Ferrochelatase (overview)

Reverse activity of FECH is triggered by fatty acids

Previous studies (1,5,21) have found that FECH activity increases owing to various lipids including fatty acids and phospholipids. To determine the effects of fatty acids on iron-removal reverse activity, we added sodium palmitate to the reaction mixture.

Upon increasing the concentration of sodium palmitate to 100 mg/ml, the forward and reverse activities increased concentration dependently, and the rates of the forward and reverse reactions increased 2.5-fold and 2.0-fold in the presence of 100 mg/ml sodium palmitate, respectively.

Other fatty acids such as stearic acid and oleic acid showed activities similar to those of palmitic acid.

At 100 mg/ml, phosphatidylcholine, the rate of forward activity increased while that of the reverse reaction decreased. Lysophosphatidylcholine slightly activated forward activity, but inhibited reverse activity. Sphingomyelin and lysophosphatidic acid markedly inhibited reverse but not forward activity. (R1)

Some drugs can trigger reverse reaction

The enzyme ferrochelatase (FECH) catalyzes the conversion of protoporphyrin IX into heme and was recently found to be an off-target of the BRAF inhibitor Vemurafenib, likely explaining the phototoxicity associated with this drug in melanoma patients. This raises the question of whether FECH binding is a more general feature of kinase inhibitors. (R3)

Localization inside and outside of mitochondria

To clarify the role of the reverse reaction of ferrochelatase in cells, mouse liver mitochondria were fractionated to examine the localization of ferrochelatase, and it was found that the enzyme localizes not only to the inner membrane, but also to the outer membrane.

Observations by immunoelectron microscopy confirmed the dual localization of ferrochelatase in ferrochelatase-expressing human embryonic kidney cells and mouse liver mitochondria.

The conventional (zinc-insertion) activities of the enzyme in the inner and outer membranes were similar, whereas the iron-removal activity was high in the outer membrane.

2D gel analysis revealed that two types of the enzyme with different isoelectric points were present in mitochondria, and the acidic form, which was enriched in the outer membrane, was found to be phosphorylated. Mutation of human ferrochelatase showed that serine residues at positions 130 and 303 were phosphorylated, and serine at position 130 may be involved in the balance of the reversible catalytic reaction. (R2)

Open questions

• Which enzyme phosphorylates FECH and what triggers it?