Polyhydroxyalkanoate film formation and synthase activity during in vitro and in situ polymerization on hydrophobic...

In vitro and in situ enzymatic polymerization of polyhydroxyalkanoate (PHA) on two hydrophobic surfaces, a highly oriented pyrolytic graphite (HOPG) and an alkanethiol self-assembled monolayer (SAM), was studied by atomic force microscopy (AFM) and quartz crystal microbalance (QCM), using purified Ralstonia eutropha PHA synthase (PhaC(Re)) as a biocatalyst. (R)-Specific enoyl-CoA hydratase was used to prepare R-enantiomer monomers [(R)-3-hydroxyacyl-CoA] with an acyl chain length of 4-6 carbon atoms. PHA homopolymers with different side-chain lengths, poly[(R)-3-hydroxybutyrate] [P(3HB)] and poly[(R)-3-hydroxyvalerate] [P(3HV)] were successfully synthesized from such R-enantiomer monomers on HOPG substrates. After the reaction, the surface morphologies were analyzed by AFM, revealing a nanometer thick PHA film. The same biochemical polymerization process was observed on an alkanethiol (C18) SAM surface fabricated on a gold electrode using QCM. This analysis showed that a complex sequence of PhaC(Re) adsorption and PHA polymerization has occurred on the hydrophobic surface. On the basis of these observations, the possible mechanisms of the PhaC(Re)-catalyzed polymerization reaction on the surface of hydrophobic substrates are proposed.