Endogenous F(1)F(o)-ATPase inhibitor limiting ATP depletion when the mitochondrial membrane potential falls below a threshold and the F(1)F(o)-ATP synthase starts hydrolyzing ATP to pump protons out of the mitochondrial matrix. Required to avoid the consumption of cellular ATP when the F(1)F(o)-ATP synthase enzyme acts as an ATP hydrolase (By similarity). Indirectly acts as a regulator of heme synthesis in erythroid tissues: regulates heme synthesis by modulating the mitochondrial pH and redox potential, allowing fech to efficiently catalyze the incorporation of iron into protoporphyrin IX to produce heme (PubMed:23135403). Homodimer; represents the active form and is present at a pH value below 6.5. Homotetramer; represents the inactive form and is present at a pH value above 7.0 (By similarity). Forms an alpha-helical dimer with monomers associated via an antiparallel alpha-helical coiled coil composed of residues 73-105, leaving each N-terminal inhibitory region (residues 22-51) accessible for interaction with an F1 catalytic domain. The inhibitory N-terminal region (residues 22-51) binds the alpha(ADP-bound)-beta(ADP-bound) (ATP5F1A-ATP5F1B) interface of F1-ATPase, and also contact the central gamma subunit (ATP5F1C). This dimeric state is favored by pH values below 7.0, and at higher values the dimers associate to form inactive homotetramer, where the inhibitory region is occluded, masking its inhibitory activity (By similarity). Defects in circulating erythroid cells. Embryos are anemic despite normal expression of the erythroid cell markers. The erythrocytes from embryos that survive to adult stage exhibit hypochromic, microcytic anemia. Histological analysis of adult haematopoietic tissues does not show morphological defects. Defects are due to changes in the mitochondrial pH-and consequently the redox potential-to change to a level that reduces [2Fe-2S] cluster-containing fech activity, thereby reducing heme synthesis, resulting in hypochromic anemia. Belongs to the ATPase inhibitor family.