Discovery and biosynthesis of karnamycins as angiotensin-converting enzyme inhibitors

Angiotensin-converting enzyme inhibitors are widely used for treatment of hypertension and related diseases. Here, six karnamycins E1–E6 (1–6), which bear fully substituted hydroxypyridine and thiazole moieties, are characterized from the rare actinobacterium Lechevalieria rhizosphaerae NEAU-A2. Through isotopic labeling, genome mining, and enzymatic characterization, the programmed assembly of the fully substituted hydroxypyridine moiety in karnamycin is proposed to result from a hybrid polyketide synthase-nonribosomal peptide synthetase, two regiospecific flavoprotein hydroxylases, and a methyltransferase. Based on AlphaFold structural predictions, molecular docking, and site-directed mutagenesis, two pyridine hydroxylases deploy distinct active-site residues relative to other flavoprotein monooxygenases to achieve chemo- and regiospecific hydroxylation of the pyridine nucleus. Karnamycins show significant angiotensin-converting enzyme inhibitory activity with IC50 values ranging from 0.24 to 5.81 μM, suggesting potential use for treating hypertension and related diseases.