Polymerization nonenzymatic

Lenthionine (53) is one of flavoring substances in Lentinus edodes (Berk.) Sing (67CPB988). It was proposed that 53 is produced by polymerization of the parent dithiirane (54), which was formed from lentinic acid (55) by enzymatic and then nonenzymatic processes (76TL3129 77MI1). 

Dopachrome also undergoes a nonenzymatic reaction to form dihidroxyindole (DHI), the precursor of DHI-eumelanins. For the formation of phaeomelanins, dopaquinone is first transformed in cysteinil-DOPA and then in cysteinyl-dopaquinone which suffers a nonenzymatic polymerization. The polymerization of monomers and the association of melanins with proteins is not yet completely elucidated and may involve other intermediates. ... 

Fmctose is a highly reactive molecule. When stored in solution at high temperatures, fmctose not only browns rapidly but also polymerizes to dianhydrides [38837-99-9], [50692-21-2], [50692-22-3], [50692-23 4], [50692-24-5]. Fmctose also reacts rapidly with amines and proteins in the nonenzymatic or Maillard browning reaction (5). This is a valued attribute in baked food products where cmst color is important. An appreciation of these properties allows the judicious choice of conditions under which fmctose can be used successfully in food applications. 

Fibrin polymerization is initiated by the enzymatic cleavage of the fibrinopeptides, converting fibrinogen to fibrin monomer (Fig. 1). Then, several nonenzymatic reactions yield an orderly sequence of macromolec-ular assembly steps. Several other plasma proteins bind specifically to the resulting fibrin network. The clot is stabilized by covalent ligation or crosslinking of specific amino acids by a transglutaminase, Factor XHIa. 

Motai, H. Nonenzymatic oxidative browning Polymerization of melanoidins. Kagaku To Seibutsu 1975, 13, 292-4. 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Enzymes present in melanosomes synthesize two types of melanin, eumelanin and pheomelanin. Figure 2 illustrates the proposed biosynthetic pathways of eumelanin and pheomelanin. The synthesis of eumelanin requires tyrosinase, an enzyme located in melanosomes. Tyrosinase catalyzes the conversion of tyrosine to dopa, which is further oxidized to dopaquinone. Through a series of enzymatic and nonenzymatic reactions, dopaquinone is converted to 5,6-indole quinone and then to eumelanin, a polymer. This polymer is always found attached to proteins in mammalian tissues, although the specific linkage site between proteins and polymers is unknown. Polymers affixed to protein constitute eumelanin, but the exact molecular structure of this complex has not been elucidated. Pheomelanin is also synthesized in melanosomes. The initial steps in pheomelanin synthesis parallel eumelanin synthesis, since tyrosinase and tyrosine are required to produce dopaquinone. Dopaquinone then combines with cysteine to form cysteinyldopa, which is oxidized and polymerized to pheomelanin. The exact molecular structure of pheomelanin also has not been determined. 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

The oxidative polymerization of polyphenols in soils is regttrded as one of the main processes in the formation of humic substances (Scheffer and Ulrich, 1960 Felbeck, 1965 Kononova, 1966 Hurst and Burges, 1967 Martin and Haider, 1971 Schnitzer and Khan, 1972 Flaig et al., 1975 Wang et ah, 1986). The polymerization causes a darkening of polyphenols that can be accelerated nonenzymatically as well as enzymatically (Scheffer et al., 1959 Scheffer and Ulrich, 1960 Kyuma and Kawaguchi, 1964 Kumada and Kato, 1970 Haider et al., 1975 Wang et ah, 1978 Kumada, 1981). 

Tyrosinase or polyphenol oxidase (EC 1.14.18.1) is a bifunctional, copper-containing enzyme widely distributed on the phylogenetic tree. This enzyme uses molecular oxygen to catalyze the oxidation of monophenols to their corresponding o-diphenols (cresolase activity) as well as their subsequent oxidation to o-quinones (catecholase activity). The o-quinones thus generated polymerize to form melanin, through a series of subsequent enzymatic and nonenzymatic reactions [1-3]. 

While most enzymatically-catalyzed polymerizations in supercritical C02 have been conducted by solution or precipitation polymerization mechanisms, the ability to perform emulsion-like polymerization has been extensively reported for nonenzymatic initiation [61-68]. 

Zamora, R., Alaiz, M., and Hidalgo, F.J. 2000. Contribution of pyrrole formation and polymerization to the nonenzymatic browning produced by amino-carbonyl reaction. Journal of Agricultural and Food Chemistry 48 3152-3158. 

Nonenzymatic examples of intramolecular migrations are known, such as the migration of tritium during oxidative polymerization of... 

There are also many uses for nonenzymatic polymeric catalysts. For instance, polymer-bound crown ethers, cryptates, and channel compounds behave as polymeric phase-transfer catalysts. The catalytic activity is based on selective complex formation. An example is the use of polystyrene-attached oxygen heterocycles [18]-crown-6 or a cryptand[222] to catalyze replacements of bromine in n-octyl bromide by an iodine or by a cyanide groups... 

Due to the nonenzymatic nature of polymerization in lignin, the linkage pattern is irregular (Brett and Waldron, 1996). Lignin continues to form as long as there is space in the cell wall. It fills all spaces in the cell wall and replaces water (Brett and Waldron, 1996). The lignin deposition in the cell wall is well planned and controlled. It starts at different rates in different cell corners (Davin et al., 2009). The effect... 

Blocking of this nonenzymatic detoxification process, developed by the parasites, results in a toxicity of the non-polymerized hematin [232,233]. Other hypothetical scenarios explaining activity of quinine are discussed [232, 233]. 

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