Nomenclature for stereoisomers

Nomenclature for stereoisomers

Nomenclature for stereoisomers: D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehydes

 

For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

D & L sugars are mirror images of one another. They have the same name. For example, D-glucose and L-glucose

Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2 n, where n is the number of asymmetric centers. The six-carbon aldoses have 4 asymmetric centers, and thus 16 stereoisomers (8 D-sugars and 8 L-sugars

An aldehyde can react with an alcohol to form a hemiacetal

Similarly a ketone can react with an alcohol to form a hemiketal

 

Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl on one of the distal carbons

E.g., glucose forms an intra-molecular hemiacetal by reaction of the aldehyde on C1 with the hydroxyl on C5, forming a six-member pyranose ring, named after the compound pyran

The representations of the cyclic sugars below are called Haworth projections.

 

 

Fructose can form either: 

  • a six-member pyranose ring, by reaction of the C2 keto group with the hydroxyl on C6
  • a 5-member furanose ring, by reaction of the C2 keto group with the hydroxyl on C5.

 

 

Cyclization of glucose produces a new asymmetric center at C1, with the two stereoisomers called anomers, α & β

 

Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1 extending either:

  • below the ring (α)
  • above the ring (β).

Because of the tetrahedral nature of carbon bonds, the cyclic form of pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar

Related Questions Carbohydrate metabolism

Cori Cycle

The Cori Cycle operates during exercise, when aerobic metabolism in muscle cannot keep up with energy needs.

For a brief burst of ATP utilization, muscle cells utilize ~P stored as phosphocreatine. For more extended exercise, ATP is mainly provided by Glycolysis.

Lactate, produced from pyruvate, passes via the blood to the liver where it is converted to glucose. The glucose may travel back to the muscle to fuel Glycolysis.

The Cori Cycle costs 6 P in liver for every 2P made available in muscle. The net cost is 4 P Although costly in terms of "high energy" bonds, the Cori Cycle allows the organism to accommodate to large fluctuations in energy needs of skeletal muscle between rest and exercise.

Nomenclature for stereoisomers: D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehydes

 

For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

D & L sugars are mirror images of one another. They have the same name. For example, D-glucose and L-glucose

Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2 n, where n is the number of asymmetric centers. The six-carbon aldoses have 4 asymmetric centers, and thus 16 stereoisomers (8 D-sugars and 8 L-sugars

An aldehyde can react with an alcohol to form a hemiacetal

Similarly a ketone can react with an alcohol to form a hemiketal

 

Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl on one of the distal carbons

E.g., glucose forms an intra-molecular hemiacetal by reaction of the aldehyde on C1 with the hydroxyl on C5, forming a six-member pyranose ring, named after the compound pyran

The representations of the cyclic sugars below are called Haworth projections.

 

 

Fructose can form either: 

  • a six-member pyranose ring, by reaction of the C2 keto group with the hydroxyl on C6
  • a 5-member furanose ring, by reaction of the C2 keto group with the hydroxyl on C5.

 

 

Cyclization of glucose produces a new asymmetric center at C1, with the two stereoisomers called anomers, α & β

 

Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1 extending either:

  • below the ring (α)
  • above the ring (β).

Because of the tetrahedral nature of carbon bonds, the cyclic form of pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar

Monosaccharides: Aldoses (e.g., glucose) have an aldehyde at one end

They are classified acc to the number of carbon atoms present

Trioses, tetroses, pentose ( ribose, deoxyribose), hexoses  (glucose, galactose, fructose) Heptoses (sedoheptulose)

Glyceraldehyde simplest aldose

Ketoses (e.g., fructose) have a keto group, usually at C 2.

Dihydroxyacetone simplest Ketoses

The higher sugar exists in ring form rather than chain form

Furan  : 4 carbons and 1 oxygen

Pyrans : 5 carban and 1 oxygen

 These result from formation of hemiacital linkage b/w carbonyl and an alcohol group

Carbohydrates (glycans) have the  basic composition

  • Monosaccharides - simple sugars,  with multiple hydroxyl groups. Based on the number of carbons (e.g., 3, 4, 5, or 6) a monosaccharide is a triose, tetrose, pentose, or hexose, etc.
  • Disaccharides - two monosaccharides covalently linked
  • Oligosaccharides - a few monosaccharides covalently linked.
  • Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units

Glycogen Storage Diseases are genetic enzyme deficiencies associated with excessive glycogen accumulation within cells.

  • When an enzyme defect affects mainly glycogen storage in liver, a common symptom is hypoglycemia (low blood glucose), relating to impaired mobilization of glucose for release to the blood during fasting.
  • When the defect is in muscle tissue, weakness and difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.

Various type of Glycogen storage disease are

Type

Name

Enzyme Deficient

I

Von Geirke’s Disease

Glucose -6-phosphate

II

Pompe’s Disease

(1, 4)glucosidase

III

Cori’s Disease

Debranching Enzymes

IV

Andersen’s Disease

Branching Enzymes

V

McArdle’s Disease

Muscles Glycogen Phosphorylase