Component of 1,2-phenylacetyl-CoA epoxidase multicomponent enzyme system which catalyzes the reduction of phenylacetyl-CoA (PA-CoA) to form 1,2-epoxyphenylacetyl-CoA. The subunit C may be essential for structural integrity of the alpha subunit. Aromatic compound metabolism; phenylacetate degradation. Forms a stable heterotetramer (dimer of heterodimers) with PaaA and a stable heterodimer with PaaB. Activated by cAMP receptor protein (CRP), integration host factor (IHF) and by phenylacetyl-coenzyme A (PA-CoA) that prevents PaaX from binding its target sequences. Inhibited by PaaX.