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20-September-2008 09:29:09 - Cofactor biochemistry The bound heme cofactor of succinate dehydrogenase, an enzyme in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. The bound heme cofactor of succinate dehydrogenase, an enzyme in the mitochondrial electron transfer chain. The large semi-transparent sphere indicates the location of the iron ion. A cofactor is a non-protein chemical compound that is bound either tightly or loosely to an enzyme and is required for catalysis.1 They can be considered helper molecules/ions that assist in biochemical transformations. Certain substances such as water and various abundant ions may be bound tightly by enzymes, but are not considered to be cofactors since they are ubiquitous and rarely limiting. Some sources limit the use of the term cofactor to inorganic substances.23 Cofactors can be divided into two broad groups: coenzymes and prosthetic groups. Coenzymes are small organic non-protein molecules that carry chemical groups between enzymes. These molecules are not bound tightly by enzymes and are released as a normal part of the catalytic cycle. In contrast, prosthetic groups form a permanent part of the protein structure. Contents 1 Apoenzymes and holoenzymes 2 Metal ion cofactors 3 Cofactors and coenzymes 4 See also 5 References 6 External links Apoenzymes and holoenzymes An enzyme without a cofactor is referred to as an apoenzyme, and the completely active enzyme in addition to the cofactor is called a holoenzyme. Apoenzyme + cofactor = Holoenzyme Metal ion cofactors Further information: Metalloproteins Metal ions are common cofactors. The study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list commonly includes iron, manganese, cobalt, copper, zinc, selenium, and molybdenum.4 Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified.56 Iodine is also an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor.7 Calcium is another special case, in that it is required as a component of the human diet, and it is needed for the full activity of many enzymes: such as nitric oxide synthase, protein phosphatases or adenylate kinase, but calcium activates these enzymes in allosteric regulation, often binding to these enzymes in a complex with calmodulin.8 Calcium is therefore a cell signaling molecule, and not usually considered as a cofactor of the enzymes it regulates.9 Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter,10 tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus,11 and even cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii.1213 In many cases, the cofactor includes both an inorganic and organic component. One diverse set of examples are the haem proteins, which consists of a porphyrin ring coordinated to iron. Ion Examples of enzymes containing this ion Cupric Cytochrome oxidase Ferrous or Ferric Catalase Cytochromevia Heme Nitrogenase Hydrogenase Magnesium Glucose 6-phosphatase Hexokinase Manganese Arginase Molybdenum Nitrate reductase Nickel Urease Selenium Glutathione peroxidase Zinc Alcohol dehydrogenase Carbonic anhydrase DNA polymerase Cofactors and coenzymes Cofactors vary in their location and the tightness of their binding to the host enzyme. When bound tightly to the enzyme, cofactors are called prosthetic groups. Loosely-bound cofactors typically associate in a similar fashion to enzyme substrates. These are better described as coenzymes, which are organic substances that directly participate as substrates in an enzyme reaction. Vitamins can serve as precursors to coenzymes e.g. vitamins B1, B2, B6, B12, niacin, folic acid or as coenzymes themselves e.g. vitamin C. See also Enzyme catalysis Inorganic chemistry References ^ de Bolster, M.W.G. 1997. Glossary of Terms Used in Bioinorganic Chemistry: Cofactors. International Union of Pure and Applied Chemistry. Retrieved on 2007-10-30. ^ coenzymes and cofactors. Retrieved on 2007-11-17. ^ Enzyme Cofactors. Retrieved on 2007-11-17. ^ Aggett PJ 1985. Physiology and metabolism of essential trace elements: an outline. Clin Endocrinol Metab 14 3: 513-43. doi:10.1016/S0300-595X8580005-0. PMID 3905079. ^ Stearns DM 2000. Is chromium a trace essential metal?. Biofactors 11 3: 149-62. PMID 10875302. ^ Vincent JB 2000. The biochemistry of chromium. J. Nutr. 130 4: 715-8. PMID 10736319. ^ Cavalieri RR 1997. Iodine metabolism and thyroid physiology: current concepts. Thyroid 7 2: 177-81. PMID 9133680. ^ Clapham DE 2007. Calcium signaling. Cell 131 6: 1047-58. doi:10.1016/j.cell.2007.11.028. PMID 18083096. ^ Niki I, Yokokura H, Sudo T, Kato M, Hidaka H 1996. Ca2+ signaling and intracellular Ca2+ binding proteins. J. Biochem. 120 4: 685-98. PMID 8947828. ^ Eady RR 1988. The vanadium-containing nitrogenase of Azotobacter. Biofactors 1 2: 111-6. PMID 3076437. ^ Chan MK, Mukund S, Kletzin A, Adams MW, Rees DC 1995. Structure of a hyperthermophilic tungstopterin enzyme, aldehyde ferredoxin oxidoreductase. Science 267 5203: 1463-9. doi:10.1126/science.7878465. PMID 7878465. ^ Lane TW, Morel FM 2000. A biological function for cadmium in marine diatoms. Proc. Natl. Acad. Sci. U.S.A. 97 9: 4627-31. doi:10.1073/pnas.090091397. PMID 10781068. ^ Lane TW, Saito MA, George GN, Pickering IJ, Prince RC, Morel FM 2005. Biochemistry: a cadmium enzyme from a marine diatom. Nature 435 7038: 42. doi:10.1038/435042a. PMID 15875011. External links Cofactor at eMedicine Dictionary MeSH Enzyme+cofactors v d e Proteins: enzymes Topics Active site - Allosteric regulation - Binding site - Catalytically perfect enzyme - Coenzyme - Cofactor - Cooperativity - EC number Enzyme catalysis - Enzyme inhibitor - Enzyme kinetics - Lineweaver-Burk plot - Michaelis-Menten kinetics - List of enzymes Types EC1 Oxidoreductases/list - EC2 Transferases/list - EC3 Hydrolases/list - EC4 Lyases/list - EC5 Isomerases/list - EC6 Ligases/list v d e Enzyme cofactors Coenzymes vitamins: NAD+ B3 | NADP+ B3 | Coenzyme A B5 | THF / H4F B9, DHF, MTHF | Ascorbic acid C | Menaquinone K | Coenzyme F420 non-vitamins: ATP | CTP | SAM | PAPS | GSH | Coenzyme B | Coenzyme M | Coenzyme Q | Methanofuran | BH4 | H4MPT Organic prosthetic groups vitamins: TPP / ThDP B1 | FMN, FAD B2 | PLP / P5P B6 | Biotin B7 | Methylcobalamin, Cobamamide B12 non-vitamins: Haem / Heme | Lipoic acid | Molybdopterin | PQQ Metal prosthetic groups Ca2+ | Cu2+ | Fe2+, Fe3+ | Mg2+ | Mn2+ | Mo | Ni2+ | Se | Zn2+ Major families of biochemicals Saccharides | Carbohydrates | Glycosides | | Amino acids | Peptides | Proteins | Glycoproteins | | Lipids | Terpenes | Steroids | Carotenoids Alkaloids | Nucleobases | Nucleic acids | | Enzyme cofactors | Flavonoids | Polyketides | Tetrapyrroles Retrieved from http://en..org/wiki/Cofactor_biochemistry Categories: Enzymes | Cofactors Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages БългарÑ?ки Català Česky Español Ù?ارسی Français עברית Lietuvių 日本語 ‪Norsk bokmÃ¥l‬ Polski ä¸æ–‡ This page was last modified on 10 July 2008, at 10:37
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