08/23/2017

bio chemistry indian pg entrance important CARBOHYDRATES OF PHYSIOLOGIC SIGNIFICANCE

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CARBOHYDRATES OF PHYSIOLOGIC SIGNIFICANCE / 105

nucleic acids, and several coenzymes (Table 13–2).

Glucose, galactose, fructose, and mannose are physiologically

the most important hexoses (Table 13–3). The

biochemically important aldoses are shown in Figure

13–6, and important ketoses in Figure 13–7.

In addition, carboxylic acid derivatives of glucose are

important, including D-glucuronate (for glucuronide

formation and in glycosaminoglycans) and its metabolic

derivative, L-iduronate (in glycosaminoglycans)

(Figure 13–8) and L-gulonate (an intermediate in the

uronic acid pathway; see Figure 20–4).

Sugars Form Glycosides With Other

Compounds & With Each Other

Glycosides are formed by condensation between the hydroxyl

group of the anomeric carbon of a monosaccharide,

or monosaccharide residue, and a second compound

that may—or may not (in the case of an aglycone)—be

another monosaccharide. If the second group is a hydroxyl,

the O-glycosidic bond is an acetal link because it

results from a reaction between a hemiacetal group

(formed from an aldehyde and an .OH group) and an-

Table 13–2. Pentoses of physiologic importance.

Sugar Where Found Biochemical Importance Clinical Significance

D-Ribose Nucleic acids. Structural elements of nucleic acids and

coenzymes, eg, ATP, NAD, NADP, flavoproteins.

Ribose phosphates are intermediates

in pentose phosphate pathway.

D-Ribulose Formed in metabolic processes. Ribulose phosphate is an intermediate in

pentose phosphate pathway.

D-Arabinose Gum arabic. Plum and cherry gums. Constituent of glycoproteins.

D-Xylose Wood gums, proteoglycans, Constituent of glycoproteins.

glycosaminoglycans.

D-Lyxose Heart muscle. A constituent of a lyxoflavin isolated from

human heart muscle.

L-Xylulose Intermediate in uronic acid pathway. Found in urine in essential

pentosuria.

Table 13–3. Hexoses of physiologic importance.

Sugar Source Importance Clinical Significance

D-Glucose Fruit juices. Hydrolysis of starch, cane The “sugar” of the body. The sugar carried Present in the urine (glycosuria)

sugar, maltose, and lactose. by the blood, and the principal one used in diabetes mellitus owing to

by the tissues. raised blood glucose (hyperglycemia).

D-Fructose Fruit juices. Honey. Hydrolysis of Can be changed to glucose in the liver Hereditary fructose intolerance

cane sugar and of inulin (from the and so used in the body. leads to fructose accumulation

Jerusalem artichoke). and hypoglycemia.

D-Galactose Hydrolysis of lactose. Can be changed to glucose in the liver Failure to metabolize leads

and metabolized. Synthesized in the to galactosemia and cataract.

mammary gland to make the lactose of

milk. A constituent of glycolipids and

glycoproteins.

D-Mannose Hydrolysis of plant mannans and A constituent of many glycoproteins.

gums.

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