A novel poly-3-hydroxybutyrate depolymerase was identified in Azotobacter vinelandii. This enzyme, now designated PhbZ1, is associated to the poly-3-hydroxybutyrate (PHB) granules and when expressed in Escherichia coli, it showed in vitro PHB depolymerizing activity on native or artificial PHB granules, but not on crystalline PHB. Native PHB (nPHB) granules isolated from a PhbZ1 mutant had a diminished endogenous in vitro hydrolysis of the polyester, when compared to the granules of the wild-type strain. This in vitro degradation was also tested in the presence of free coenzyme A. Thiolytic degradation of the polymer was observed in the nPHB granules of the wild type, resulting in the formation of 3-hydroxybutyryl-CoA, but was absent in the granules of the mutant. It was previously reported that cultures of A. vinelandii OP grown in a bioreactor showed a decrease in the weight average molecular weight (Mw) of the PHB after 20 h of culture, with an increase in the fraction of polymers of lower molecular weight. This decrease was correlated with an increase in the PHB depolymerase activity during the culture. Here, we show that in the phbZ1 mutant, neither the decrease in the Mw nor the appearance of a low molecular weight polymers occurred. In addition, a higher PHB accumulation was observed in the cultures of the phbZ1 mutant. These results suggest that PhbZ1 has a role in the degradation of PHB in cultures in bioreactors and its inactivation allows the production of a polymer of a uniform high molecular weight.
Keywords: Bioplastic; Depolymerase; Molecular mass; Polyhydroxybutyrate.