Involved in siroheme-dependent heme b biosynthesis. Catalyzes the decarboxylation of siroheme into didecarboxysiroheme. 2 H(+) + siroheme = 12,18-didecarboxysiroheme + 2 CO2 Binds heme b (PubMed:24669201, PubMed:24865947). The redox state of the heme b modulates the activity of the enzyme (PubMed:24865947). Activity is stimulated by sodium dithionite (PubMed:24865947). Porphyrin-containing compound metabolism; protoheme biosynthesis. Forms an heterodimer composed of AhbA and AhbB. Belongs to the Ahb/Nir family.