Laboratory of Biochemistry, Department of Chemistry and Biotechnology, Faculty of Science and Technology, National University Corporation Kochi University, Kochi, 780-8520, Japan.
J Mol Evol. 2023 Dec;91(6):912-921. doi: 10.1007/s00239-023-10144-x. Epub 2023 Nov 26.
Tryptophan indole-lyase (TIL), a pyridoxal-5-phosphate-dependent enzyme, catalyzes the hydrolysis of L-tryptophan (L-Trp) to indole and ammonium pyruvate. TIL is widely distributed among bacteria and bacterial TILs consist of a D2-symmetric homotetramer. On the other hand, TIL genes are also present in several metazoans. Cephalopods have two TILs, TILα and TILβ, which are believed to be derived from a gene duplication that occurred before octopus and squid diverged. However, both TILα and TILβ individually contain disruptive amino acid substitutions for TIL activity, and neither was active when expressed alone. When TILα and TILβ were coexpressed, however, they formed a heterotetramer that exhibited low TIL activity. The loss of TIL activity of the heterotetramer following site-directed mutagenesis strongly suggests that the active heterotetramer contains the TILα/TILβ heterodimer. Metazoan TILs generally have lower k values for L-Trp than those of bacterial TILs, but such low TIL activity may be rather suitable for metazoan physiology, where L-Trp is in high demand. Therefore, reduced activity may have been a less likely target for purifying selection in the evolution of cephalopod TILs. Meanwhile, the unusual evolution of cephalopod TILs may indicate the difficulty of post-gene duplication evolution of enzymes with catalytic sites contributed by multiple subunits, such as TIL.
色氨酸吲哚-裂解酶（TIL）是一种依赖吡哆醛-5-磷酸的酶，催化 L-色氨酸（L-Trp）水解为吲哚和丙酮酸铵。TIL 在细菌中广泛分布，细菌 TIL 由 D2 对称的四聚体组成。另一方面，TIL 基因也存在于几种后生动物中。头足类动物有两种 TIL，TILα 和 TILβ，它们被认为是在章鱼和鱿鱼分化之前发生的基因复制的结果。然而，TILα 和 TILβ 各自都含有对 TIL 活性有破坏作用的氨基酸取代，单独表达时都没有活性。然而，当 TILα 和 TILβ 共表达时，它们形成了一个具有低 TIL 活性的杂四聚体。通过定点突变使杂四聚体失去 TIL 活性强烈表明，活性杂四聚体包含 TILα/TILβ 异二聚体。后生动物 TIL 对 L-Trp 的 k 值通常比细菌 TIL 低，但这种低 TIL 活性可能更适合后生动物生理学，因为 L-Trp 的需求量很大。因此，在头足类动物 TIL 的进化过程中，活性降低可能不是纯化选择的一个不太可能的目标。同时，头足类动物 TIL 的异常进化可能表明，对于具有由多个亚基贡献的催化位点的酶，如 TIL，在后基因复制进化方面存在困难。
