Final enzyme in salinosporamide A biosynthesis characterized

The natural product salinosporamide A is in clinical trials as a treatment for brain cancer. The bacterial compound crosses the blood-brain barrier and inhibits proteasomes, enzyme complexes that degrade target proteins. This inhibition kills cancer cells. Salinosporamide A was first discovered nearly 20 years ago, but the enzymatic assembly of the complex bicyclic molecule has remained a puzzle.

Now, Bradley S. Moore and Katherine D. Bauman of the University of California, San Diego and coworkers have characterized the enzyme, called SalC, responsible for the bicyclization in the final step of salinosporamide A biosynthesis (Nat. Chem. Biol. 2022, DOI: 10.1038/s41589-022-00993-w). The researchers presented the work on Sunday at the ACS Spring 2022 meeting, Moore in an award presentation in the Division of Organic Chemistry and Bauman in a poster session in the Division of Biological Chemistry.

The molecule has the unusual structure of a β-lactone fused to a γ-lactam. SalC catalyzes the formation of both rings. “The active site looks like what people call a non-elongating ketosynthase. A lot of people might look at this enzyme and think that it’s nonfunctional, that it’s not active at all in the pathway,” Bauman told C&EN. “Instead, it just turns out it’s doing very different chemistry than a normal ketosynthase might do.”

The precursor to salinosporamide A is covalently tethered to the enzyme, so both cyclizations happen in the same active site. The formation of the second ring triggers the release of the completed molecule from the active site. At this point, the researchers don’t know whether the rings form sequentially or in a simultaneous, concerted reaction.

“This paper establishes the SalC [ketosynthase] domain as a true β-lactone synthase, the first of its kind, and provides a chemical and genetic blueprint for β-lactone biosynthesis,” Timothy Wencewicz, a natural product chemist at Washington University in St. Louis, said in an email. “This finding will aid genome mining efforts and chemoenzymatic methods for producing salinosporamide analogs as promising anticancer agents.”

The researchers hope to engineer SalC to produce analogs of salinosporamide A that can target different proteasomes, such as those in the human immune system or in parasites.