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Monosaccharide derivativespage 1
page 2
1. Monosaccharide derivatives are important metabolic
products,although excesses or deficiencies of some contribute to
pathogenicconditions.
2. Sugar acidsa. Ascorbic acid (vitamin C) is required in the
synthesis of collagen.
Scurvy: vitamin C deficiency produces abnormalcollagen.
(1) Prolonged deficiency of vitamin C causes scurvy
(i.e.,perifollicular petechiae, corkscrew hairs, bruising,
gingivalinflammation, and bleeding).
b. Glucuronic acid reacts with bilirubin in the liver,
formingconjugated (direct) bilirubin, which is water soluble.
Glucuronic acid: reacts with bilirubin to produceconjugated
bilirubin
c. Glucuronic acid is a component of glycosaminoglycans
(GAGs),which are major constituents of the extracellular
matrix.
3. Deoxy sugarsa. 2-Deoxyribose is an essential component of
the
deoxyribonucleotide structure.
2-Deoxyribose: component of deoxyribonucleotidestructure
4. Sugar alcohols (polyols)a. Glycerol derived from hydrolysis
of triacylglycerol is
phosphorylated in the liver to form glycerol phosphate,
whichenters the gluconeogenic pathway.
(1) Liver is the only tissue with glycerol kinase to
phosphorylate glycerol.
Glycerol 3-phosphate: substrate forgluconeogenesis and for
synthesizingtriacylglycerol
b. Sorbitol derived from glucose is osmotically active and
isresponsible for damage to the lens (cataract formation),
Schwanncells (peripheral neuropathy), and pericytes (retinopathy),
all
associated with diabetes mellitus.c. Galactitol derived from
galactose contributes to cataract formationin galactosemia.
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Sorbitol: cataracts, neuropathy, and retinopathy indiabetes
mellitus
5. Amino sugarsa. Replacement of the hydroxyl group with an
amino group yields
glucosamine and galactosamine.b. N-acetylated forms of these
compounds are present in GAGs.
6. Sugar estersa. Sugar forms glycosidic bonds with phosphate or
sulfate.b. Phosphorylation of glucose after it enters cells
effectively traps it
as glucose-6-phosphate, which is further metabolized.
Phosphorylation of glucose: traps it in cells for
furthermetabolism
7. Glycosylation
Glycosylation of basement membranes of small vesselsrenders them
permeable to proteins.
a. Refers to the reaction of sugar aldehyde with protein
aminogroups to form a nonreversible covalent bond.
b. Excessive glycosylation in diabetes leads to
endothelialmembrane alteration, producing microvascular
disease.
c. In arterioles, glycosylation of the basement membrane
rendersthem permeable to protein, producing hyaline
arteriolosclerosis.
Hemoglobin A1c
: formed by glucose reaction with
terminal amino groups and used clinically as a measureof
long-term blood glucose concentration
Table 1-1. Monosaccharides Common in MetabolicProcesses
CLASS/SUGAR*CARBONYLGROUP MAJOR METABOLIC ROLE
Triose (3 Carbons)
Glyceraldehyde Aldose Intermediate in glycolytic andpentose
phosphate pathways
Dihydroxyacetone Ketose Reduced to glycerol (used in
fatmetabolism); present in glycolyticpathway
Tetrose (4 Carbons)Erythrose Aldose Intermediate in pentose
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phosphate pathway
Pentose (5 Carbons)
Ribose Aldose Component of RNA; precursor ofDNA
Ribulose Ketose Intermediate in pentosephosphate pathway
Hexose (6 Carbons)
Glucose Aldose Absorbed from intestine with Na+
and enters cells; starting point ofglycolytic pathway;
polymerized toform glycogen in liver and muscle
Fructose Ketose Absorbed from intestine byfacilitated diffusion
and enterscells; converted to intermediatesin glycolytic pathway;
derivedfrom sucrose
Galactose Aldose Absorbed from intestine with Na+
and enters cells; converted toglucose; derived from lactose
Heptose (7 Carbons)
Sedoheptulose Ketose Intermediate in pentose
phosphate pathway
*Within cells, sugars usually are phosphorylated, which prevents
them from diffusing out of the cell.
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