Reexamination of the binding site for pyridoxal 5'-phosphate in ribulosebisphosphate carboxylase/oxygenase from Rhodospirillum rubrum

Abstract
The high specificity of pyridoxal 5''-phosphate (PLP) for an essential lysyl residue of R. rubrum ribulosebisphosphate carboxylase/oxygenase was confirmed, but half-of-sites reactivity was not observed in contrast to an earlier report [Robison, P. D., et al. (1980)]. Subsequent to reduction with [3H]borohydride and tryptic digestion of the enzyme inactivated by PLP, the sole labeled peptide was purified by successive chromatography on DEAE-cellulose, SP-Sephadex and Sephadex G-25. The peptide, recovered in good yield, appeared essentially homogeneous by amino acid analysis, peptide mapping and sequencing. Automated Edman degradation established the peptide''s sequence as Val-Leu-Gly-Arg-Pro-Glu-Val-Asp-Gly-Gly-Leu-Val-Val-Gly-Thr-Ile-Ile-(PLP)Lys-Pro-Lys instead of Ala-Leu-Gly-Arg-Pro-Glu-Val-Asp-(PLP)Lys-Gly-Thr-Leu-Val-Ile-Lys as reported by Robison et al. (1980). The sequence -Ile-Lys-Pro-Lys- in the former is identical with that encompassing Lys-175 in the carboxylase/oxygenase from spinach, which reacts preferentially with PLP and 2 other affinity labels. This finding of homology supports the hypothesis that Lys-175 in the spinach enzyme and the corresponding lysyl residue in the R. rubrum enzyme are active-site residues. It is also likely that these Lys residues are functional in catalysis.