Together with Xaut_5043, catalyzes the reversible isomerization between pivalyl-CoA and isovaleryl-CoA, using radical chemistry. Does not exhibit isobutyryl-CoA mutase (ICM) activity. 3-methylbutanoyl-CoA = 2,2-dimethylpropanoyl-CoA kcat is 14 min(-1) (measured in the presence of the small and large subunits). Monomer in the absence of the PCM large subunit. Weakly interacts with the PCM large subunit; an alpha(2)beta(2) stoichiometry seems to represent the active state of the enzyme. This enzyme and the derivative PCM-F, a variant in which the alpha and beta subunits are fused into a single polypeptide via a short 11-amino acid linker, have potential applications in bioremediation of pivalic acid found in sludge, in stereospecific synthesis of C5 carboxylic acids and alcohols, and in the production of potential commodity and specialty chemicals. Belongs to the acyl-CoA mutase small subunit family.