Page 57 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: GTQ Final
Encyclopedia of Physical Science and Technology EN006P-81 June 29, 2001 21:48

Glycoconjugates and Carbohydrates 841

CH 2 OH
C OH O
HO H
HIO 4 2 C O HIO 3 H 2 O H
4 OH
C OH H
H O
H HO O O HN
C OH C O O C O
( H 2 O) O P O P O O N
IO 4 I O IO 3
C OH C O O C O O O CH 2 O
UDP-galactose
H H
FIGURE 16 Action of periodate on vicinal diols. The proposed H H
cyclic intermediate suggests that rigid structures with trans- UDP-glucose
hydroxyls will react poorly, a prediction confirmed experimentally. NAD 4-epimerase OH OH

energy is harvested via chloroplasts and used to pro- CH 2 OH
vide chemical potential in the form of adenosine triphos- H O H
phate, and reducing equivalents. The key intermediate, D- 4 H H
OH
ribulose 1,5-bisphosphate, fixes carbon dioxide (the dark HO O
reaction) yielding products that are ultimately converted to
H HO O O
D-glucose via a series of reactions of phosphorylated sugar HN
intermediates. All other naturally occurring sugars are de- O P O P O
O N
rived from glucose in transformations that involve phos- O O CH 2 O
phorylated or nucleotide-linked sugars. Thus, glucose-6- UDP-glucose
phosphate is converted to fructose-6-phosphate, which H H
H H
in turn is converted to mannose-6-phosphate; fructose-
6-phosphate is also aminated to form 2-deoxy 2-amino OH OH
glucose-6-phosphate. Galactose is formed by epimeriza- FIGURE 17 Biosynthesis of D-galactose by epimerization of uridi-
tion at C-4 of uridine diphosphoglucose (Fig. 17), fucose ine diphosphoglucose.
by a series of reactions initiating with guanosine diphos-
phomannose, etc. Thus, the diversity in saccharides seen production of antibiotics. The sweetness of sucrose, and
in the biosphere stems from a single precursor, D-glucose. other simple sugars, is a major aspect of their commercial
This is, therefore, the only required dietary saccharide for importance. The fact that sucrose is a nonreducing sugar
man. contributes to its stability and market value.
Lactose (β-D-galactopyranosyl-1-4-D-glucopyranose)
is the major sugar of milk (Fig. 19). It is of interest that
II. OLIGOSACCHARIDES the ability to utilize either sucrose or lactose as a source of
calories is dependent on their enzymatic hydrolysis to the
The ability of the anomeric hydroxyl group of a sugar to constituent monosaccharides in the intestine. About one-
be substituted with another sugar (via one of its hydroxyl third of the oriental population lacks the requisite galac-
groups), and for this process to be iterated leads to forma- tosidase and is thus lactose intolerant; this same problem
occurs in some infants (developmentally related) and re-
tionofdi-andhighersaccharides.Becauseseveralhydrox-
sults in a “colicky” baby.
ylsareavailableforsuchlinkagesandtheanomericconfig-
uration can vary as well, the number of possible structures
6 CH 2 OH
grows exponentially even for oligomers derived from the 1
O HOCH 2 O
same sugar. This may explain, in part, why many biolog- H 5 H H
ical recognition events involve saccharides as a ligand. H
4 OH H 1 2 H HO 5
Two disaccharides are widely distributed in nature:
HO CH 2 OH
sucrose and lactose. Sucrose (α-D-glucopyranosyl-β-D- 3 2 O 3 4 6
H OH OH H
fructofuranoside) is the table sugar of commerce and a
Sucrose
major industrial product (Fig. 18); primary sources are -D-fructofuranosyl -D-glucopyranoside
sugar cane and beets. This saccharide and its source mate- Fru( 1 )Glc
2
rial (cane molasses) serve as the basis for rum production; FIGURE 18 Structure of sucrose. Note that this sugar is
cruder precursors are important medium additives for the nonreducing.