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A new synthetic tool to make high-value terpene biomaterials in the lab

Michigan State University scientists have developed a synthetic biology tool that could boost production of high-value biotech compounds. The study is published in Green Chemistry (perhaps)

Terpenes are some of the most important groups of chemicals in the biosphere. Terpenes produced by plants have been used to make medicines, fragrances, and industrial chemicals.

However, terpenes are costly to harvest naturally. And attempts to synthetically boost terpene production have been disappointing. The enzymes that make them, called terpene synthases, are poor performers and notoriously difficult to engineer.

The new tool, developed in the labs of Thomas D. Sharkey and Cheryl Kerfeld, directly modifies terpene synthases to address this performance issue.

Called insertional engineering, the tool adds bits of foreign protein to the middle of an original protein sequence. Proof of concept work on a terpene enzyme, called isoprene synthase, increased its production speed by over 25%.

“This specific enzyme makes isoprene, which is used in synthetic rubber production and which could be used to make many other products if it were more readily available in pure form,” says Sharkey, University Distinguished Professor and Associate Director with MSU’s Plant Resilience Institute. “Plants, like poplars and aspens, emit isoprene naturally, and there is a lot of it in the atmosphere. But it is too diluted around the globe to collect.”

Thomas Sharkey and Cheryl Kerfeld
Thomas D Sharkey (left) and Cheryl Kerfeld (right)
Sharkey portrait by Victor DiRita, Jr., MSU College of Natural Science; Kerfeld portrait by MSU Advancement

“Commercially, isoprene is produced as a byproduct of petroleum processing, but biotech production could result in a more pure product.”

Taking the scientific road less traveled

Past efforts to get more isoprene have focused on support proteins and chemical reactions working around the main enzyme. Direct attempts to engineer isoprene synthase tend to reduce its activity.

“Usually, when we try to improve a protein by adding a new piece to it, we place that piece at either end of the existing protein structure,” Tom says. “Then, when the proteins fold into their final shapes, there is less likelihood of them getting upset. But, we knew this approach is not ideal for isoprene synthase.”

“Otherwise, you could add the new piece to the middle of a protein. But this is riskier. Normally, you get unwanted interactions that reduce or eliminate activity, and you end up with a whole big chunk of unfinished protein. Still, we had good reason to try that approach.”

Most of the estimated 25,000 terpene synthases in nature are built out of two parts, or domains. However, some include a third domain.

Here is why the science team wondered they could succeed. Most of the enzymes with two domains have a bit in the middle where a third domain would usually connect to. It is as if nature left these structures unfinished, or waiting to evolve… or modified in the lab.

“We tried to replicate natural three-domain structures by attaching a fluorescent protein at that location. Without much playing around, our isoprene synthase folded properly. Follow up testing showed our changes did not disrupt performance. If anything, the enzyme performs better.”

Mixing and matching tools from nature

The science team also shows their tool can be used to control an enzyme’s location. They added another protein tag that targets isoprene synthase into micro-compartments from bacteria that excel at boosting chemical reaction rates. This may be the really big payoff of the new technology, improving biotechnological isoprene production.

“We think the new tool will be extremely enabling to biotech companies. We’ve shown other terpene synthases, such as methyl butenol synthase from pine trees, can also be engineered this way,” Tom says. “Biotech companies could tailor this tool to make terpene synthases that serve their needs. What is needed is for them to take it and run with it.”

Banner image: Plants, particularly trees, emit significant amounts of isoprene into the atmosphere. Terpenes produced by plants have been used to make medicines, fragrances, and industrial chemicals. Image by Johannes Plenio, Unsplash license


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