Bioretrosynthesis is a technique for synthesizing organic chemicals from inexpensive precursors and evolved enzymes.[1] The technique builds on the retro-evolution hypothesis proposed in 1945 by geneticist Norman Horowitz.[2]
The technique works backwards from the target to identify a precursor molecule and an enzyme that converts it into the target, and then a second precursor that can produce the first and so on until a simple, inexpensive molecule becomes the beginning of the series.[1] For each precursor, the enzyme is evolved using induced mutations and natural selection to produce a more productive version. The evolutionary process can be repeated over multiple generations until acceptable productivity is achieved.[1] The process does not require high temperature, high pressure, the use of exotic catalysts or other elements that can increase costs.[1] The enzyme "optimizations" that increase the production of one precursor from another are cumulative in that the same precursor productivity improvements can potentially be leveraged across multiple target molecules.[1]
In 2014 the technique was used to produce the HIV drug didanosine:[2] a simpler molecule was identified that can be converted into didanosine when subjected to a specific chemical transformation in the presence of a specific enzyme.[2] The gene that creates the enzyme was then "copied", adding random mutations to each copy using ribokinase engineering.[2] The mutant genes were inserted into Escherichia coli bacteria and used to produce (now-mutant) enzymes. The enzymes were then mixed with the precursor and the mutant enzymes that produced the greatest amount of didanosine were retained and replicated. One mutant stimulated a 50x increase in didanosine production.[2] The first step was repeated, using the first precursor in place of didanosine, finding a yet simpler precursor and an enzyme to produce it. One mutated enzyme produced a 9,500x increase in nucleoside production.[2] A third retrogression allowed them to start with the simple and inexpensive sugar named dideoxyribose and produce didanosine in a three-step sequence.[2]
