68
1
General Principles
teins are synthesized and assembled. 2-Deoxy-D-ribofuranose-5-phosphates are the backbone
components of deoxyribonucleotides (DNA), which primarily act as the carrier of the genetic
information, necessary for the formation of proteins involved in life processes. Like RNA,
these molecules have two purines (adenine and guanine) and two pyrimidines (uracil and
cytosine for RNA and thymidine and cytosine for DNA) that are linked β to carbon-1 of the
D-ribofuranose-phosphate units to give N-glycosides.
D-Xylopyranose occurs as one of the major components in the hemicelluloses (see > Sect. 6
on cellulose and > Sect. 7 on hemicelluloses and > Chap. 6.3 on cellulose).
3.7 Occurrence of Hexoses
Of the eight D-hexoses in > Fig. 1, only three occur to any extent: D-glucose, its 2-isomer,
D-mannose, and its 4-isomer, D-galactose. Another hexose that is found is D-fructose, which
is a keto-sugar that can be derived from dihydroxy acetone, as the aldehyde carbohydrates were
derived from D-glyceraldehyde. D-Fructose is formed when D-glucose or D-mannose are treated with alkali, which isomerizes carbons 1 and 2 of the two D-hexoses [3]. The diphosphate of
D-fructose, D-fructose-1,6-bis-phosphate is the first hexose that is formed in the photosynthetic process and it is rapidly converted into D-fructose-6-phosphate, and then into D-glucose6-phosphate.
4 Properties and Occurrence of D-Glucose
4.1 D-Glucose in the Free State
Free D-glucose occurs primarily in the blood of many higher animals, where it serves as an
immediate source of energy and as a stabilizer of the osmotic pressure, and a precursor for the
formation of glycogen and fat in muscle tissue. In normal humans, the concentration of blood
glucose is 80–120 mg/100 mL −1 or 5–7 mM. This can increase to 200–300 mg/100 mL −1 or
11–20 mM after a high-carbohydrate meal, but then is relatively rapidly decreased by the action
of insulin and often goes below 80 mg/mL −1 and then goes slowly up to normal levels. In
uncontrolled diabetics the glucose often goes much higher, to 140–1100 mg/100 mL −1 or 8–
60 mM. In controlled diabetics, the glucose will often be slightly higher than normal, for example, 120–140 mg/100 mL −1 .
D-Glucose is also found in the free state in honey, grapes, and raisins. It is produced by
the action of glucoamylase (amyloglucosidase) on starch, where glucoamylase hydrolyzes
the α-(1→4) linkage of the glucose units at the nonreducing-ends of the starch chains, giving inversion of the configuration, forming β-D-glucose. Glucoamylase will also hydrolyze
α-(1→6) branch linkages, although at a rate about 0.1 that of the α-(1→4) linkage. Eventually glucoamylase will completely convert all of the starch into D-glucose.
4.2 D-Glucose in the Combined State
D-Glucose is found in a combined form in which its hemiacetal group has reacted with
a hydroxyl on another glucose or another carbohydrate to form an acetal or glucosidic link-
1
General Principles
teins are synthesized and assembled. 2-Deoxy-D-ribofuranose-5-phosphates are the backbone
components of deoxyribonucleotides (DNA), which primarily act as the carrier of the genetic
information, necessary for the formation of proteins involved in life processes. Like RNA,
these molecules have two purines (adenine and guanine) and two pyrimidines (uracil and
cytosine for RNA and thymidine and cytosine for DNA) that are linked β to carbon-1 of the
D-ribofuranose-phosphate units to give N-glycosides.
D-Xylopyranose occurs as one of the major components in the hemicelluloses (see > Sect. 6
on cellulose and > Sect. 7 on hemicelluloses and > Chap. 6.3 on cellulose).
3.7 Occurrence of Hexoses
Of the eight D-hexoses in > Fig. 1, only three occur to any extent: D-glucose, its 2-isomer,
D-mannose, and its 4-isomer, D-galactose. Another hexose that is found is D-fructose, which
is a keto-sugar that can be derived from dihydroxy acetone, as the aldehyde carbohydrates were
derived from D-glyceraldehyde. D-Fructose is formed when D-glucose or D-mannose are treated with alkali, which isomerizes carbons 1 and 2 of the two D-hexoses [3]. The diphosphate of
D-fructose, D-fructose-1,6-bis-phosphate is the first hexose that is formed in the photosynthetic process and it is rapidly converted into D-fructose-6-phosphate, and then into D-glucose6-phosphate.
4 Properties and Occurrence of D-Glucose
4.1 D-Glucose in the Free State
Free D-glucose occurs primarily in the blood of many higher animals, where it serves as an
immediate source of energy and as a stabilizer of the osmotic pressure, and a precursor for the
formation of glycogen and fat in muscle tissue. In normal humans, the concentration of blood
glucose is 80–120 mg/100 mL −1 or 5–7 mM. This can increase to 200–300 mg/100 mL −1 or
11–20 mM after a high-carbohydrate meal, but then is relatively rapidly decreased by the action
of insulin and often goes below 80 mg/mL −1 and then goes slowly up to normal levels. In
uncontrolled diabetics the glucose often goes much higher, to 140–1100 mg/100 mL −1 or 8–
60 mM. In controlled diabetics, the glucose will often be slightly higher than normal, for example, 120–140 mg/100 mL −1 .
D-Glucose is also found in the free state in honey, grapes, and raisins. It is produced by
the action of glucoamylase (amyloglucosidase) on starch, where glucoamylase hydrolyzes
the α-(1→4) linkage of the glucose units at the nonreducing-ends of the starch chains, giving inversion of the configuration, forming β-D-glucose. Glucoamylase will also hydrolyze
α-(1→6) branch linkages, although at a rate about 0.1 that of the α-(1→4) linkage. Eventually glucoamylase will completely convert all of the starch into D-glucose.
4.2 D-Glucose in the Combined State
D-Glucose is found in a combined form in which its hemiacetal group has reacted with
a hydroxyl on another glucose or another carbohydrate to form an acetal or glucosidic link-