X-Ray snapshots of a pyridoxal enzyme: a catalytic mechanism involving concerted [1,5]-hydrogen sigmatropy in methionine γ-lyase

• Published in: Scientific reports 2017-07, Vol.7 (1), p.4874-10, Article 4874
• Main Authors: Sato, Dan, Shiba, Tomoo, Karaki, Tsuyoshi, Yamagata, Wataru, Nozaki, Tomoyoshi, Nakazawa, Takashi, Harada, Shigeharu
• Format: Article
• Language: English
• Subjects: 631/45/173; 631/535/1266; Amino acids; Catalysis; Conformation; Enzymes; Humanities and Social Sciences; Methanethiol; Methionine; Migration; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-017-05032-6

Pyridoxal 5′-phosphate (PLP)-enzymes are essentially involved in amino acid and amine metabolism of a wide variety of organisms. Despite their extensive biochemical studies, there are little evidence and structural data to comprehensively elaborate the catalytic mechanism. We obtained X-ray snapshots of l-methionine γ-lyase from Entamoeba histolytica (EhMGL), a PLP-enzyme catalyzing the γ-elimination reaction of methionine. Here, we suggest a catalytic mechanism of EhMGL by using the X-ray snapshots covering all stages of this multistep catalysis reaction. Initial formation of a Michaelis complex is followed by the migration of double bond from the C4′=Nα–Cα moiety in an intermediate PLP-methionine imine to C4′–Nα=Cα in pyridoxamine 5′-phosphate (PMP)-α,β-dehydromethionine imine without intervention of a putative quinonoid intermediate. The enzyme can facilitate the subsequent γ-elimination of methanethiol by the possible general acid-base catalysis of Tyr108 for the E1cB mechanism, enabling to form the ene-imine C4′–Nα=Cα–Cβ=Cγ structure with the s- cis conformation, which is prerequisite for the non-enzymatic symmetry-allowed suprafacial [1,5]-hydrogen shift to complete the catalytic cycle by releasing α-ketobutyrate. The mechanism based on the X-ray snapshots is consistent with the reactivity of MGL toward methionine analogues. The generality of such a mechanism involving non-enzymatic concerted reaction in other PLP enzymes is discussed.