Page 205 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN017G-116 August 2, 2001 18:14

Vitamins and Coenzymes 513

FIGURE 7 Vitamin B 12 (cobalamin), the first discovered carbon-cobalt compound and its coenzyme forms methyl-
cobalamin and 5 -deoxyribosylcobalamin.

is a growth factor needed by a common species of flour freshly ground active yeast cells still fermented sugar.
beetle, but is synthesized in adequate amounts by most hu- Pasteur and others had previously maintained that fer-
man individuals. However, a few children require dietary mentation required intact cells. Dialysis of the yeast
carnitine. Inositol is an essential growth factor for yeast juice stopped the fermentation, but the material that dif-
and is sometimes regarded as a vitamin. Flavonoid com- fused out, which was called cozymase, could be added
pounds, abundant in citrus fruit and other plant sources, back with restoration of the fermentation ability of the
have also sometimes been classified as a vitamin. Several juice. Cozymase was soon found to consist of a mix-
amino acids, common constituents of proteins, must also ture of the nicotinamide-containing compound now called
be present in the human diet. These are needed in relatively NAD (Fig. 8), magnesium ions, and thiamin diphosphate
large amounts and are not classified as vitamins. The role (Fig. 2). NAD was quickly recognized as a component of a
of small amounts of another amino acid taurine, which is “respiratory chain” in animal and yeast cells. Since about
essential for cats, in human nutrition is of current interest. 1910 this chain has been recognized as beginning with the
The essential omega-3 and omega-6 fatty acids are also hemoglobin-like oxygen-binding protein cytochrome ox-
necessary dietary constituents, as are numerous metallic idase and an intensely yellow flavin compound that was
elements. identified as the riboflavin derivative FAD (Fig. 9). The
The discovery of coenzymes and catalytic prosthetic cooperative functioning of these vitamin-containing com-
groups came in part from biochemical studies of yeast, pounds, together with associated proteins, established the
of alcoholic fermentation, and of respiration. Buchner concept of coenzymes. Additional compounds, isolated
in 1899, discovered that a cell-free juice prepared from from natural materials, containingvitamin B 6 , pantothenic