Cobalamins vitamin

Cobalamin (vitamin B12) Streptomyces olivaceus Food supplements... 

In cobalamin, vitamin Bu, one of the six ligands forming an octahedral structure around a cobalt atom is an organic molecule attached through a carbon-cobalt bond (red). The bond is weak and easily broken. 

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Marcuard SP, Albemaz L, Khazanie PG Omeprazole therapy causes malabsorption of cyano-cobalamin (vitamin B12). Ann Intern Med 1994 120 211-215. 

The vitamin cobalamin (vitamin Bjj) is reduced and activated in the body to two forms, adeno-sylcobalamin, used by methylmalonyl CoA mutase, and methylcobalamin, formed from methyl-THF in the N-methyl THF-homocysteine methyltransferase reaction. These are the only two enzymes that use vitamin (other than the enzymes that reduce and add an adenosyl group to it). 

Researchers studying the metalloenzyme hydrogenase would like to design small compounds that mimic this enzyme s ability to reversibly reduce protons to H2 and H2 to 2H+, using an active center that contains iron and nickel. Cobalamins (vitamin and its derivatives) contain an easily activated Co-C bond that has a number of biological functions, one of which is as a methyl transferase, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR). This enzyme converts homocysteine (an amino acid that has one more CH2 group in its alkyl side chain than cysteine see Figure 2.2) to methionine as methylcobalamin is converted to cobalamin. 

The loss of a methyl group from AdoMet in each of the reactions yields S-ad-enosylhomocysteine (AdoHcy) and this is subsequently hydrolysed to adenosine and Hey by AdoHcy-hydrolase. Hey sits at a metabolic branch point and can be remethylated to methionine by way of two reactions. One is the 5-methyltetrahydrofo-late dependent reaction catalysed by methionine synthase, which itself is reductively methylated by cobalamin (vitamin B12) and AdoMet, requiring methionine synthase reductase. 5-Methyltetrahydrofolate is generated from 5,10-methylenetetrahydrofo-late (MTHF) by MTHF reductase. The second remethylation reaction is catalysed by betaine methyltransferase, which is restricted to the liver, kidney and brain, while methionine synthase is widely distributed. 

The human body contains only about 1.5 mg of cobalt, almost all of it is in the form of cobalamin, vitamin B12. Ruminant animals, such as cattle and sheep, have a relatively high nutritional need for cobalt and in regions with a low soil cobalt content, such as Australia, cobalt deficiency in these animals is a serious problem. This need for cobalt largely reflects the high requirement of the microorganisms of the rumen (paunch) for vitamin B12. All bacteria require vitamin B12 but not all are able to synthesize it. For example, E. coli lacks one enzyme in the biosynthetic... 

The major vitamins are described in separate alphabetical entries in tins book. Titles used for these entries have been selected on tlie basis of the most frequently used designations as of tlie early 1980s. In alphabetical order, the vitamins described in this book are Ascorbic Acid (Vitamin C) Biotin Choline and Cholinesterase Folic Acid Inositol Niacin Pantothenic Acid Vitamin Bj (Riboflavin) Thiamine (Vitamin Bj) Vitamin A Vitamin B (Pyridoxine) Vitamin B12 (Cobalamin) Vitamin D Vitamin E and Vitamin K. 

RH Allen, SP Stabler, DG Savage, J Lindenbaum. Metabolic abnormalities in cobalamin (vitamin B12) and folate deficiency. FASEB J 7 1344-1353, 1993. 

The structures of the biologically active forms of B12 were solved relatively recently (1961) (78) and were shown to contain a cobalt atom surrounded by a corrin ring as shown in Fig. 16 (80). The crystal structure also showed a cobalt-carbon a bond which was quite surprising since the few compounds with cobalt-carbon a- bonds known at that time were quite unstable (79). The corrin ring is similar to the porphyrin ring, but its greater saturation imports less rigidity than the porphyrin. Corrinoids with the axial 5,6-dimethylbenzimidazole substituent are called cobalamins. Vitamin B12 with Co(III) and CN in the top axial position is... 

The catalytic isomerization of meso-epoxides to allylic alcohols has been achieved with chiral cobalt complexes, in particular with cobalamin (vitamin B12) [47, 48]. 

Coenzymes can be either inorganic species such as coenzyme F450 and or purely organic as in coenzyme A (2.7), the coenzyme that carries acyl groups in the synthesis and oxidation of fatty acids, and is responsible for oxidation of pyruvate in the citric acid cycle. Vitamins are commonly coenzymes or their precursors. Coenzyme A is vitamin B5 and we will look in detail at cobalamin, vitamin B12, by way of an example in the next section. Some examples of vitamin and non-vitamin coenzymes are shown in Table 2.3. 

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