Interpretation of electron spin resonance spectra due to some B12-dependent enzyme reactions

Abstract

Previous studies by a number of authors have shown that during a number of B₁₂-dependent enzyme reactions, e.g., catalysis of amino-2,1-propanol to propionaldehyde in the presence of ethanolamine ammonia lyase as reported by Babior et al., a very characteristic e.s.r. spectrum is observed. It consists of a broad line at g∼ 2.3 and a pair of lines in the intensity ratio of approximately 2:1, split by 7–14 mT, centred about g= 2. Schepler et al. have given a partial explanation for this and four other similar results, basing their interpretation on isotropic exchange coupling between a radical and the cobalt (II) from B₁₂. In this paper we have interpreted the results on the basis of a coupling model which includes both isotropic exchange and dipolar interactions, and which represents a logical extension of previous work of two of the the authors on dissimilar ion coupling. The presence of dipolar coupling brings about an important improvement in the computer simulated lineshapes for the “radical” part of the spectrum, especially for ethanolamine ammonia lyase. The spectra are also sensitive to the sign of the exchange coupling when second order effects are included in the line positions and, more particularly, the intensities. From comparison between computed and experimental spectra, it is concluded that a lower limit for the cobalt (II)-radical separation of 10 Å exists in all cases and that it is in the range 10–12 Å for ethanolamine ammonia lyase. Clearly this new information regarding distance must be borne in mind in the development of models for the reactions of such enzyme–substrate–coenzyme systems.