Page 47 - Color Atlas of Biochemistry

P. 47

38 Biomolecules

Monosaccharides and disaccharides Sugar alcohols (6)suchas sorbitol and
mannitol do not play an important role in
animal metabolism.
A. Important monosaccharides
Only the most important of the large number
of naturally occurring monosaccharides are B. Disaccharides
mentioned here. They are classified according When the anomeric hydroxyl group of one
to the number of C atoms (into pentoses, monosaccharide is bound glycosidically with
hexoses, etc.) and according to the chemical one of the OH groups of another, a disaccha-
nature of the carbonyl function into aldoses ride is formed. As in all glycosides, the glyco-
and ketoses. sidic bond does not allow mutarotation. Since
The best-known aldopentose (1), D-ribose, this type of bond is formed stereospecifically
is a component of RNA and of nucleotide by enzymes in natural disaccharides, they are
coenzymes and is widely distributed. In these only found in one of the possible configura-
compounds, ribose always exists in the fura- tions (α or β).
nose form (see p. 34). Like ribose, D-xylose and Maltose (1) occurs as a breakdown product
L-arabinose are rarely found in free form. of the starches contained in malt (“malt
However, large amounts of both sugars are sugar”; see p.148) and as an intermediate in
foundasconstituentsof polysaccharides in intestinal digestion. In maltose, the anomeric
the walls of plant cells (see p. 42). OH group of one glucose molecule has an α-
The most important of the aldohexoses (1) glycosidic bond with C-4 in a second glucose
is D-glucose. A substantial proportion of the residue.
biomass is accounted for by glucose polymers, Lactose (“milk sugar,” 2)is the most impor-
above all cellulose and starch. Free D-glucose tant carbohydrate in the milk of mammals.
is found in plant juices (“grape sugar”) and as Cow’s milk contains 4.5% lactose, while hu-
“blood sugar” in the blood of higher animals. man milk contains up to 7.5%. In lactose, the
As a constituent of lactose (milk sugar), D- anomeric OH group of galactose forms a β-
galactose is part of the human diet. Together glycosidic bond with C-4 of a glucose. The
with D-mannose, galactose is also found in lactosemoleculeis consequentlyelongated,
glycolipids and glycoproteins (see p. 44). and bothofits pyranrings lie inthe same
Phosphoric acid esters of the ketopentose plane.
D-ribulose (2) are intermediates in the pen- Sucrose (3) servesin plants asthe form in
tose phosphate pathway (see p.152) and in which carbohydrates are transported, and as a
photosynthesis (see p.128). The most widely soluble carbohydrate reserve. Humans value
distributed of the ketohexoses is D-fructose.In it because of its intensely sweet taste. Sources
free form, it is present in fruit juices and in used for sucrose are plants that contain par-
honey. Bound fructose is found in sucrose (B) ticularly high amounts of it, such as sugar
and plant polysaccharides (e. g., inulin). cane and sugar beet (cane sugar, beet sugar).
In the deoxyaldoses (3), an OH group is Enzymatic hydrolysis of sucrose-containing
replaced by a hydrogen atom. In addition to flower nectar in the digestive tract of bees—
2-deoxy-D-ribose, a component of DNA (see catalyzed by the enzyme invertase—produces
p. 84) that is reduced at C-2, L-fucose is shown honey, a mixture of glucose and fructose. In
as another example of these. Fucose, a sugar sucrose, the two anomeric OH groups of glu-
in the λ series (see p. 34) is reduced at C-6. cose and fructose have a glycosidic bond; su-
The acetylated amino sugars N-acetyl-D- crose is therefore one of the non-reducing
glucosamine and N-acetyl-D-Galactosamine sugars.
(4) are often encountered as components of
glycoproteins.
N-acetylneuraminic acid (sialic acid, 5), is a
characteristic component of glycoproteins.
Other acidic monosaccharides such as D-glu-
curonic acid, D-galacturonic acid,and liduronic
acid, are typical constituents of the glycosa-
minoglycans found in connective tissue.

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