zum Directory-modus

Reaktionsmechanismus der SHCHC-Synthase-Reaktion

Abb.1
Reaktion von Isochorisminsäure und 2-Oxo-glutarsäure, katalysiert durch die SHCHC-Synthase

Die SHCHC-Synthase katalysiert eine zweistufige Reaktion: Zunächst wird 2-Oxo-glutarsäure unter CO2-Abspaltung in den den Succinyl-thiamin-diphosphat-ester überführt. Thiamin-diphosphat fungiert als aktivierende Gruppe, um im 2. Schritt die C-C-Knüpfung zwischen Isochorisminsäure und dem Succinyl-Rest zu ermöglichen. Im letzten Schritt wird Brenztraubensäure (Pyruvat) eliminiert und damit ein stabiles System aus drei konjugierten Doppelbindungen erzeugt.

Literatur

Sieminska, E. A. L.; Macova, A.; Palmer, D. R. J.; Sanders, D. A. R. (2005): Crystallization and preliminary X-ray analysis of (1R,6R)-2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase (MenD) from Escherichia coli. In: Acta Crystallogr. Sect. F Struct. Biol. Cryst. Commun.. 61 (Pt 5) , 489-492
Titel des Artikels
Crystallization and preliminary X-ray analysis of (1R,6R)-2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase (MenD) from Escherichia coli
Abstract
(1R,6R)-2-Succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase, also called MenD, participates in the menaquinone (vitamin K2) biosynthetic pathway. The enzyme is a part of the superfamily of ThDP-dependent enzymes; however, it is the only enzyme known to catalyze a Stetter-like 1,4-addition of a ThDP adduct to the β-carbon of an unsaturated carboxylate. This is the first reported crystallization of the apoenzyme and holoenzyme forms of MenD. The apoenzyme crystals were obtained by sitting-drop vapour diffusion with 70% MPD. However, the crystals were too small to collect diffraction data and a search for better conditions was not successful. Single crystals of the holoenzyme with ThDP and Mn2+ as cofactors were obtained by the hanging-drop vapour-diffusion method with 35% ethylene glycol as precipitant. Diffraction data were collected on a cryocooled crystal to a resolution of 2.0 Å at BioCARS, Advanced Photon Source (APS), Chicago, IL, USA. The crystal was found to belong to space group P212121, with unit-cell parameters a = 106.86, b = 143.06, c = 156.85 Å, α = β = γ = 90°.