Chemical structure of Vitamin B12
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Cyanocobalamin

The name vitamin B12 (or B12 for short) is used in two different ways. In a broader sense it refers to a group of Co-containing compounds known as cobalamins - cyanocobalamin (an artifact formed as a result of the use of cyanide in the purification procedures), hydroxocobalamin and the two coenzyme forms of B12, methylcobalamin (MeB12) and 5-deoxyadenosylcobalamin (adenosylcobalamin - AdoB12). more...

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In a more specific way, the term B12 is used to refer to only one of these forms, cyanocobalamin, which is the principal B12 form used for foods and in nutritional supplements.

Pseudo-B12 refers to B12-like substances which are found in certain organisms, such as Spirulina spp. (blue-green algae, cyanobacteria). However, these substances do not have B12 biological activity for humans.

Structure

B12 is the most chemically complex of all the vitamins. B12's structure is based on a corrin ring, which, although similar to the porphyrin ring found in heme, chlorophyll, and cytochrome, has two of the pyrrole rings directly bonded. The central metal ion is Co (cobalt). Four of the six coordinations are provided by the corrin ring nitrogens, and a fifth by a dimethylbenzimidazole group. The sixth coordination partner varies, being a cyano group (-CN), a hydroxyl group (-OH), a methyl group (-CH₃) or a 5'-deoxyadenosyl group (here the C5' atom of the deoxyribose forms the covalent bond with Co), respectively, to yield the four B12 forms mentioned above. The covalent C-Co bond is the only carbon-metal bond known in biology.

Synthesis

B12 cannot be made by plants or by animals, as the only type of organisms that have the enzymes required for the synthesis of B12 are bacteria and archaea.

Functions

Coenzyme B12's reactive C-Co bond participates in two types of enzyme-catalyzed reactions.

  1. Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcochol, or an amine.
  2. Methyl (-CH₃) group transfers between two molecules.

In humans there are only two coenzyme B12-dependent enzymes:

  1. MUT which uses the AdoB12 form and reaction type 1 to catalyze a carbon skeleton rearrangement (the X group is -COSCoA). MUT's reaction converts MMl-CoA to Su-CoA, an important step in the extraction of energy from proteins and fats (for more see MUT's reaction mechanism)
  2. MTR, a methyl transfer enzyme, which uses the MeB12 and reaction type 2 to catalyzes the conversion of the amino acid Hcy into Met (for more see MTR's reaction mechanism).

Read more at Wikipedia.org


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Antidotes to anxiety: here's our guide to 8 calming supplements that can soothe a fretful mind
From Natural Health, 7/1/05 by Lorie Parch

aS FREUD SAID, "The act of birth is the first experience with anxiety." For many of us, feelings of distress remain constant companions: About 19 million Americans suffer from serious anxiety conditions like obsessive-compulsive disorder and social phobia, reports the National Institute of Mental Health, while virtually everyone else deals with less debilitating but still harmful levels of anxiety.

Relief is as close as your supplement shelf. "There are many manifestations and degrees of anxiety," says David Bunting, director of botanical and regulatory affairs at Herb Pharm, an herbal remedy company in Williams, Ore. "My tendency would be to use botanical dietary supplements for any degree--but in severe cases only as a medically supervised complement to appropriate treatment."

Certain supplements are best taken for a limited duration. "For something that's short-term--say, anxiety related to an exam or an illness--kava and valerian are good options," says Cathy Wong, N.D., a Boston-based naturopathic doctor and About.com's alternative-medicine expert. "But they're not good for managing long-term anxiety, like a high-stress job."

Remedies for ongoing, generalized anxiety always circle back to lifestyle solutions. Regular workouts and a balanced diet that contains adequate amounts of magnesium and B vitamins can do a world of tensionrelieving good. "One of my first recommendations for someone who has a lot of anxiety is aerobic exercise, as well as calming exercise like yoga or tat chi," Wong says. Both types will help stave off the burnout caused by chronically elevated levels of the stress hormone cortisol.

For occasional anxiety, try these supplements, which may soothe a temporarily troubled mind.

COPYRIGHT 2005 Weider Publications
COPYRIGHT 2005 Gale Group

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