Gluconate catabolism in cowpea rhizobia: evidence for a ketogluconate pathway

A comparative proteomic evaluation of culture grownvs nodule isolatedBradyrhizobium japonicum

Total protein extract of Bradyrhizobium japonicum cultivated in HM media were resolved by 2-D PAGE using narrow range IPG strips. More than 1200 proteins were detected, of which nearly 500 proteins were analysed by MALDI-TOF and 310 spots were tentatively identified. The present study describes at the proteome level a significant number of metabolic pathways related to important cellular events in free-living B. japonicum. A comparative analysis of proteomes of free-living and nodule residing bacteria revealed major differences and similarities between the two states. Proteins related to fatty acid, nucleic acid and cell surface synthesis were significantly higher in cultured cells. Nitrogen metabolism was more pronounced in bacteroids whereas carbon metabolism was similar in both states. Relative percentage of proteins related to global functions like protein synthesis, maturation & degradation and membrane transporters were similar in both forms, however, different proteins provided these functions in the two states.

Purification and kinetic characterization of hexokinase and glucose-6-phosphate dehydrogenase from Schizosaccharomyces pombe

Hexokinase and D-glucose-6-phosphate dehydrogenase (G6PDH) from Schizosaccharomyces pmbe have been purified 250-fold by an identical three-step. Both enzymes are dimeric with a molecular mass of 88 kDa for the kinase and 112 kDa for the dehydrogenase. Steady-state kinetic studies were performed on hexokinase and G6PDH, which form the glucose phosphate branch of the oxidative pentose phosphate pathway of S. pombe (fission yeast). Hexokinase promotes Mg(2+)-activated phosphorylation of D-glucose by the equilibrium random Bi Bi mechanism with formation of the abortive enzyme-ADP-glucose complex. ADP inhibits the kinase competitively versus ATP and noncompetitively versus D-glucose. The Mg2+ activation of hexokinase is associated with an increase in the maximal velocity by its interaction with the ternary complex to facilitate the transfer of the phosphoryl group. G6PDH catalyzes NADP(+)-linked oxidation of D-glucose-6-phosphate by the ordered Bi Bi mechanism with NADP+ as the leading reactant. High NADP+ concentration inhibits the dehydrogenase by forming the dead-end ternary complex. In addition, G6PDH is also subjected to product inhibition by NADPH and noncompetitive inhibition by A(G)TP. Thus, the oxidative pentose phosphate pathway in S. pombe may be regulated via inhibition of hexokinase by ADP in conjunction with inhibition of G6PDH by NADPH and ATP.

Tricarboxylic acid cycle and anaplerotic enzymes in rhizobia

Rhizobia are a diverse group of Gram-negative bacteria comprised of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and Azorhizobium. A unifying characteristic of the rhizobia is their capacity to reduce (fix) atmospheric nitrogen in symbiotic association with a compatible plant host. Symbiotic nitrogen fixation requires a substantial input of energy from the rhizobial symbiont. This review focuses on recent studies of rhizobial carbon metabolism which have demonstrated the importance of a functional tricarboxylic acid (TCA) cycle in allowing rhizobia to efficiently colonize the plant host and/or develop an effective nitrogen fixing symbiosis. Several anaplerotic pathways have also been shown to maintain TCA cycle activity under specific conditions. Biochemical and physiological characterization of carbon metabolic mutants, along with the analysis of cloned genes and their corresponding gene products, have greatly advanced our understanding of the function of enzymes such as citrate synthase, oxoglutarate dehydrogenase, pyruvate carboxylase and malic enzymes. However, much remains to be learned about the control and function of these and other key metabolic enzymes in rhizobia.

Enzymes of glucose metabolism in Frankia sp

Enzymes of glucose metabolism were assayed in crude cell extracts of Frankia strains HFPArI3 and HFPCcI2 as well as in isolated vesicle clusters from Alnus rubra root nodules. Activities of the Embden-Meyerhof-Parnas pathway enzymes glucokinase, phosphofructokinase, and pyruvate kinase were found in Frankia strain HFPArI3 and glucokinase and pyruvate kinase were found in Frankia strain HFPCcI2 and in the vesicle clusters. An NADP+-linked glucose 6-phosphate dehydrogenase and an NAD-linked 6-phosphogluconate dehydrogenase were found in all of the extracts, although the role of these enzymes is unclear. No NADP+-linked 6-phosphogluconate dehydrogenase was found. Both dehydrogenases were inhibited by adenosine 5-triphosphate, and the apparent Km's for glucose 6-phosphate and 6-phosphogluconate were 6.86 X 10(-4) and 7.0 X 10(-5) M, respectively. In addition to the enzymes mentioned above, an NADP+-linked malic enzyme was detected in the pure cultures but not in the vesicle clusters. In contrast, however, the vesicle clusters had activity of an NAD-linked malic enzyme. The possibility that this enzyme resulted from contamination from plant mitochondria trapped in the vesicle clusters could not be discounted. None of the extracts showed activities of the Entner-Doudoroff enzymes or the gluconate metabolism enzymes gluconate dehydrogenase or gluconokinase. Propionate- versus trehalose-grown cultures of strain HFPArI3 showed similar activities of most enzymes except malic enzyme, which was higher in the cultures grown on the organic acid. Nitrogen-fixing cultures of strain HFPArI3 showed higher specific activities of glucose 6-phosphate and 6-phosphogluconate dehydrogenases and phosphofructokinase than ammonia-grown cultures.