Figure 1: A schematic of microbial PHA production pathways. For scl-PHA, 2 molecules of acetyl-CoA are ligated by a β-ketothiolase (1) to form acetoacetyl-CoA, which is reduced by acetoacetyl-CoA reductase (2) to form 3-hydroxybutyryl-CoA (3HB-CoA). The 3HB-CoA is polymerized by a PHA synthase enzyme (3) to produce PHB (black dashed box). Acetyl-CoA for PHB biosynthesis can be produced by a turn of the fatty acid β-oxidation cycle (enzymes 4-7). Substrates for mcl-PHA can also come from β-oxidation, via an (R)-specific enoyl-CoA hydratase (8). These medium chain-length hydroxyacyl-CoA molecules are polymerized by a PHA synthase (3) to produce mcl-PHA (grey box). Medium chain-length monomers can also be produced through fatty acid biosynthesis (not shown). Enzyme designations: fatty acyl-CoA dehydrogenase (4), 2-enoyl-CoA hydratase (5), 3-hydroxyacyl-CoA dehydrogenase (6), β-ketothiolase (7).