Page 377 - Color Atlas of Biochemistry

368 Nutrition

Water-soluble vitamins II Vitamin C is L-ascorbic acid (chemically:
2-oxogulonolactone). The two hydroxyl
groups have acidic properties. By releasing a
A. Water-soluble vitamins II
proton, ascorbic acid therefore turns into its
Vitamin B 6 consists of three substituted pyr- anion, ascorbate. Humans, apes, and guinea
idines—pyridoxal, pyridoxol, and pyrid- pigs require vitamin C because they lack the
oxamine. The illustration shows the structure enzyme L-gulonolactone oxidase (1.1.3.8),
of pyridoxal, which carries an aldehyde group which catalyzes the final step in the conver-
(–CHO) at C-4. Pyridoxol is the corresponding sion of glucose into ascorbate.
alcohol (–CH 2 OH), and pyridoxamine the Vitamin C is particularly abundant in fresh
amine (–CH 2 NH 2 ). fruit and vegetables. Many soft drinks and
The active form of vitamin B 6 , pyridoxal foodstuffs also have synthetic ascorbic acid
phosphate, is the most important coenzyme added to them as an antioxidant and flavor
in the amino acid metabolism (see p. 106). enhancer. Boiling slowly destroys vitamin C.
Almost all conversion reactions involving In the body, ascorbic acid serves as a reducing
amino acids require pyridoxal phosphate, in- agent in variations reactions (usually hydrox-
cluding transaminations, decarboxylations, ylations). Among the processes involved are
dehydrogenations, etc. Glycogen phosphory- collagen synthesis, tyrosine degradation, cate-
lase, theenzymefor glycogen degradation, cholamine synthesis, and bile acid biosynthesis.
also contains pyridoxal phosphate as a cofac- The daily requirement for ascorbic acid is
tor. Vitamin B 6 deficiency is rare. about 60 mg, a comparatively large amount
Vitamin B 12 (cobalamine) is one of the most for a vitamin. Even higher doses of the vita-
complex low-molecular-weight substances min have a protective effect against infec-
occurring in nature. The core of the molecule tions. However, the biochemical basis for
consists of a tetrapyrrol system (corrin), with this effect has not yet been explained. Vitamin
cobalt as the central atom (see p. 108). The C deficiency only occurs rarely nowadays; it
vitamin is exclusively synthesized by micro- becomes evident after a few months in the
organisms. It is abundant in liver, meat, eggs, form of scurvy, with connective-tissue dam-
and milk, but not in plant products. As the age, bleeding, and tooth loss.
intestinal flora synthesize vitamin B 12, strict Vitamin H (biotin) is present in liver, egg
vegetarians usually also have an adequate yolk, and other foods; it is also synthesized by
supply of the vitamin. the intestinal flora. In the body, biotin is co-
Cobalamine can only be resorbed in the valently attached via a lysine side chain to
small intestine when the gastric mucosa se- enzymes that catalyze carboxylation reac-
creteswhatisknown as intrinsic factor—a tions. Biotin-dependent carboxylases include
glycoprotein that binds cobalamine (the ex- pyruvate carboxylase (see p. 154) and acetyl-
trinsic factor) and thereby protects it from CoA carboxylase (see p. 162). CO 2 binds, using
degradation. In the blood, the vitamin is up ATP, to one of the two N atoms of biotin,
bound to a special protein known as trans- from which it is transferred to the acceptor
cobalamin. The liver is able to store vitamin (see p. 108).
B 12 in amounts suf cient to last for several Biotin binds with high af nity
months. Vitamin B 12 deficiency is usually (K d =10 –15 M) and specificity to avidin,a pro-
due to an absence of intrinsic factor and the tein found in egg white. Since boiling dena-
resulting resorption disturbance. This leads to tures avidin, biotin deficiency only occurs
a disturbance in blood formation known as when egg whites are eaten raw.
pernicious anemia.
In animal metabolism, derivatives of cobal-
amine are mainly involved in rearrangement
reactions. For example, they act as coenzymes
in the conversion of methylmalonyl-CoA to
succinyl-CoA (see p. 166), and in the formation
ofmethioninefromhomocysteine(seep. 418).
In prokaryotes, cobalamine derivatives also
play a part in the reduction of ribonucleotides.

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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