Metazoan tryptophan indole-lyase: Are they still active?

Tryptophan indole-lyase (TIL), also known as tryptophanase, is a pyridoxal-5 '-phosphate dependent bacterial enzyme that catalyzes the reversible hydrolytic cleavage of L-tryptophan (L-Trp) to indole and ammonium py-ruvate. TIL is also found in some metazoans, and they may have been acquired by horizontal gene transfer. In this study, two metazoans, Nematostella vectensis (starlet sea anemone) and Bradysia coprophila (fungus gnat) TILs were bacterially expressed and characterized. The kcat values of metazoan TILs were low, < 1/200 of the kcat of Escherichia coli TIL. By contrast, metazoan TILs showed lower Km values than the TILs of common bacteria, indicating that their affinity for L-Trp is higher than that of bacterial TILs. Analysis of a series of chimeric en-zymes based on B. coprophila and bacterial TILs revealed that the low Km value of B. coprophila TIL is not accidental due to the substitution of a single residue, but is due to the cooperative effect of multiple residues. This suggests that high affinity for L-Trp was positively selected during the molecular evolution of metazoan TIL. This is the first report that metazoan TILs have low but obvious activity.

Automated summary

This study characterized the Tryptophan indole-lyase (TIL) enzymes from two metazoans, Nematostella vectensis and Bradysia coprophila, and found that they have low but significant activity with higher affinity for L-Trp compared to bacterial TILs. Analysis of chimeric enzymes revealed that the high affinity of Bradysia coprophila TIL is not accidental but due to the cooperative effect of multiple residues.