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Heme (American English), or haem (Commonwealth English, both pronounced /hi:m/ HEEM), is a ring-shaped iron-containing molecular component of hemoglobin, which is necessary to bind oxygen in the bloodstream. It is composed of four pyrrole rings with 2 vinyl and 2 propionic acid side chains. [1] Heme is biosynthesized in both the bone marrow and ...
Heme is a major source of dietary iron in humans and other mammals, and its synthesis in the body is well understood, but heme pathways are not as well understood. It is likely that heme is tightly regulated for two reasons: the toxic nature of iron in cells, and the lack of a regulated excretory system for excess iron.
In enzymology, a heme-transporting ATPase (EC 3.6.3.41) is an enzyme that catalyzes the chemical reaction ATP + H 2 O + hemein ⇌ {\displaystyle \rightleftharpoons } ADP + phosphate + hemeout The 3 substrates of this enzyme are ATP , H 2 O , and heme , whereas its 3 products are ADP , phosphate , and heme .
Haem or Heme carrier protein 1 (HCP1) was originally identified as mediating heme-Fe transport although it later emerged that it was the SLC46A1 folate transporter. [2] [3]HCP1 is a protein found in the small intestine that plays a role in the absorption of dietary heme, a form of iron that is only found in animal products.
Heme B or haem B (also known as protoheme IX) is the most abundant heme. [1] Hemoglobin and myoglobin are examples of oxygen transport proteins that contain heme B. The peroxidase family of enzymes also contain heme B. The COX-1 and COX-2 enzymes (cyclooxygenase) of recent fame, also contain heme B at one of two active sites.
The number of heme C units bound to a holoprotein is highly variable. For vertebrate cells one heme C per protein is the rule but for bacteria this number is often 2, 4, 5, 6 or even 16 heme C groups per holoprotein. It is generally agreed the number and arrangement of heme C groups are related and even required for proper holoprotein function.
Heme is bound to the protein either covalently or noncovalently or both. [2] The heme consists of iron cation bound at the center of the conjugate base of the porphyrin, as well as other ligands attached to the "axial sites" of the iron. The porphyrin ring is a planar dianionic, tetradentate ligand.
Summary of heme B biosynthesis—note that some reactions occur in the cytoplasm and some in the mitochondrion (yellow) Ferrochelatase catalyzes the insertion of ferrous iron into protoporphyrin IX in the heme biosynthesis pathway to form heme B. The enzyme is localized to the matrix-facing side of the inner mitochondrial membrane.