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 A is the catalytic subunit involved in the incorporation of one atom of molecular oxygen into phenylacetyl-CoA. H(+) + NADPH + O2 + phenylacetyl-CoA = 2-(1,2-epoxy-1,2-dihydrophenyl)acetyl-CoA + H2O + NADP(+) Aromatic compound metabolism; phenylacetate degradation. Forms a stable heterotetramer (dimer of heterodimers) with PaaC. 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. Truncated N-terminus.