The type of PHA produced depends on the metabolic pathway of
the particular bacteria being used. Scl-PHAs consisting of P(3HB)
homopolymers are generally biosynthesized in a three-step process
that is regulated by 3-ketothiolase (PhaA), acetoacetyl-CoA reductase
(PhaB) and the SCL PHA synthase (PhaC). The most common
metabolic pathway, found in a wide range of bacteria producing scl-
PHAs, is probably the generation of P(3HB) from acetyl-CoAs. This
pathway has been extensively studied in Cupriavidus necator. In this
bacteria two acetyl-CoA moieties are condensed to yield
acetoacetyl-CoA by 3-ketothiolase (PhaA). The product is subsequently
reduced to (R)-3HB-CoA by an NADPH-dependent acetoacetyl-
CoA reductase (PhaB). Only (R)-isomers are accepted as
substrates by PHA synthase (PhaC), the polymerizing enzyme
(Oeding and Schlegel, 1973).