Primary central carbon metabolism of C. acetobutylicum. Cofactors consumed by each reaction are listed as (−) and cofactors produced (+) (H+ ions are not shown). The following enzymes are shown in bold: (LDH) lactate dehydrogenase, (PFO) pyruvate ferredoxin oxidoreductase, (FNO) ferredoxin NAD+ oxidoreductase, (FNPO) ferredoxin NADP+ oxidoreductase, (HYDA) hydrogenase, (AAD) acetaldehyde/alcohol dehydrogenase, (PTA) phosphotransacetylase, (AK) acetate kinase, (THL) thiolase, (CoAT) acetoacetyl-CoA transferase (for acetate and butyrate), (AADC) acetoacetate decarboxylase, (BHBD) β-hydroxybutyryl-CoA dehydrogenase, (CRO) crotonase, (BCD) butyryl-CoA dehydrogenase, (PTB) phosphotransbutyrylase, (BK) butyrate kinase, (BDHA) butanol dehydrogenase A, and (BDHB) butanol dehydrogenase B. The CoAT can function with either acetate or butyrate substrate; it does not require both. The AAD can catalyze three reactions in the model. These are listed as (i) AAD_1, (ii) AAD_2, and (iii) AAD_3.