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Friday, July 12, 2024
Dr. Brown: Environmentally friendly, effective enzymes and all that plastic

The headline might sound esoteric, but the subject is not.

The planet is awash with plastic, thrown every which way from car windows, piled up in dumpsites or cluttering the bottoms of rivers, seas and oceans where it's out of sight and mind, left to generations to come to solve.

Bundling the stuff up and shipping it off to foreign countries isn’t and should never have been an option. The trouble is “us.” We consume plastic, not literally of course, but for packaging food, household cleaners, cosmetics and hundreds of other items our homes and cars are stuffed with – plastic is everywhere.

But first a word about catalysts: they accelerate chemical reactions without themselves changing.

For example, when silver is put into a beaker containing hydrogen sulphide (H2O2), the hydrogen peroxide changes into water (H2O) and oxygen (O2).

The catalytic converter in cars and trucks control toxic emissions from internal combustion engines by turning them into less-toxic pollutants by catalyzing the combination of oxygen (O2) with carbon monoxide (CO) and unburned hydrocarbons (CnHn) to produce water and carbon dioxide (CO2) using metals such as platinum, rhodium or palladium.

Before the year 2000, catalysts belonged to one of two groups: metals or organic enzymes. Metals are excellent catalysts because they borrow and share electrons willingly. However, to work, they need an oxygen- and water-free environment, and many are heavy, toxic metals.

The beauty of life’s catalysts (organic and carbon-based) is that they are very efficient, precise and do not incorporate a metal. They also work side by side with other enzymes to form chains, passing off their products to the next metabolic reaction in the chain, much like relay racers pass batons, to build complex molecules or generate energy such as ATP in oxidative phosphorylation, our prime source of energy at the cellular level.

One way to deal with plastics would be to break them down with environmentally friendly organic enzymes. Which brings us to two recent Nobel Prizes.

In 2018, Frances Arnold was awarded a Nobel for harnessing evolution to make more effective organic enzymes and, this year, Benjamin List and David MacMillan won a Nobel for equally clever ways of developing organic enzymes.

Like other scientists and engineers, Arnold considered building enzymatic proteins from scratch, one amino acid after another, but soon realized that would be a herculean task well beyond her laboratory’s or any other’s capabilities. Then she had her better idea – why not harness evolution to create better organic enzymes?

She and her colleagues began with a single organic enzyme and set out to improve it by introducing random mutations into the gene that coded for the enzyme. Then they compared the effectiveness of the enzymes produced by the mutant genes and chose the most effective.

After several similar rounds, the best enzymes were much more effective than the original, and all achieved by harnessing the power of evolution. It was an inspired idea and reason enough for her Nobel win.

This year’s winners, List and MacMillan, had other clever ideas. List’s was based on the fact that the active site in enzymes was usually only a small part of the much larger molecule (as small as a single amino acid or at the most a few amino acids).

So why not try a single amino acid, such as proline, which is simple, cheap, and environmentally friendly, to see whether such a bared-boned molecule could catalyze organic reactions? It could. That was List’s breakthrough and with hindsight, it’s hard to see why such a simple, straightforward idea hadn’t been tried before. Even List was surprised with his success.

MacMillan’s bright idea involved designing simple organic molecules, which like metals could accommodate electrons, but were far more durable than metals and unlike many metals, not toxic.

To catalyze the reaction he was interested in, he needed to form an iminium ion that contains a nitrogen atom, which has an inherent affinity for electrons. He chose several organic molecules with the right properties and tested their ability to drive a reaction that chemists use to build rings of carbon (the Diels-Alder reaction). It worked.

All three were clever ideas that worked to simplify organic enzymes, make them more effective and cheaper to produce. That’s a huge plus for cleaning up plastics and toxins in the environment and a boon for pharmaceutical companies creating tailor made drugs.

* Our fifth annual Review of the 2021 Nobel Prizes begins with physics on Nov. 8, followed by the chemistry on Nov. 15, economics on Nov. 22, medicine on Nov. 29, the Peace Prize on Dec. 6 and, finally, the literature prize on Dec. 13. All are at 11 a.m., with Zoom as our medium. Reserve your spot through the NOTL Public Library.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.  


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