A clue into making of biodegradable plastic

Biodegradable plastic is used in niche areas, as in this field, where it gives a protective cover to plants.   | Photo Credit: CATHAL MCNAUGHTON

For the first time, scientists from MIT have succeeded in decoding the structure of an enzyme that is used by bacteria to make polymer chains which is used for storing carbon when going through nutrient deficits. What is special about these polymers is that they make up a range of “biodegradable” plastics.

Known as Polyhydroxyalkanoic acids (PHAs), these polymer chains have properties ranging from thermoplastics to elastomers depending on the type of monomers attached to them. Their property of being biodegradable renders them of great value in producing environment-friendly plastics for industrial use; however, the process is not cost-effective.

One such PHA is polyhydroxybutyrate (PHB). Knowing the structure of the enzyme PHB synthase can greatly help in developing the process further. While the present work does not promise biodegradable plastic at competitive rates just yet, it is a step in that direction.

In making these polymers, a culture of a gram negative bacteria — Cupriavidis necator — is allowed to multiply significantly on a substrate. Then the colony is put through a deficiency of a particular nutrient, while being supplied carbon. In response, the bacteria produce these long polymer chains to store the available carbon. The polymers can be extracted from the bacteria and used. A bacterium can produce polymers that measure up to 85 per cent of its weight.

“One of the keys to being able to understand a system in enough detail to engineer it is being able to visualize the components that are required to produce a desired product. For close to 20 years, groups around the world have been working towards determining the structure of the enzyme responsible for producing PHAs,” says Catherine Drennan, professor of biology and chemistry at MIT, the author of a paper published in Journal of Biological Chemistry.

Elizabeth Wittenborn, a graduate student and first author of the paper, came up with the structure of the enzyme through a process of crystallography. Her analysis revealed a “dimer” configuration of Pha synthase — namely that it was made up of two identical subunits. Each of this has an active site where the polymerisation occurs.

This differed from earlier surmises that the active site resided in the interfaces of the subunits. This analysis also revealed that the enzyme had two openings through which the starting material entered and the formed polymer emerged.

Prof. Drennan says that now that the structure is known, it is possible that thermal and mechanical properties may be improved by changing the polymer composition.

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Printable version | Jan 22, 2022 6:05:11 AM |

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