The natural world has long been a source of useful compounds. Aspirin, quinine and morphine all have beneficial therapeutic properties and are often found in plants. Being able to identify, extract and synthesise biologically active ingredients can lead to larger, cheaper amounts being produced.
For compounds with relatively simple chemical structures, such as aspirin, this is straightforward. Others with more complex structures such as opioids – the family of medicines that includes morphine and codeine – it’s extremely difficult.
The complexity of opioids means their active compounds are still sourced from the opium poppy. But now researchers at Stanford have cleverly used genetically modified baker’s yeast to convert sugar into an opioid called hydrocodone. In a study published in Science, a second strain also produced thebaine, a precursor to other opioids.
In nature, the opium poppy has enzymes that are able to synthesise complex molecule structures in a cell. By reprogramming their genetic machinery, the researchers were able to mimic this process in yeast cells, with the added advantage that it also speeded up the process. While production using plans could take up to a year from farm to factory, the Stanford researchers claim a speed of three to five days. Although still in very minute amounts, their proof of concept has the potential for a much wider impact in synthesising other structurally complicated molecules.
Finding active compounds
Biologically active compounds are produced by living organisms in tiny amounts as part of their normal life; that they may have a beneficial effect on us is a side effect but one we have exploited through the ages. The challenge for the chemist is not only to isolate these useful compounds but to obtain a useful amount.
A single plant, for example, only makes microgram quantities of a bioactive compound either because the effect of the molecule on the plant is so potent that a small amount is required by the plant or because it’s a byproduct that the plant doesn’t need. For example a Pacific yew tree only produces minute amounts of taxol, thought to play an antifungal role in protecting the tree. However, for us, taxol has been shown to treat a range of cancers. To obtain enough taxol to produce a dose to treat cancer in us would require several hundred yew trees. So once the compound has been isolated and its structure determined, more of it can be synthesised in the laboratory.
Creating a chemical factory
Many biologically active compounds are the result of a complex series of reactions occurring within cells and driven by nature’s own chemical reagents – enzymes. These naturally occurring proteins are produced within cells encoded by their DNA and allow quite lengthy and complex biosynthesis to be carried out and allows a cell to synthesise complex compounds such as opioids efficiently.
In the lab it can involve as many as 20 or 30 chemical reactions each with a separation and purification step to replicate this – so you can see why it is so hard to scale up to produce useful, cheaper amounts of the compound.
The ability of enzymes to carry out chemical reactions with high degrees of selectivity and efficiency have made them useful chemical tools. This efficiency comes at a price though as, outside their cell the reactivity of enzymes can be drastically reduced. It is much better to keep them inside the cell. The trouble is that no single organism will have all the enzymes you need to carry out a lengthy chemical synthesis.
Yeast has always been a good chemical factory and our long experience over several thousand years in handling it to make bread and alcoholic drinks have given us a good understanding of how it carries out simple reactions such as making sugars into alcohol. It’s a relatively simple organism compared to a poppy plant but the sequencing of its entire genome in 1996 offered a means of rewriting yeast’s DNA and increasing the number of reactions it could process.
To allow yeast cells to produce more complex molecules, however, it needs more enzymes to work with. To do this you need to tack the machinery to produce enzymes onto yeast cells by reprogramming its DNA to tell it how to make the enzymes it needs.
In their work, the Stanford group took more than 20 genes from five different organisms – the California poppy, rat, goldthread, bacteria, and opium poppy – and engineered them into the yeast genome to create the conditions to make hydrocodone, all without disabling the yeast cell. This is an impressive achievement in itself, as putting these different enzymes together in such a sequence runs the risk of a product of one enzyme reaction poisoning or inhibiting another crucial enzyme in the sequence.
A question of scale
The research gives us another way to synthesise natural products, which sits between a purely lab-based and wholly natural approach to producing compounds. However, despite the proof of concept, the researchers still have the same problem with plant sources. The yields per yeast are still too low to be useful (they say it will still take it would take more 16,000 litres of bioengineered yeast to produce a single dose of pain relief). This is partially offset by the shorter production times as yeast cultures can be grown and harvested in a matter of days as opposed to a longer annual growing cycle for plants.
Given the number of genes they’ve been able to incorporate into the yeast cells, an increase in efficiency is certainly not impossible. There is no question that yeast cells have the potential to be chemical factories taking in simple chemical feedstock and producing complex useful compounds efficiently and specifically.
Synthesising a complex molecule in the laboratory is a satisfying exercise for a chemist but as the Stanford researchers pointed out, many of these syntheses are impractical to scale up. Their approach was focused on the end product rather than the practicality of the synthesis itself. To have real-life application, we now need to find a way to scale up the process.