Novel Extracellular PHB Depolymerase from Streptomyces ascomycinicus: PHB Copolymers Degradation in Acidic Conditions

The ascomycin-producer strain Streptomyces ascomycinicus has been proven to be an extracellular poly(R)-3-hydroxybutyrate (PHB) degrader. The fkbU gene, encoding a PHB depolymerase (PhaZ(Sa)), has been cloned in E. coli and Rhodococcus sp. T104 strains for gene expression. Gram-positive host Rhodococcus sp. T104 was able to produce and secrete to the extracellular medium an active protein form. PhaZ(Sa) was purified by two hydrophobic interaction chromatographic steps, and afterwards was biochemically as well as structurally characterized. The enzyme was found to be a monomer with a molecular mass of 48.4 kDa, and displayed highest activity at 45°C and pH 6, thus being the first PHB depolymerase from a gram-positive bacterium presenting an acidic pH optimum. The PHB depolymerase activity of PhaZ(Sa) was increased in the presence of divalent cations due to non-essential activation, and also in the presence of methyl-β-cyclodextrin and PEG 3350. Protein structure was analyzed, revealing a globular shape with an alpha-beta hydrolase fold. The amino acids comprising the catalytic triad, Ser(131)-Asp(209)-His(269), were identified by multiple sequence alignment, chemical modification of amino acids and site-directed mutagenesis. These structural results supported the proposal of a three-dimensional model for this depolymerase. PhaZ(Sa) was able to degrade PHB, but also demonstrated its ability to degrade films made of PHB, PHBV copolymers and a blend of PHB and starch (7∶3 proportion wt/wt). The features shown by PhaZ(Sa) make it an interesting candidate for industrial applications involving PHB degradation.