Bioinformatic and functional analysis of a PHB polymerase (PhbC) from Azospirillum baldaniorum

Background

Azospirillum baldaniorum Sp245 produces poly-β-hydroxybutyrate, a biodegradable polymer with characteristics similar to synthetic thermoplastics, including polypropylene. In the synthesis pathway, the poly-β-hydroxybutyrate synthase enzyme uses thioesters of 3-hydroxy butyryl-CoA as a substrate and catalyzes their polymerization with HS-CoA release.

Methods

A study was conducted using in silico analysis of the two phbC genes of A. baldaniorum Sp245. One was selected for amplification and cloning into the pEXP5- CT/TOPO® vector, which was analysed by restriction pattern, polymerase chain reaction, and sequencing. SDS-PAGE analysis determined the molecular weight of the PhbC1 protein from Azospirillum baldaniorum (AbPhbC1). The presence of the protein was confirmed by Western blotting using anti-polyhistidine monoclonal antibodies. The enzymatic activity in the crude extract of AbPhbC1 was determined by measuring the concentration of sulfhydryl groups using the Ellman method. A UV–Vis assay was performed. To confirm the presence of the poly-β-hydroxybutyrate product, an NMR assay was performed.

Results

In silico analyses, it is revealed that AbPhbC1 and the PhbC2 protein from Azospirillum baldaniorum (AbPhbC2) retain the poly-β-hydroxybutyrate polymerase and α/β hydrolase domain. The Cys-His-Asp catalytic triad is highly conserved in all four polyβ-hydroxyalkanoate synthases in the central subdomain, structurally similar to the reported crystallized proteins. The dimerization subdomain is different; in AbPhbC1, it is in the closed form; in AbPhbC2, it is in the open form; and in AbPhbC2, it lacks the EC region as class III and IV poly-β-hydroxyalcanoate synthases. In vitro, the molecular weight of AbPhbC1 was 68 kDa. The polymerization of PHB by AbPhbC1 was detected by the release of HS-CoA from the quantification of SH. The UV–Vis scan showed a characteristic peak at 264 nm. A comparison of the NMR spectra of the bacterial and commercial poly-β-hydroxybutyrate samples suggested their presence.

Conclusion

In silico analyses suggested that AbPhbC1 and AbPhbC2 are structurally functional, except that AbPhbC2 might require the PhaR subunit for its activity; this strongly suggests that it could be a class IV poly-β-hydroxyalcanoate synthase. UV–Vis scanning and NMR spectroscopy revealed the synthesis of poly-β-hydroxybutyrate by the A. baldaniorum enzyme AbPhbC1, indicating that the enzyme is functional.