Opioid analgesics—pain killers like morphine and codeine—are indispensable to modern medicine; they make recovery from surgical procedures slightly more bearable, and they alleviate pain in cancer patients. Opioid chemical relatives are also hugely important and include antibiotics, muscle relaxants, and cough suppressants.

But these compounds are complex and difficult to synthesize, and labs can't make them nearly as well as biological systems—specifically poppies—can. Manufacturing opioids would be so much easier if we could simply engineer microbes to just spit them out, as we've done for insulin and artemisin.

Now, researchers have done exactly that.

The color test

The first stage of the process required to make morphine has been achieved in bacteria, and the rest of the process has been done in yeast, but large-scale production requires that the entire process be performed within a single microorganism. That has proven difficult because the enzymes required don't work well in yeast. But now, researchers have optimized an enzyme from a completely different source—beets—so it can perform the first stage, enabling the whole process to proceed in one yeast strain.

Opioids start out simply, with the amino acid tyrosine. An enzyme called tyrosine hydroxylase converts tyrosine into L-DOPA, a vital intermediate on the pathway to the opioid drugs. However, tyrosine hydroxylase also transforms L-DOPA into another, unwanted compound through a side reaction.

John Dueber, Vincent Martin, and their colleagues wanted to find a variant of tyrosine hydroxylase that (a) works in yeast, (b) makes a lot of L-DOPA, and (c) has minimal side activity. To do so, they used a very clever biosensor to monitor the activity of the enzymes they were testing. They harnessed a protein from the flowering plant Mirabilis jalapa that turns L-DOPA into a yellow pigment and the unwanted side product into a purple pigment. Using this protein, they could put tyrosine hydroxylase variants from different plant species into yeast and simply see which yeast colony turned the most yellow.

Tyrosine hydroxylase from beets worked best, and the researchers improved its efficiency by subjecting it to a few rounds of random mutagenesis. The resulting tyrosine hydroxylase, combined with enzymes that had been tested earlier in yeast, created a way to fully manufacture opioids in a single microorganism.

While the result is amazing, the real revelation of this work was just how effective a biosensor can be to help identify and then optimize an enzymatic activity of interest.

The coming heroin boom?

Because narcotics are a controlled substance, the growth of poppies is monitored worldwide. Even so, people manage to grow them illegally to fuel the global heroin trade. But yeast is commonplace; any home brewer or baker has access to it. And while a strain of yeast that can make morphine from glucose has not yet been made, now that all of the steps have been worked out, experts expect one within the year.

Because such a yeast strain could provide a much bigger boon to illegal drug dealers than to pharmaceutical companies, Dueber and Martin consulted with bioethicists before publishing their work. Their paper was also accompanied by commentary recommending public policy regulations.

Hopefully Dueber and Martin will start a trend not only of inventing nifty biosensors but also of thinking through the ramifications of their work—and treading carefully to get ahead of unwanted consequences.

Nature Chemical Biology, 2015. DOI: 10.1038/NCHEMBIO1816 (About DOIs).