Vitamin synthesis, riboflavin

Methylenetetrahydrofolate is then reduced to CH3-THF by the vitamin B2 (riboflavin)-dependent enzyme 5,10-methylenetetrahydrofolate reductase (MTHFR), using NADPH as cosubstrate. MTHFR is the key enzyme for diverting 5,10-methylentetrahydrofolate to methylation of homocysteine or to DNA synthesis though the conversion of uracil to thymidine. 

The 1930s were a golden age for the discoveries of structures and functions of other vitamins. In 1935, the laboratories of both Kuhn and Karrer reported synthesis of vitamin B2 (riboflavin, see the strucmre below). Two years earlier Warburg found a yellow oxidative enzyme in bottom yeasts and Kuhn identified it as vitamin B2. Its REDOX role in the metabolism of carbohydrates, fats, and proteins would soon be under-... 

If substances spare the B vitamins by increasing vitamin synthesis, or by reducing metabolic requirements, they should act equally well when given parenterally. Ekman and Strcimbeck (1949) reported that rats survived a very short while longer on riboflavin-deficient diets when they were injected with small doses of ascorbic acid, though there was no difference in the appearance of the signs of deficiency. If these results are confirmed, they would imply that ascorbic acid can indeed reduce the metabolic need for riboflavin, but only to a very limited extent. 

Isolation of tobacco mosaic virus under the electron microscope Suggested that an organism could be produced by transplanting the nucleus (i.e. the DNA) of a cell from one human to the egg of another Synthesis of Vitamin B2 (Riboflavin)... 

The conventional process for the production of 4000 tons/year of vitamin B2 (riboflavin) consists of eight chemical and biochemical steps. The first step is a fermentation using Bacillus bacteria. A biotechnological synthesis of riboflavin in a single fermentation step with the help of bacteria, yeast, or fungi (Roche, ADM, and BASF, respectively) has reduced the production cost by 40%. The... 

Vitamins are organic compounds required as vital nutrients in small amounts by a given organism. Vitamins cannot be synthesized in sufficient quantities by the body and have to be provided with the diet to avoid characteristic diseases (e.g., scurvy in the case of human shortage of vitamin C). The three most important vitamins (by their industrial production capacities) are vitamin C, vitamin B3 (nicotinic acid amide) and vitamin E (tocopherol). These three vitamins represent, from their stmcture and synthesis, typical fine chemicals. However, they are produced on a multi-10000 ton-scale per year. To give an example that also fulfils the capacity criterion of fine chemicals. Table 5.3.19, shows vitamin B2 (riboflavin). The latter is produced both by chemical synthesis and fermentation on the order of 10 000 tons yr . Riboflavin is required for a wide variety of cellular processes and is used for therapeutic purposes and as food additive. 

In 1933, R. Kuhn and his co-workers first isolated riboflavin from eggs in a pure, crystalline state (1), named it ovoflavin, and deterrnined its function as a vitamin (2). At the same time, impure crystalline preparations of riboflavin were isolated from whey and named lyochrome and, later, lactoflavin. Soon thereafter, P. Karrer and his co-workers isolated riboflavin from a wide variety of animal organs and vegetable sources and named it hepatoflavin (3). Ovoflavin from egg, lactoflavin from milk, and hepatoflavin from Hver were aU. subsequently identified as riboflavin. The discovery of the yeUow en2yme by Warburg and Christian in 1932 and their description of lumiflavin (4), a photochemical degradation product of riboflavin, were of great use for the elucidation of the chemical stmcture of riboflavin by Kuhn and his co-workers (5). The stmcture was confirmed in 1935 by the synthesis by Karrer and his co-workers (6), and Kuhn and his co-workers (7). 

Biotransformations are carried out by either whole cells (microbial, plant, or animal) or by isolated enzymes. Both methods have advantages and disadvantages. In general, multistep transformations, such as hydroxylations of steroids, or the synthesis of amino acids, riboflavin, vitamins, and alkaloids that require the presence of several enzymes and cofactors are carried out by whole cells. Simple one- or two-step transformations, on the other hand, are usually carried out by isolated enzymes. Compared to fermentations, enzymatic reactions have a number of advantages including simple instmmentation reduced side reactions, easy control, and product isolation. 

Riboflavin, or vitamin B2, is a constituent and precursor of both riboflavin 5 -phosphate, also known as flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). The name riboflavin is a synthesis of the names for the molecule s component parts, ribitol and flavin. The structures of riboflavin. 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

Other mediators which have been used in combination with diaphorase for the regeneration of NAD+ are riboflavin and Vitamin K3, which is 2,3-dimethyl-1,4-naphthoquinone. However, especially riboflavin is not stable enough for synthetic applications [40]. Better stability is exhibited by phenanthrolindiones as mediators. In combination with diaphorase, Ohshiro [41] showed the indirect electrochemical oxidation of cyclohexanol to cyclohexanone using the NAD+ dependent HLADH with a turnover frequency of two per hour. For an effective enzymatic synthesis, this turnover frequency, however, would be too small. In our own studies, we were able to accelerate the NAD(P)+ regeneration considerably by lowering the electron density within the... 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

This group includes the coenzyme forms of water-soluble vitamin B2 or riboflavin. Synthesis occurs by initial cyclohydrolase action on the guanine ring of GTP and subsequent steps lead to the synthesis of the isoalloxazine ring structure (see structures below). 

Alcohol is distilled up to a content of 96% in one or more stages. About 1 % of ethanol consists of fusel oils (degradation products of amino acids) which can be used as solvents for lacquers and resins. Solids from the processed liquor containing proteins, carbohydrates, mineral salts, riboflavin and other vitamins are used in poultry, swine and cattle feeds. C02 and H2 produced in butanol-acetone-butyric acid production can be used for the chemical synthesis of methanol and ammonia, or are burned. 

Riboflavin is produced by Clostridium, Ascomycetes and Candida species. The yield can be as high as 5 g 1 1 after 7 days 60). Gibberella fujikuroi is utilized for the synthesis of gibberellins, a group of plant hormones used for plant growth promotion. Glucose, molasses, lipid (corn oil) are usually used as carbon sources. Vitamin B12 may also be synthesized from alcohols and hydrocarbons. 

Dietary deficiency is relatively widespread, yet is apparently never fatal there is not even a clearly characteristic riboflavin deficiency disease. In addition to intestinal bacterial synthesis of the vitamin, there is very efficient conservation and reutilization of riboflavin in tissues. Flavin coenzymes are tightly enzyme bound, in some cases covalently, and control of tissue flavins is largely at the level of synthesis and catabolism of flavin-dependent enzymes. 

In bacteria, flavin adenine dinucleotide (FAD) is the prosthetic group of the photolyases that catalyze reductive repair of light-induced pyrimidine dimers in DNA. Riboflavin is the light-emitting molecule in some bioluminescent fungi and bacteria, and is the precursor for synthesis of the dimethylbenzimidazole ring of vitamin B12 (Section 10.7.3). 

Intestinal bacteria synthesize riboflavin, and fecal losses of the vitamin may be five- to six-fold higher than intake. It is possible that bacterial synthesis makes a significant contribution to riboflavin intake, because there is carrier-mediated uptake of riboflavin into colonocytes in culture. The activity of the carrier is increased in riboflavin deficiency and decreased when the cells are cultured in the presence of high concentrations of riboflavin. The same carrier mechanism seems to be involved in tissue uptake of riboflavin (Said et al., 2000). 

Control over tissue concentrations of riboflavin coenzymes seems to be largely by control of the activity of flavokinase, and the synthesis and catabolism of flavin-dependent enzymes. Almost all the vitamin in tissues is enzyme bound, and free riboflavin phosphate and FAD are rapidly hydrolyzed to riboflavin. If this is not rephosphorylated, it rapidly diffuses out of tissues and is excreted. 

A number of fungi have a failure of the normal regulation of riboflavin synthesis and are overproducers of the vitamin. Mutants of Ashbya gossypii may accumulate up to 150 /xmol of riboflavin per gram of protein, compared with a normal content of 0.25 /xmol per gram of protein. They can produce and excrete so much that riboflavin crystallizes in the culture medium. Such fungi are used for the commercial production of riboflavin by fermentation, as an alternative to chemical synthesis. 

In addition to the role of flavoproteins in iron metabolism, it is possible that the anemia associated with riboflavin deficiency is a consequence of the impairment of vitamin Be metabolism in riboflavin deficiency. Pyridoxine oxidase is a flavoprotein and, like glutathione reductase, is very sensitive to riboflavin depletion (McCormick, 1989). Vitamin Be deficiency can result in hypochromic anemia as a result of impaired porphyrin synthesis. Although riboflavin depletion decreases the oxidation of dietary vitamin Be to pyridoxal (Section 9.2), it is not clear to what extent there is secondary vitamin Be deficiency in riboflavin deficiency This is partly because vitamin Be nutritional status is commonly... 

In species for which ascorbate is not a vitamin, riboflavin deficiency can also lead to considerably reduced synthesis and low tissue concentrations of ascorbate, since gulonolactone oxidase, the key enzyme in ascorbate synthesis (Section 13.2), is a flavoprotein. 

An alternative which is attractive for large scale work is the electrochemical reduction of aldonolac-tones.42 43 Particular attention has been paid to the electroreductive synthesis of ribose from ribonolac-tone because of the importance of the former in the synthesis of riboflavin (vitamin 62). Processes generally involve a mercury cathode and maintenance of an acidic pH, often with the assistance of a phosphate or borate buffer. It has been reported that alkali metal ions are also necessary, suggesting that the reduction occurs via metal amalgam formation. However, other accounts make no mention of metal... 

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