Heme

Heme is an iron-containing molecular group in which an iron ion is held by a porphyrin ring. It enables proteins to bind gases and participate in electron transfer and chemical reactions.

Porphyrin: A large, ring-shaped molecule made of four linked pyrrole units that can bind a metal ion. Its nitrogen atoms hold heme’s iron at the center of the ring.
Heme b: A common heme variant consisting of iron protoporphyrin IX, found in hemoglobin and many other proteins. It is the best-known heme structure and a standard reference for comparing variants.
Hemoglobin: A red blood cell protein that transports oxygen and helps carry carbon dioxide in vertebrates. Its four heme groups bind oxygen for transport through the blood.
Heme biosynthesis: The multistep cellular pathway that produces heme from simple metabolic precursors. It assembles the porphyrin ring and inserts iron to make heme.
Chlorophyll: A family of magnesium-containing pigments that captures light energy in photosynthesis. Chlorophyll shares a related ring architecture but uses magnesium rather than iron.
Iron: A metallic chemical element that occurs in biological molecules in several oxidation states. Heme’s iron changes its binding and electronic behavior with oxidation state.
Protoporphyrin IX: A porphyrin molecule that serves as the organic framework of heme b. It provides the ring structure that binds heme b’s iron.
Myoglobin: An oxygen-binding protein in muscle that stores and facilitates oxygen diffusion. Its single heme group binds oxygen in muscle tissue.
Ferrochelatase: An enzyme that inserts ferrous iron into protoporphyrin IX in the final step of heme synthesis. It performs the last chemical step that turns the ring precursor into heme.
Cobalamin: A family of cobalt-containing compounds that includes vitamin B12 and its coenzymes. Cobalamin is another metal-bearing biological cofactor, but its central metal is cobalt.

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