Biosynthesis of Violacein: Structure and Function of L-Tryptophan Oxidase VioA from Chromobacterium Violaceumna

Violacein is a natural purple pigment of Chromobacterium violaceum with potential medical applications as antimicrobial, antiviral and anticancer drugs. The initial step of violacein biosynthesis is the oxidative conversion of L-tryptophan into the corresponding α-imine catalyzed by the flavoenzyme L-tryptophan oxidase (VioA). A substrate-related (3-(1H-indol-3-yl)-2-methylpropanoic acid, IAA) and a product-related (2-(1H-indol-3-ylmethyl)prop-2-enoic acid, IEA) competitive VioA inhibitor was synthesized for subsequent kinetic and X-ray crystallographic investigations. Structures of the binary VioA/FADH2 and of the ternary VioA/FADH2/IEA complex were resolved. VioA forms a `loosely associated` homodimer as indicated by small-angle X-ray scattering experiments. VioA belongs to the GR2 family of FAD-dependent oxidoreductases according to the structurally conserved cofactor binding domain. The substrate-binding domain of VioA is mainly responsible for the specific recognition of L-tryptophan. Other canonical amino acids were efficiently discriminated with a minor conversion of L-phenylalanine. Furthermore, 7-aza-tryptophan, 1-methyl-tryptophan, 5-methyl-tryptophan and 5-fluoro-tryptophan were efficient substrates of VioA. The ternary product-related VioA structure indicated involvement of protein domain movement during enzyme catalysis. Extensive structure-based mutagenesis in combination with enzyme kinetics (using L-tryptophan and substrate analogs) identified Arg$^{64}$, Lys$^{269}$ and Tyr$^{309}$ as key catalytic residues of VioA. An increased enzyme activity of protein variant H163A in the presence of L-phenylalanine indicated a functional role of the residues in substrate-binding. The combined structural and mutational analyses lead to the detailed understanding of VioA substrate recognition. Related strategies for the in vivo synthesis of novel violacein derivatives are discussed