The heme in HtaA-CR2 is coordinated by an axial tyrosine. Mutation of the axial ligand and a potential hydrogen-bonding partner His reduced heme binding. Reconstitution of the protein after full unfolding significantly altered the heme binding pocket. HtaA-CR2 is very stable toward heat and treatment with guanidinium hydrochloride. Bound heme plays an essential role in the stability of HtaA-CR2.