Page 375 - Color Atlas of Biochemistry

366 Nutrition
Water-soluble vitamins I
The B group of vitamins covers water-soluble
vitamins, all of which serve as precursors for
coenzymes. Their numbering sequence is not
continuous, as many substances that were
originally regarded as vitamins were not later
confirmed as having vitamin characteristics.
A. Water-soluble vitamins I
Vitamin B 1 (thiamine) contains two heterocy-
clic rings—a pyrimidine ring (a six-membered
aromatic ring with two Ns) and a thiazole ring
(a five-membered aromatic ring with N and
S), which are joined by a methylene group.
Theactiveform of vitamin B 1 is thiamine
diphosphate (TPP), which contributes as a
coenzyme to the transfer of hydroxyalkyl res-
idues (active aldehyde groups). The most im-
portant reactions of this type are oxidative
decarboxylation of 2-oxoacids (see p. 134)
and the transketolase reaction in the pentose
phosphate pathway (see p. 152). Thiamine
was the first vitamin to be discovered, around
100 years ago. Vitamin B 1 deficiency leads to
beriberi, a disease with symptoms that in-
clude neurological disturbances, cardiac in-
suf ciency, and muscular atrophy.
Vitamin B 2 is a complex of several vita-
mins: riboflavin, folate, nicotinate, and pan-
tothenic acid.
Riboflavin (from the Latin flavus, yellow)
serves in the metabolism as a component of
the redox coenzymes flavin mononucleotide
(FMN) and flavin adenine dinucleotide (FAD;
see p.104). As prosthetic groups, FMN and FAD
are cofactors for various oxidoreductases (see
p. 32). No specific disease due to a deficiency Further information
of this vitamin is known.
Folate, the anion of folic acid, is made up of The requirement for vitamins in humans and
three different components—a pteridine
derivative, 4-aminobenzoate, and one or
more glutamate residues. After reduction to
tetrahydrofolate (THF), folate serves as a
coenzyme in the C 1 metabolism (see p. 418).
Folate deficiency is relatively common, and
leads to disturbances in nucleotide biosynthe-
sis and thus cell proliferation. As the precur- ganisms and plants whose nutrition is mainly
sors for blood cells divide particularly rapidly, autotrophic have to produce all of these com-
disturbances of the blood picture can occur,
with increased amounts of abnormal precur-
sors for megalocytes (megaloblastic anemia).
Later, general damage ensues as phospholipid
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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synthesis and the amino acid metabolism are
affected.
In contrast to animals, microorganisms are
able to synthesize folate from their own com-
ponents. The growth of microorganisms can
therefore be inhibited by sulfonamides, which
competitively inhibit the incorporation of 4-
aminobenzoate into folate (see p. 254). Since
folate is not synthesized in the animal organ-
ism, sulfonamides have no effect on animal
metabolism.
Nicotinate and nicotinamide, together re-
ferred to as “niacin,” are required for biosyn-
thesis of the coenzymes nicotinamide ad-
+
enine dinucleotide (NAD ) and nicotinamide
+
adenine dinucleotide phosphate (NADP ).
These both serve in energy and nutrient me-
tabolism as carriers of hydride ions (see
pp. 32, 104). The animal organism is able to
convert tryptophan into nicotinate, but only
with a poor yield. Vitamin deficiency there-
fore only occurs when nicotinate, nicotin-
amide, and tryptophan are all simultaneously
are lacking in the diet. It manifests in the form
of skin damage (pellagra), digestive distur-
bances, and depression.
Pantothenic acid is an acid amide consist-
ing of β-alanine and 2,4-dihydroxy-3,3 -di-
methylbutyrate (pantoic acid). It is a precur-
sor of coenzyme A, which is required for acti-
vation of acyl residues in the lipid metabolism
(see pp. 12, 106). Acyl carrier protein (ACP; see
p.168) also contains pantothenic acid as part
of its prosthetic group. Due to the widespread
availability of pantothenic acid in food (Greek
pantothen = “from everywhere”), deficiency
diseases are rare.
other animals is the result of mutations in the
enzymes involved in biosynthetic coenzymes.
As intermediates of coenzyme biosynthesis
are available in suf cient amounts in the
diet of heterotrophic animals (see p. 112),
the lack of endogenous synthesis did not
have unfavorable effects for them. Microor-
pounds themselves in order to survive.