proviolacein

Proviolacein is a red pigment and bisindole derivative produced by Chromobacterium violaceum. It was historically proposed as a biosynthetic intermediate of violacein because oxygenation at the 2-position of the right-side indole nucleus was thought to lead to formation of the final product. However, subsequent studies demonstrated that proviolacein is an artifact and a "false intermediate" formed through the non-enzymatic spontaneous autooxidation and decarboxylation of the labile true intermediate, protoviolaceinic acid [97].

Proviolacein accumulates in metabolic systems or cell-free extracts when the final enzyme in the pathway, VioC, a flavin-dependent oxygenase, is absent or inactive because of a lack of the essential cofactor NADPH. Its identification as an artifact was critical for correctly elucidating the violacein biosynthetic pathway, establishing that the violacein scaffold is constructed through the sequential action of five enzymes, VioABCDE, followed by non-enzymatic oxidative decarboxylation [97].