Involved in siroheme-dependent heme b biosynthesis. Catalyzes the decarboxylation of siroheme into didecarboxysiroheme. 2 H(+) + siroheme = 12,18-didecarboxysiroheme + 2 CO2 Porphyrin-containing compound metabolism; protoheme biosynthesis. Forms an heterodimer composed of AhbA and AhbB. Belongs to the Ahb/Nir family.