Glycosidic bond
Overview
 
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a glycosidic bond is a type of covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 that joins a carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

 (sugar) molecule to another group, which may or may not be another carbohydrate.

A glycosidic bond is formed between the hemiacetal
Hemiacetal
Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...

 group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some organic compound such as an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

. If the group attached to the carbohydrate residue is not another saccharide it is referred to as an aglycone
Aglycone
An aglycone is the non-sugar compound remaining after replacement of the glycosyl group from a glycoside by a hydrogen atom. The spelling aglycon is sometimes encountered .Classes of phytochemicals found in the aglycone and glycosides forms :...

.
Encyclopedia
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a glycosidic bond is a type of covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 that joins a carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

 (sugar) molecule to another group, which may or may not be another carbohydrate.

A glycosidic bond is formed between the hemiacetal
Hemiacetal
Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...

 group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some organic compound such as an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

. If the group attached to the carbohydrate residue is not another saccharide it is referred to as an aglycone
Aglycone
An aglycone is the non-sugar compound remaining after replacement of the glycosyl group from a glycoside by a hydrogen atom. The spelling aglycon is sometimes encountered .Classes of phytochemicals found in the aglycone and glycosides forms :...

. If it is another saccharide, the resulting units can be termed as being at the reducing end or the terminal end of the structure. This is a relative nomenclature where the reducing end of the di-
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...

 or polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

 is towards the last anomeric carbon of the structure, and the terminal end is in the opposite direction.

In the literature, the bond between an amino group or other nitrogen-containing group and the sugar is often referred to as a glycosidic bond (although IUPAC seems to suggest that the term is a misnomer). For example, the sugar-base bond in a nucleoside
Nucleoside
Nucleosides are glycosylamines consisting of a nucleobase bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage...

 may be referred to as a glycosidic bond. A substance containing a glycosidic bond is a glycoside
Glycoside
In chemistry, a glycoside is a molecule in which a sugar is bound to a non-carbohydrate moiety, usually a small organic molecule. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme...

.




S-, N-, C-, and O-glycosidic bonds

Glycosidic bonds of the form discussed above known as O-glycosidic bonds, in reference to the glycosidic oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 that links the glycoside to the aglycone or reducing end sugar. In analogy, one also considers S-glycosidic bonds (which form thioglycosides), where the oxygen of the glycosidic bond is replaced with a sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 atom. In the same way, N-glycosidic bonds, have the glycosidic bond oxygen replaced with nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

. Substances containing N-glycosidic bonds are also known as glycosylamine
Glycosylamine
Glycosylamine is a class of biochemical compounds consisting of an amine with a β-N-glycosidic bond to a carbohydrate, thus forming a cyclic hemiaminal ether bond .Examples include nucleosides such as adenosine....

s; the term "N-glycoside" is considered a misnomer by IUPAC and is discouraged. C-glycosyl bonds have the glycosidic oxygen replaced by a carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

. All of these modified glycosidic bonds have different susceptibility to hydrolysis, and in the case of C-glycosyl structures, they are typically more resistant to hydrolysis.

Numbering, and α/β distinction of glycosidic bonds


One distinguishes between α- and β-glycosidic bonds based on the relative stereochemistry of the anomeric position and the stereocentre furthest from C1 in the saccharide. In D-hexose
Hexose
In organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2....

 sugars in their pyranose
Pyranose
Pyranose is a collective term for carbohydrates that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds...

 forms, an α-glycosidic bond is formed in an axial orientation, whereas a β-glycosidic bond will be oriented equatorially.

Pharmacologists often join substances to glucuronic acid
Glucuronic acid
Glucuronic acid is a carboxylic acid. Its structure is similar to that of glucose. However, glucuronic acid's sixth carbon is oxidized to a carboxylic acid...

 via glycosidic bonds in order to increase their water solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

; this is known as glucuronidation
Glucuronidation
Glucuronidation is the addition of glucuronic acid to a substrate. Glucuronidation is often involved in xenobiotic metabolism of substances such as drugs, pollutants, bilirubin, androgens, estrogens, mineralocorticoids, glucocorticoids, fatty acid derivatives, retinoids, and bile acids...

. Many other glycoside
Glycoside
In chemistry, a glycoside is a molecule in which a sugar is bound to a non-carbohydrate moiety, usually a small organic molecule. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme...

s have important physiological functions.



Glycoside hydrolases

Glycoside hydrolases (or glycosidases), are enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s that break glycosidic bonds. Glycoside hydrolases typically can act either on α- or on β-glycosidic bonds, but not on both.

Before monosaccharide units are incorporated into glycoproteins, polysaccharides, or lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

s in living organisms, they are typically first "activated" by being joined via a glycosidic bond to the phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

 group of a nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

 such as uridine diphosphate
Uridine diphosphate
Uridine diphosphate, abbreviated UDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside uridine. UDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase uracil.-See also:* Nucleoside...

 (UDP), guanosine diphosphate
Guanosine diphosphate
Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase guanine....

 (GDP), thymidine diphosphate
Thymidine diphosphate
Thymidine diphosphate, abbreviated dTDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside thymidine. dTDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase thymine...

 (TDP), or cytidine monophosphate
Cytidine monophosphate
Cytidine monophosphate, also known as 5'-cytidylic acid or simply cytidylate, and abbreviated CMP, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid with the nucleoside cytidine...

 (CMP). These activated biochemical intermediates are known as sugar nucleotide
Nucleotide sugar
Nucleotide sugars are the activated forms of monosaccharides. Nucleotide sugars act as glycosyl donors in glycosylation reactions. Those reactions are catalyzed by a group of enzymes called glycosyltransferases.-History:...

s or sugar donors. Many biosynthetic pathways use mono- or oligosaccharides activated by a diphosphate linkage to lipids, such as dolichol
Dolichol
Dolichol refers to any of a group of long-chain mostly unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an alcohol functional group.-Functions:...

. These activated donors are then substrates for enzymes known as glycosyltransferase
Glycosyltransferase
Glycosyltransferases are enzymes that act as a catalyst for the transfer of a monosaccharide unit from an activated nucleotide sugar to a glycosyl acceptor molecule, usually an alcohol....

s, which transfer the sugar unit from the activated donor to an accepting nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

 (the acceptor substrate).

External links

  • Definition of glycosides, from the IUPAC Compendium of Chemical Terminology, the "Gold Book
    Gold Book
    The Compendium of Chemical Terminology is a book published by the International Union of Pure and Applied Chemistry containing internationally accepted definitions for terms in chemistry...

    "
  • Varki A et al. Essentials of Glycobiology. Cold Spring Harbor Laboratory Press; 1999. Searchable online

See Also

Mannan Oligosaccharides (MOS)
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK