INSIGHT INTO BIOPLASTICS SYNTHESIS IN BACTERIA
27 July 2018 — Finally we have an idea how bioplastics are made in bacteria.
Many bacteria can produce a certain type of bioplastics, polyhydroxyalkanoates (PHAs), which the bacteria actually use to store carbon and energy. The key enzyme is the PHA synthase PhaC, which combines hydroxyalkanoate (HA) monomers into long PHA chains. PhaC had been studied since the 1980s, but its mechanism had been eluding scientists until now.
One way to understand how a protein functions is to determine its three-dimensional structure. Recently two PhaC structures had been determined for the first time, but unfortunately the structures were in a closed form — no tunnel was observed for the PHA chains to exit the enzyme — and hence the mechanism was still not fully revealed.
CCB's Structural Biology team then constructed a model of the PhaC enzyme from the bacterium Aquitalea sp. USM4, and successfully identified a three-branched channel that could serve as the entry for the HA monomers (blue) and exit for the PHA polymers (cyan in the figure above). Docking of a growing PHA chain into the catalytic site further suggested a unique elongation mechanism, which requires a ∼180° rotation of the monomer being added to the chain.
The properties of the PHA bioplastics — flexible, brittle, hard, etc — depend on the types and length of the HA monomers. For example, the PHA made up of the shortest monomer is brittle and stiff, but adding longer polymers increases its flexibility. Our model has also been able to explain how the PhaC enzyme recognizes monomers of different length.
With these fresh insights to further facilitate PhaC engineering, we hope we are now closer to designer bioplastics that can cater for our daily needs and, more importantly, also help reduce the pollution created by humans.
(Link valid until 17 September)