A plastic pathway

Metab. Eng., published online 23 June 2011, doi:10.1016/j.ymben.2011.06.005

Styrene is an important precursor for a variety of polymer products. Its production from petroleum is highly energy intensive, so alternative synthetic routes are economically and environmentally desirable. Styrene can be produced naturally by yeast and plants, but specific biosynthetic routes have not been identified, and yields are too low to offer any commercial relevance. Metabolic engineering pathways producing related functionalized monoaromatic compounds have also been developed, but these have primarily used tyrosine as a precursor, resulting in phenolic products. Instead, McKenna and Nielsen envisioned converting phenylalanine to styrene by a two-step pathway of deamination and decarboxylation, with trans-cinnamic acid (tCA) as the intermediate. To identify a suitable deaminase, they screened a number of phenylalanine ammonia lyases (PALs) from different species and identified two Arabidopsis thaliana enzymes that could produce up to 918 mg L⁻¹ tCA with no conversion of tyrosine. The authors then searched for a phenylacrylate decarboxylase, identifying one protein from Saccharomyces cerevisiae—FDC1—that could perform the reaction in high yields and with preference for tCA over the tyrosine-based intermediate. When both genes were introduced into an E. coli host, 260 mg L⁻¹ styrene was detected in the aqueous phase, close to the observed toxicity limit of 300 mg L⁻¹. Beyond concerns of toxicity, the authors identified PAL turnover and availability of phenylalanine as possible limits to styrene production, pointing to several future directions to optimize output.