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d-Ribulose 1,5-Diphosphate Carboxylase from Rhodospirillum rubrum

1974; Elsevier BV; Volume: 249; Issue: 11 Linguagem: Inglês

10.1016/s0021-9258(19)42595-7

1083-351X

F. Robert Tabita, Bruce A. McFadden,

Porphyrin Metabolism and Disorders

Abstract Homogeneous ribulose 1,5-diphosphate carboxylase from Rhodospirillum rubrum has a molecular weight of 114,000 and is composed of two subunits of approximately 56,000 daltons. No evidence for the presence of smaller polypeptide chains was obtained. The amino acid composition of the R. rubrum enzyme resembles that of the large subunit from the spinach enzyme and of the enzyme from the hydrogen bacteria. Four of nine half-cystines of the R. rubrum enzyme are titratable with 5,5'-dithiobis(2'-nitrobenzoic acid) in the presence of 8 m urea. Hyperbolic velocity versus substrate concentration curves are obtained when ribulose 1,5-diphosphate is the varied substrate. However, in the presence of EDTA or in the presence of EDTA and dithiothreitol, sigmoidal curves are obtained when CO2 is the varied substrate. A sigmoidal response is not seen with enzyme activated by dithiothreitol alone. Interaction coefficients are approximately one for the enzyme activated by dithiothreitol, EDTA or EDTA plus dithiothreitol at higher (0.33 to 1.1 mm) CO2 concentrations. However, at low levels of CO2 (0.11 to 0.33 mm CO2) interaction coefficients are 3.8 and 2.6 for the enzyme activated by EDTA or EDTA plus dithiothreitol, respectively. Initial velocity studies with the dithiothreitol-activated enzyme are compatible with an ordered reaction catalyzed by ribulose 1,5-diphosphate carboxylase. The R. rubrum ribulose 1,5-diphosphate carboxylase is immunologically unusual in that antibodies do not form precipitates in immunodiffusion with the enzyme from any source outside of the Athiorhodaceae.