![]() ![]() When the situation is reversed (low pH and high carbon dioxide concentrations), hemoglobin will release oxygen into the tissues. Hemoglobin reversibly binds to oxygen in the lungs when the pH is high, and the carbon dioxide concentration is low. For example, the ability of hemoglobin to effectively deliver oxygen to tissues is due to specific amino acid residues located near the heme molecule. Hemoproteins achieve their remarkable functional diversity by modifying the environment of the heme macrocycle within the protein matrix. ![]() It has been speculated that the original evolutionary function of hemoproteins was electron transfer in primitive sulfur-based photosynthesis pathways in ancestral cyanobacteria-like organisms before the appearance of molecular oxygen. In general, diatomic gases only bind to the reduced heme, as ferrous Fe(II) while most peroxidases cycle between Fe(III) and Fe(IV) and hemeproteins involved in mitochondrial redox, oxidation-reduction, cycle between Fe(II) and Fe(III). During the detection of diatomic gases, the binding of the gas ligand to the heme iron induces conformational changes in the surrounding protein. In the transportation or detection of diatomic gases, the gas binds to the heme iron. In peroxidase reactions, the porphyrin molecule also serves as an electron source, being able to delocalize radical electrons in the conjugated ring. The heme iron serves as a source or sink of electrons during electron transfer or redox chemistry. ![]() Hemoproteins have diverse biological functions including the transportation of diatomic gases, chemical catalysis, diatomic gas detection, and electron transfer. The large semi-transparent sphere indicates the location of the iron ion. The heme group of succinate dehydrogenase bound to histidine, an electron carrier in the mitochondrial electron transfer chain. The word haem is derived from Greek αἷμα haima meaning "blood". Hemes are most commonly recognized as components of hemoglobin, the red pigment in blood, but are also found in a number of other biologically important hemoproteins such as myoglobin, cytochromes, catalases, heme peroxidase, and endothelial nitric oxide synthase. Among the metalloporphyrins deployed by metalloproteins as prosthetic groups, heme is one of the most widely used and defines a family of proteins known as hemoproteins. In biochemical terms, heme is a coordination complex "consisting of an iron ion coordinated to a porphyrin acting as a tetradentate ligand, and to one or two axial ligands." The definition is loose, and many depictions omit the axial ligands. Heme is biosynthesized in both the bone marrow and the liver. Heme ( American English), or haem ( Commonwealth English, both pronounced / hi:m/ HEEM), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Binding of oxygen to a heme prosthetic group. ![]()
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