Formation of a 5′-deoxyadenosyl radical. The methylmalonyl CoA mutase reaction begins with the homolytic cleavage of the bond joining Co³⁺ of coenzyme B₁₂ to a carbon atom of the ribose of the adenosine moiety of the enzyme. The cleavage generates a 5′-deoxyadenosyl radical and leads to the reduction of Co³⁺ to Co²⁺. The letter R represents the 5′-deoxyadenosyl component of the coenzyme, and the green oval represents the remainder of the coenzyme.