| Salinosporamide A, a highly bioactive beta-lactone from the marine bacterium Salinispora tropica, originates from three biosynthetic building blocks, namely acetate, chloroethylmalonyl-CoA, and the nonproteinogenic amino acid cyclohexenylalanine. The unexpected and unprecedented pathway to chloroethylmalonyl-CoA was illuminated by a multidisciplinary approach involving genetics, organic synthesis, and protein biochemistry, where S-adenosyl-L-methionine (SAM) is converted to chloroethylmalonyl-CoA in a series of reactions catalyzed by pathway-specific enzymes evolved from primary metabolic homologs. Besides the discovery of the first functionalized polyketide synthase (PKS) extender unit chloroethylmalonyl-CoA, three new PKS building blocks were identified, namely propylmalonyl-CoA, bromoethylmalonyl-CoA, and fluoroethylmalonyl-CoA, which are involved in the biosynthesis of different members of the salinosporamide family of potent anticancer agents. This discovery doubles the number of coenzyme A-based PKS extender units from four to eight that give rise to the polyketide family that number in the thousands. Moreover it also suggests a new paradigm in PKS biochemistry employing alpha, beta-unsaturated carboxylic acid substrates. The partial characterized pathway to cyclohexenylalanine, another building block of salinosporamide A, not only represents a new branch in the phenylalanine pathway, but also affords ready access to new fermentation-based salinosporamide A variants for SAR studies through rational metabolic engineering. |
