Metabolism of sugars and 2 , 3 - butanediol production by P . carbinolicus. The phosphotransferase system (right) consists of two enzymes (PtsI, PtsP) that take a phosphate (circled P) from phosphoenolpyruvate, a phosphocarrier protein (PtsH), two signal output proteins (PtsII A), and a tripartite transporter complex (PtsII BCD) that imports and phosphorylates sugars yet to be identified. Glycolysis of a sugar (e.g. glucose-6-phosphate) yields NADH, ATP and two phosphoenolpyruvate molecules, which can either bring in two sugar molecules or yield two ATP or enter a third pathway through phosphonopyruvate (bottom right). The oxidative pentose-phosphate pathway from glucose-6-phosphate to ribulose-5-phosphate (which is recycled into glycolytic intermediates) yields no ATP, but by making NADPH instead of NADH, it may facilitate production of hydrogen/formate. If pyruvate:ferredoxin/flavodoxin oxidoreductase does not function catabolically in P. carbinolicus (middle), there will be insufficient acetyl-CoA for ethanol production to dispose of NADH from glycolysis, and the only source of doubly reduced ferredoxin (Fd2e) for hydrogen/formate production will be the Rnf complex. Production of acetoin and 2,3-butanediol (left), which P. carbinolicus can degrade to acetyl-CoA later, disposes of half the NADH from glycolysis.