2-(4-Bromoacetamido)anilino-2-deoxypentitol 1,5-bisphosphate, a new affinity label for ribulose bisphosphate carboxylase/oxygenase from Rhodospirillum rubrum. Determination of reaction parameters and characterization of an active site peptide.

ABSTRACT

A new affinity label for ribulose bisphosphate carboxylase/oxygenase from Rhodospirillum rubrum, 2-(4-bromoacetamido)anilino-2-deoxypentitol 1,5-bisphosphate, has been prepared, Reductive amination of ribulose-P2 with p-phenylenediamine in the presence of sodium cyanoborohydride yielded an epimeric mixture which was resolved by chromatography on quaternary aminoethyl-Sephadex. Subsequent bromoacetylation of the isolated amino bisphosphates gave reagents A and B (ribo and arabino epimers of 2-(4-bromoacetamido) anilino-2-deoxypentitol 1,5-bisphosphate) which were competitive inhibitors of the carboxylase with Ki values of 705 and 104 microM, respectively. Reagent A exhibited no time-dependent effects on the carboxylase in either the deactivated or activated state. Incubation of the enzyme with reagent B in the presence of the essential activators CO2 and Mg2+, however, resulted in an irreversible, time-dependent loss of activity, with a Kinact of 125 microM and a minimal half-time of 7.3 min. Covalent incorporation of [14C]reagent B was directly proportional to the loss of activity, with total inactivation correlating with an incorporation of 1.1 mol of reagent/mol of subunit. Inclusion of the competitive inhibitor 2-carboxyribitol 1,5-bisphosphate protected against inactivation with a concomitant reduction in incorporation. Neither reagent affected the activity of spinach carboxylase. Fractionation of [14C]reagent B-modified enzyme on DEAE-cellulose, subsequent to carboxymethylation and tryptic digestion, revealed two major radioactive peaks of approximately equal area. Digestion of each peak with alkaline phosphatase and rechromatography on DEAE-cellulose resulted in pure peptides I and II. The peptides were identical except in the site of labeling peptide I contained a modified cysteinyl residue while peptide II contained a modified histidyl residue. Automated Edman degradation established the sequence as (sequence in text) which is located near the NH2 terminus of the enzyme. The lack of reactivity with the spinach enzyme is explained by the deletion of the histidyl residue and the replacement of cysteine by tryptophan in the eukaryotic species. Although the nonconservation of the modified residues argues against a functional role other than maintenance of structural integrity, the extensive homology in this region among seven different species of carboxylase is compatible with the region comprising a portion of the active site.