English Views: 10 Author: Site Editor Publish Time: 2023-02-02 Origin: Site
Heme is the precursor to hemoglobin,which is required to bind oxygen in the blood.Heme is biosynthesized in the bone marrow and liver.In biochemical terms, heme is a coordination complex "coordinated by an iron ion to porphyrin as a tetradentate ligand, and one or two axial ligands."Loosely defined , many descriptions omit the axial ligand.Among the metalloporphyrins deployed by metalloproteins as prosthetic groups, heme is the most widely used one, which defines a family of proteins called heme proteins.Heme is most commonly considered a component of hemoglobin (the red pigment in blood), but is also present in many other biologically important hemoglobins such as myoglobin, cytochromes, catalase, heme peroxidation enzymes and endothelial nitric oxide synthase.
The word haem is derived from the Greek αἷμα haima, meaning "blood"".
Function 
Hemoglobin has a variety of biological functions, including diatomic gas transport, chemical catalysis, diatomic gas detection, and electron transfer. Heme iron acts as an electron source or electron sink during electron transfer or redox chemistry. In the peroxidase reaction, the porphyrin molecule also acts as an electron source capable of delocalizing the free radical electrons in the conjugated ring.In the transport or detection of diatomic gases, the gas is bound to heme iron.In the detection of diatomic gases, the binding of gas ligands to heme iron induces conformational changes in surrounding proteins.Typically, diatomic gases are only bound to reduced heme, such as ferrous Fe(II), whereas most peroxidases cycle between Fe(III) and Fe(IV), heme proteins involved in mitochondrial redox, Redox, cycling between Fe(II) and Fe(II) iron (III).The original evolutionary function of the heme protein was postulated to be electron transfer in the original sulfur-based photosynthetic pathway in primitive cyanobacteria before the advent of molecular oxygen.Heme proteins achieve their remarkable functional diversity by altering the environment of the heme macrocycle within the protein matrix.For example, hemoglobin's ability to efficiently transport oxygen to tissues is due to specific amino acid residues located near the heme molecule.Hemoglobin reversibly binds to oxygen in the lungs when the pH is high and the carbon dioxide concentration is low.When conditions are reversed (low pH and high carbon dioxide concentration), hemoglobin releases oxygen into the tissues.This phenomenon, which shows that the oxygen-binding affinity of hemoglobin is inversely proportional to acidity and carbon dioxide concentration, is known as the Bohr effect.The molecular mechanism behind this effect is the spatial structure of the globin chain; histidine residues located near the heme group are positively charged under acidic conditions (caused by dissolved carbon dioxide, etc. in working muscles), from the heme prime group releases oxygen.
Other lineages
The carbon numbering system for porphyrins below is an older numbering system used by biochemists, rather than the 1-24 numbering system recommended by IUPAC.
Heme l is a derivative of heme B that is covalently attached to the proteins of lactoperoxidase, eosinophil peroxidase,and thyroid peroxidase. Addition of peroxide to glutamyl-375 and aspartyl-225 of lactoperoxidase forms ester bonds between these amino acid residues and the heme 1- and 5-methyl groups, respectively.Similar ester linkages to these two methyl groups are thought to form in eosinophils and thyroid peroxidase.Heme l is an important property of animal peroxidases; plant peroxidases contain heme B.Lactoperoxidase and eosinophil peroxidase are protective enzymes responsible for destroying invading bacteria and viruses.Thyroid peroxidase is an enzyme that catalyzes the biosynthesis of important thyroid hormones.Because lactoperoxidase destroys invading organisms in the lungs and excreta, it is considered an important protective enzyme.
Heme m is a derivative of heme B covalently bound to the active site of myeloperoxidase.Heme m contains two ester bonds on the heme 1- and 5-methyl groups and is also present in heme l of other mammalian peroxidases such as lactoperoxidase and eosinophil peroxidase enzyme. In addition, a unique sulfonamide ionic bond is formed between the sulfur of the methionyl amino acid residue and the heme 2-vinyl group, giving the enzyme the ability to readily oxidize chloride and bromide ions to hypochlorite and hypobromous acid Salt's unique ability.Myeloperoxidase is present in mammalian neutrophils and is responsible for the destruction of invading bacterial and viral agents.It may have been "wrongly" synthesized hypobromite.Both hypochlorite and hypobromite are very reactive substances responsible for the production of halogenated nucleosides, which are mutagenic compounds.
Heme D is another derivative of heme B,but the propionic acid side chain at carbon 6 is also hydroxylated to form γ-spironolactone.Ring III is also hydroxylated at position 5 to form a new lactone group in the trans conformation.Heme D is the site where various bacteria reduce oxygen to water under low oxygen tension.
Heme S is related to heme B because there is a formal group at position 2 in place of the 2-vinyl group.Heme S is present in the hemoglobin of several marine worms. The correct structures of heme B and heme S were first elucidated by German chemist Hans Fischer.The names of cytochromes usually (but not always) reflect the kind of heme they contain: cytochrome a contains heme A,cytochrome c contains heme C, and so on.
