion pair between Cys-12 and a basic residue was investigated. Enzymatic activity was rapidly inactivated by the histidine-modifying reagent diethylpyrocarbonate. The codons for the two histidine residues in ArsC, His-8 and His-88, were changed by site-directed mutagenesis. Cells expressing arsCH88R, arsCH88S, arsCH88W, or arsCH88V genes retained arsenate resistance, and the purified proteins had wild type level of reductase activity. Cells expressing arsCH8P, arsCH8S, arsCH8G, or arsCH8R genes were each sensitive to arsenate, and the purified H8P, H8G, and H8R proteins each lacked enzymatic activity. Using the single histidine proteins it was shown that both histidines react with diethylpyrocarbonate but that only reaction with His-8 resulted in inactivation. The pKa value of Cys-12 was determined to be 6.3 in the H8R enzyme and 8.3 in the H8G enzyme. These results indicate that His-8 is essential for catalytic activity and that a positively charged residue is required at position 8 to lower the pKa of the cysteine thiolate at position 12.