Catalytic subunit of the adenylylsulfate reductase which catalyzes reversibly the reduction of adenosine 5'-phosphosulfate (APS) to sulfite and AMP during dissimilatory sulfate reduction. A + AMP + 2 H(+) + sulfite = adenosine 5'-phosphosulfate + AH2 Optimum pH is 7.4 for the adenylylsulfate reductase. Heterodimer composed of AprA and AprB (PubMed:19820092, PubMed:2158885). The heterodimers can dimerize to form heterotetramers (PubMed:2158885). Consists of three sections from the N- to the C-terminus: FAD-binding domain, capping domain and helical domain. The FAD-binding domain and the capping domain slightly overlap and form a channel that may transport the substrate. Belongs to the FAD-dependent oxidoreductase 2 family.