Assay and purification of the adenosylcobalamin‐dependent 2‐methyleneglutarate mutase from Clostridium barkeri

Abstract

A continuous spectrophotometric assay was developed for the adenosylcobalamin–dependent 2‐methyleneglutarate mutase from Clostridium barkeri. Thereby the product (R)‐3‐methylitaconate is converted by the Δ‐isomerase from the same organism to 2,3‐dimethylmaleate which absorbs at 240 nm, much higher than both parent compounds (Δɛ= 3.7 mM⁻¹· cm⁻¹). In addition a discontinuous assay using the facile formation of 2,3‐dimethylmaleic anhydride in aqueous solution at pH 0–1 (Δɛ= 4.0 mM⁻¹· cm⁻¹ at 256 nm) was established. The mutase and the isomerase were purified together by chromatography on quaternary‐amine–Sepharose (Q‐Sepharose) and on cyanocobalamin‐agarose. The enzymes were separated and obtained in homogenous forms by preparative PAGE in non‐denaturing buffer. Both enzymes appear to be homotetramers with subunits of 70 kDa (mutase) and 50 kDa (isomerase). The equilibrium constants for both reactions were determined at I= 0.1 M and 25°C: K_1,app= [(R)‐3‐methylitaconate] · [2‐methyleneglutarate]⁻¹= 0.26 ± 0.04, K_2,app= [2,3‐dimethylmaleate] · [(R)‐3‐methylitaconate]⁻¹= 7.40 ± 0.21