title: Properties of a multimeric protein complex from chloroplasts possessing potential activities of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase and phosphoribulokinase.
abstract: A homogeneous multimeric protein isolated from the green alga, Scenedesmus obliquus, has both latent phosphoribulokinase activity and glyceraldehyde-3-phosphate dehydrogenase activity. The glyceraldehyde-3-phosphate dehydrogenase was active with both NADPH and NADH, but predominantly with NADH. Incubation with 20 mM dithiothreitol and 1 mM NADPH promoted the coactivation of phosphoribulokinase and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase, accompanied by a decrease in the glyceraldehyde-3-phosphate dehydrogenase activity linked to NADH. The multimeric enzyme had a Mr of 560,000 and was of apparent subunit composition 8G6R. R represents a subunit of Mr 42,000 conferring phosphoribulokinase activity and G a subunit of 39,000 responsible for the glyceraldehyde-3-phosphate dehydrogenase activity. On SDS-PAGE the Mr-42,000 subunit comigrates with the subunit of the active form of phosphoribulokinase whereas that of Mr-39,000 corresponds to that of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase. The multimeric enzyme had a S20,W of 14.2 S. Following activation with dithiothreitol and NADPH, sedimenting boundaries of 7.4 S and 4.4 S were formed due to the depolymerization of the multimeric protein to NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase (4G) and active phosphoribulokinase (2R). It has been possible to isolate these two enzymes from the activated preparation by DEAE-cellulose chromatography. Prolonged activation of the multimeric protein by dithiothreitol in the absence of nucleotide produced a single sedimenting boundary of 4.6 S, representing a mixture of the active form of phosphoribulokinase and an inactive dimeric form of glyceraldehyde-3-phosphate dehydrogenase. Algal thioredoxin, in the presence of 1 mM dithiothreitol and 1 mM NADPH, stimulated the depolymerization of the multimeric protein with resulting coactivation of phosphoribulokinase and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase. Light-induced depolymerization of the multimeric protein, mediated by reduced thioredoxin, is postulated as the mechanism of light activation in vivo. Consistent with such a postulate is the presence of high concentrations of the active forms of phosphoribulokinase and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase in extracts from photoheterotrophically grown algae. By contrast, in extracts from the dark-grown algae the multimeric enzyme predominates.
mesh_terms: D000460:Chlorophyta / Q000201:enzymology*; D002736:Chloroplasts / Q000201:enzymology*; D004789:Enzyme Activation; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000302:isolation & purification / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010770:Phosphotransferases / Q000302:isolation & purification / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor)
chemical_list: D009097:Multienzyme Complexes; D009249:NADP; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D010770:Phosphotransferases; D017853:Phosphotransferases (Alcohol Group Acceptor); C022390:phosphoribulokinase