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see coenzymecoenzyme
, any one of a group of relatively small organic molecules required for the catalytic function of certain enzymes. A coenzyme may either be attached by covalent bonds to a particular enzyme or exist freely in solution, but in either case it participates intimately in
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the nonprotein component (the so-called prosthetic group) and the coloring matter of hemoglobin.

Chemically, heme is a compound of protoporphyrin and divalent iron. In the vertebrate body heme is synthesized from simpler nitrogen compounds (glycine and succinate) and from ferritin, a reserve iron-protein complex present in the spleen, liver, and bone marrow. The heme isolated from the blood of various vertebrates always has the same structure:

Free heme readily oxidizes in air to hematin, in which the iron atom is trivalent. Many years of research on the structure of heme were rewarded by the synthesis in 1929 by H. Fischer of hemin, the hydrochloride of heme.


C34H32O4N4Fe An iron-protoporphyrin complex associated with each polypeptide unit of hemoglobin.
References in periodicals archive ?
Gene IRE Function location L-Ferritin 5 Iron storage H-Ferritin 5 Iron storage Erythroid 5-ALA synthase 5 Haem synthesis Nramp2 3 Iron importer FPN (IREG I) 5 Iron exporter DMT1 3 Iron transporter TFR1 ** 3 TF binding and intracellular transport * Adopted from Sargent et al (83); ** Contains multiple iron responsive elements (IREs).
Haem iron is a digestion product of haemoglobin or myoglobin.
Although irrelevant in this case, if there is the potential for significant blood loss, circulating haemoglobin levels should be rigidly maintained with transfusions, avoiding the risk of intra-operative stimulation of haem synthesis precipitating a porphyric crisis (1).
Peroxynitrite induces haem oxygenase-I in vascular endothelial cells: a link to apoptosis.
Haem iron, which is the form that exists in red meats and offal, is relatively well absorbed (20-30 per cent).
The porphyrias are a heterogeneous group of inherited metabolic disorders associated with defects in haem biosynthesis.
Haem iron is readily absorbed by the body whereas non-haem iron is less soluble in the gut and therefore less is absorbed.
Physiologically haem and other porphyrine iron-rich compounds derived only from meat are absorbed by humans in preference to ionic forms of iron, whereas herbivorous animals cannot absorb these haem complexes and rely on absorption of ionic iron.
Lead poisoning, haem synthesis and 5-aminolaevulinic acid dehydratase.
Other forms of iron supplements, such as ferrous fumarate, ferrous gluconate, haem iron concentrate and iron glycine amino acid chelate are readily absorbed and less likely to cause side effects.
Incorporating comments from nutritionist Juliette Kellow, alongside attractive beef-based recipes, the features highlight beef's ability to boost energy Levels thanks to its high haem iron content--something which is of particular value to women who may be dieting--and the fact that it contains essential vitamins and minerals that are good for the skin as well as antioxidants which boost the immune system.
Vegetable sources of iron include prunes and spring greens, but they contain a form that is less well absorbed than the haem iron found in animal products, so don't over-boil vegetables as this decreases their iron availability.