Nature's Route To Sunscreen Revealed

A closer look at how cyanobacteria craft small molecules to shield themselves from the sun’s rays might give organic chemists some inspiration for building imines. Conjugated, imine-containing natural-product sunscreens known as mycosporine and mycosporine-like amino acids protect many a marine creature and have even made their way into commercial sunscreen ingredients such as Helioguard 365. Christopher T. Walsh and Emily P. Balskus of Harvard Medical School identified and characterized the enzymes that cyanobacteria use to make one such sunscreen (Science, DOI: 10.1126/science.1193637). They report that Anabaena variabilis relies on four enzymes to make shinorine. The first two enzymes craft shinorine’s cyclohexenone core (4-deoxygadusol) from sedoheptulose 7-phosphate, an intermediate in the pentose phosphate pathway. Two other enzymes, both of which resemble those responsible for ATP-dependent peptide bond formation, then tether glycine and L-serine amino acids to that core via imine bonds. Both enzymes use mechanisms of imine formation distinct from existing chemical and biochemical methods, Walsh and Balskus note. One enzyme activates 4-deoxygadusol via phosphorylation and then carries out 1,4-addition of glycine. The other activates L-serine via adenylation and adds it to the ring to give shinorine.