5 Ribulose-1,5-Diphosphate Carboxylase

Abstract

Publisher Summary This chapter discusses the molecular properties and catalytic process of ribulose-1,5-diphosphate carboxylase. The identification by Calvin and his colleagues of phosphoglyceric acid as the first stable radioactive product formed during a brief exposure of algae to 14CO2 led ultimately to the discovery that the carboxylation of ribulose diphosphate is the initial step in the photosynthetic carbon cycle. It was observed that the ribulose diphosphate became labeled early and that illumination caused a decrease in its steady state concentration and a concomitant rise in the concentration of 3-phosphoglycerate. When the supply of CO2 was cut off, the ribulose diphosphate level increased and the level of 3-phosphoglycerate decreased correspondingly. Ribulose diphosphate carboxylases from higher plants have complex quaternary structures as evidenced by their appearance in the electron microscope, high molecular weights, and numbers of substrate, cofactor, inhibitor, and antibody binding sites. The amino acid compositions of several ribulose diphosphate carboxylases have been reported. The molecular activity of the homogeneous spinach ribulose diphosphate carboxylase, which is typical of most of the plant carboxylases, is 1340 moles of ribulose diphosphate carboxylated per minute per mole of enzyme at pH 7.9. Ribulose diphosphate carboxylase from spinach appears to be absolutely specific for D -ribulose 1,5-diphosphate.