Function of reduced pyridine nucleotide-ferredoxin oxidoreductases in saccharolytic Clostridia.

Abstract

Abstract The physiological function of the clostridial NADH- and NADPH-ferredoxin oxidoreductases was investigated with Clostridium pasteurianum and Clostridium butyricum . The NADH-ferredoxin oxidoreductases are concluded to be catabolic enzymes required for the reduction of ferredoxin by NADH. The conclusion is based on the finding that during the entire growth phase the fermentation of glucose can be formally represented by the weighted sum of Eqns 1 and 2, Glucose + 2 H 2 O → 1 butyrate − + 2 HCO 3 − + 3 H + + 2 H 2 (1) Glucose + 4 H 2 O → 2 acetate − + 2 HCO 3 − + 4 H + + 4 H 2 (2) and that in these redox processes NADH rather than NADPH is specifically formed during glyceraldehyde phosphate dehydrogenation. This NADH can be consumed by substrate reduction in Process 1 only, while it must be reoxidized in Process 2 by the ferredoxin-dependent proton reduction to hydrogen which involves the NADH-ferredoxin oxidoreductases. The kinetic and regulatory properties of these enzymes are in line with their catabolic role: they are found with high specific activities typical for other catabolic enzymes; essentially they catalyze electron flow from NADH to ferredoxin only because the back reaction is very effectively inhibited by low concentrations of NADH. These enzymes have a key role in the coupling of the two partial processes and in regulating the overall thermodynamic efficiency of the fermentations. The NADPH-ferredoxin oxidoreductases are concluded to participate in anabolism; they are required for the regeneration of NADPH. The conclusion is based on the finding that in the two clostridia all catabolic oxidations-reductions are specific for NAD(H) and that the usual NADPH-producing processes such as the glucose 6-phosphate dehydrogenase or malate enzyme reactions are absent. The kinetic properties of the enzymes are in agreement with their anabolic function: the NADPH-ferredoxin oxidoreductases are found with sufficient specific activities; they preferentially catalyze electron transfer from reduced ferredoxin to NADP + .