Native and engineered metabolic pathways involved in hydrogen production by E. coli. On the right is shown the main multiple pathways of mixed acid fermentation. Key enzymes and enzyme complexes are indicated by either the genetic nomenclature or the commonly used pathway abbreviation: Phosphoenolpyruvate (PEP); Phosphoenolpyruvate carboxylase (PEPC); Fumarate reductase (frdC); Lactate dehydrogenase (ldhA); Pyruvate formate lyase (PFL); Formate hydrogen lyase (FHL); Hydrogenase 3 (Hyd 3); formate dehydrogenase-H (FDHH); Uptake hydrogenases; hydrogenase 1 (Hyd 1) and hydrogenase 2 (Hyd 2); fumarase (fumB); fumurate reductase (frdC). Points where NADH is produced or consumed are noted. On the left is a schematic of the SH hydrogenase, which, if functional, might consume NADH, reducing protons to hydrogen.