Synthesis in Alcaligenes eutrophus
In A. eutrophus, the synthesis of PHB involves glucose as a carbon source and three enzymes: 3-ketothiolase, acetoacetyl-CoA reductase, and PHA synthase [5]. The first enzyme, 3-ketothiolase, is responsible for carrying the condensation reaction of acetyl-CoA into acetoacetyl-CoA. Then, acetoacetyl-CoA reductase reduces acetoacetyl-CoA into R(-)-3-hydroxybutynl-CoA, with NADPH serving as the reducing agent. R(-)-3-hydroxybutynl-CoA is then polymerized into PHB with the aid of PHA synthase (Figure 4). A. eutrophus can produce an amount of PHB equal to 80% of their dry body weight when grown in sugar-rich but nitrogen/ phosphate-deprived media.
Alcaligenes eutrophus synthesizes different kind of polymers based on the carbon source it is exposed to in the media. For example, having propionic acid instead of glucose as the main carbon source leads to the production of a polymer containing 3-hydroxybutyrate a (3HB) and 3-hydroxyvalerate (3HV) instead of PHB (Fig. 4). On the other hand, completely substituting glucose with valeric acid leads to higher production of polymer in A. eutrophus— euqal to 90% of the dry body weight.