Page 43 - Color Atlas of Biochemistry
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34 Biomolecules
Overview The open-chained form of glucose shown
in (1) is found in neutral solution in less than
The carbohydrates are a group of naturally 0.1% of the molecules. The reason for this is an
occurring carbonyl compounds (aldehydes intramolecular reaction in which one of the
or ketones) that also contain several hydroxyl OH groups of the sugar is added to the alde-
groups. The carbohydrates include single sug- hyde group of the same molecule (2). This
ars (monosaccharides) and their polymers, gives rise to a cyclic hemiacetal (see p.10). In
the oligosaccharides and polysaccharides. aldohexoses, the hydroxy group at C-5 reacts
preferentially, and a six-membered pyran
ring is formed. Sugars that contain this ring
A. Carbohydrates: overview
are called pyranoses. By contrast, if the OH
Polymeric carbohydrates–above all starch, as group at C-4 reacts, a five-part furan ring is
well as some disaccharides–are important formed. In solution, pyranose forms and
(but not essential) components of food (see furanose forms are present in equilibrium
p. 360). Inthe gut, they are brokendowninto with each other and with the open-chained
monosaccharides and resorbed in this form form, while in glucose polymers only the
(see p. 272). The form in which carbohydrates pyranose form occurs.
are distributed by the blood of vertebrates is The Haworth projection (2) is usually used
glucose (“blood sugar”). This is taken up by the to depict sugars in the cyclic form, with the
cells and either brokendownto obtainenergy ring being shown in perspective as viewed
(glycolysis) or converted into other metabo- from above. Depending on the configuration,
lites (see pp.150–159). Several organs (partic- the substituents of the chiral C atoms are then
ularly the liver and muscles) store glycogen as found above or below the ring. OH groups
apolymeric reserve carbohydrate (right; see that lie on the right in the Fischer projection
p.156). The glycogen molecules are covalently (1)appear under the ring level in the Haworth
bound to a protein, glycogenin. Polysaccha- projection, while those on the left appear
rides are used by many organisms as building above it.
materials. For example, the cell walls of bac- As a result of hemiacetal formation, an ad-
teria contain murein as a stabilizing compo- ditional chiral center arises at C-1, which can
nent (see p. 40), while in plants cellulose and be present in both possible configurations
other polysaccharides fulfill this role (see (anomers)(seep. 8). To emphasizethis, the
p. 42). Oligomeric or polymeric carbohydrates corresponding bonds are shown here using
are often covalently bound to lipids or pro- wavy lines.
teins. The glycolipids and glycoproteins The Haworth formula does not take ac-
formed in this way are found, for example, count of the fact that the pyran ring is not
in cell membranes (center). Glycoproteins plain, but usually has a chair conformation.In
also occur in the blood in solute form (plasma B3, two frequent conformations of D-glucopy-
proteins; see p. 276) and, as components of ranose are shown as ball-and-stick models. In
1
proteoglycans, form important constituents of the C 4 conformation (bottom), most of the
the intercellular substance (see p. 346). OH groups appear vertical to the ring level, as
in the Haworth projection (axial or a posi-
4
tion). In the slightly more stable C 1 confor-
B. Monosaccharides: structure
mation (top), the OH groups take the equato-
The most important natural monosaccharide, rial or e position. At room temperature, each
D-glucose,is analiphatic aldehyde with six C form can change into the other, as well as into
atoms, five of which carry a hydroxyl group other conformations.
(1). Since C atoms 2 to 5 represent chiral
centers (see p. 8), there are 15 further
isomeric aldohexoses in addition to D-glucose,
although only a few of these are important in
nature (see p. 38). Most natural monosaccha-
rides have the same configuration at C-5 as
D-glyceraldehyde–they belong to the D series.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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