When 4-hydroxybutyric acid, 1,4-butyrolactone or
alkanediols of even carbon numbers is used as the sole
carbon source, 4-hydroxybutyryl-coenzyme A (CoA)
may be first formed in A. eutrophus cells (Fig. 1). A
portion of 4-hydroxybutyryl-CoA is then metabolized
into [R]-3-hydroxybutyryl-CoA from intermediates of
B-oxidation of 4-hydroxybutyric acid. A random
copolymer of 3HB and 4HB units is then synthesized by
the copolymerization of [R]-3-hydroxybutyryl-CoA
and 4-hydroxybutyryl-CoA under the action of PHA
polymerase. When ammonium sulphate and citrate are
added to A. eutrophus, acetoacetyl-CoA from 4-
hydroxybutyryl-CoA may be metabolized into acetyl-
CoA rather than into [R]-3-hydroxybutyryl-CoA under
growth conditions, resulting in an increase in the 4HB
fraction and in a decrease in the polyester content in
cells