Formation of poly(hydroxybutyrate-co-hydroxyvalerate) by Azotobacter vinelandii UWD

Abstract

Azotobacter vinelandii UWD formed polyhydroxyalkanoate (PHA) copolymers containing β-hydroxybutyrate and β-hydroxyvalerate (HV) when grown in a medium containing glucose as the primary C source and valerate (pentanoate) as a precursor. Copolymer was not formed when propionate was added to the glucose medium but was formed when heptanoate, nonanoate, or trans-2- pentenoate was present. Optimal levels of HV were formed when valerate was added at the time of maximum PHA synthesis, although HV incorporation was not dependent on glucose catabolism. HV content in the polymer was increased from 17 to 24 mol% by adding 10 to 40 mM valerate to glucose medium, but HV insertion into the polymer occurred at a fixed rate. Similarly, the addition of valerate to a fed-batch culture of strain UWD in beet molasses in a fermentor produced 19 to 22 g of polymer per liter, containing 8.5 to 23 mol% HV after 38 to 40 h. The synthesis of HV in these cultures also occurred at a fixed rate (2.3 to 2.8 mol% h-1), while the maximum PHA production rate was 1.1 g liter-1 h-1. During synthesis of copolymer in batch or fed-batch culture, the yield from conversion of glucose into PHA (Y(P/S)) remained at maximum theoretical efficiency (≥0.33 g of PHA per g of glucose consumed). Up to 45 mol% HV in the polymer was obtained by growing strain UWD in medium containing ≤50 mM valerate as the sole C source, but the PHA produced amounted to <1 g/liter. The combination of 30 mM valerate as a sole C source and 0.5 mM 4-pentenoate increased the HV content in the polymer to 52 mol%. The results strongly support a route involving β-oxidation in the production of HV in A. vinelandii PHA. The results show that strain UWD can form PHA copolymers of potential use as bioplastics and that the substrate cost per kilogram of PHA formed in beet molasses medium should be less than half of that per kilogram of PHA formed in glucose medium.