Some of us are old enough to remember when a polyester suit was fashionable. Although still used in clothing today, polyester has diminished in appeal since the 1970s. There is however, plenty of polyester still around but in a different guise as PET or polyethylene terephthalate which is the major constituent of plastic drinks bottles.
Patented in the 1940s as a thermoplastic polymer, PET provided a cheap and readily available packaging material. That economic dream however has now turned into an environmental nightmare as a mountain of discarded plastic has built up across land and sea.
The news media over the past few days have hailed the accidental discovery of a mutant enzyme that breaks down PET as the solution to the plastic pollution problem. As a biochemist I thought the general news media was somewhat short on detail and so I thought I’d take a closer look.
PET comprises repeating units of a chemical called terephthalate and ethylene glycol joined together with oxygen-containing links called esters. Esters and related chemical bonds are common in nature and are generally vulnerable to enzymes collectively called hydrolases, which include esterases, lipases, and cutinases. They all add water across chemical bonds resulting in cleavage of the individual molecules. Blood, for example, is rich in esterases that break down a variety of chemicals, including neurotransmitters such as acetyl choline. The nerve agents recently featured in the news, work by stopping esterase from breaking down acetyl choline thus interfering with normal nerve function.
The trouble with PET however, is that because of its molecular structure the bonds are not accessible to hydrolases although the exact molecular process of enzymic PET hydrolysis is not clearly understood. This doesn’t mean PET is immune from biodegradation, it just means it happens very slowly. All enzymes are what are known as catalysts, in that they don’t react directly rather than just speed up existing chemical reactions. The discovery of the mutant enzyme in the news is just such a catalyst that speeds up the rate of hydrolysis of PET. The effect is highly significant, speeding up the natural process from hundreds of years to just days.
This new mutant enzyme is called PET hydrolase (or PETase for short) and was found in a species of bacteria called Ideonella sakaiensis that lives in the plastic mountains of Japan. This bacteria can live off plastic, breaking it down and using it as a carbon source in much the same way we utilise sugar.
It is however, not the first biological weapon against plastic to be discovered. Last year there were reports that wax moth larvae could break down PET although the effect was likely to be due to bacteria living in the worm’s gut.
The big question is, will these discoveries solve the problem of plastic pollution? The answer is probably, but there is always a lag of many years from the basic science until development of effective technologies. In the meantime, now is not the time to be complacent and reducing plastic consumption remains an important endeavour.
Graham Lappin 