Tryptophan is the only amino acid with a side chain possessing two hydrocarbon (hydrogen–carbon) rings fused together to make a larger flat structure called an indole. As such, tryptophan ...

Figure 1: Biosynthetic gene cluster and structure of streptide. Figure 4: StrB catalyses Lys-to-Trp crosslink formation in StrA. Figure 5: Mechanistic model for Lys-to-Trp crosslink formation ...

Tryptophan, in comparison, has a complex structure and is comparatively rare in the protein code, like a Y or Z, leading scientists to theorize that it was one of the latest additions to the code.