Figure 2

Proposed catabolic network in C. saccharolyticus. Glucose is transported over the membrane by an ABC transport system (1) [23, 26] and oxidized to pyruvate in the EMP pathway [13, 26], with the possibility that PPi can potentially replace ATP, i.e., PPi-PFK (2) instead of ATP-PFK (3), or ADP, i.e., PPDK (5) instead of PYK (6) [71]. NADH generated through the GAPDH-catalyzed reaction (4) and Fd_red generated from the PFOR-catalyzed reaction (7) can donate electrons for the generation of molecular H₂ catalyzed by cytosolic NADH-dependent Fe-only hydrogenase and membrane-bound Fd-dependent NiFe-hydrogenase, respectively [26]. Acetate is formed by the consecutive actions of phosphotransacetylase (PTA; 8) and acetate kinase (AK; 9), with the generation of ATP. Alternative routes for NADH reoxidation are the formation of lactate (LDH; 11), ethanol (ADH; 10) or succinate (12) [16]. The solid lines represent metabolic routes, whereas the dashed lines represent metabolite activation (+) or inhibition (-) of enzyme activities. PPi generated from anabolic reactions [71] is a strong inhibitor of both PYK and LDH activities [47, 71].