por The glycosidic bonds They are the covalent bonds that occur between sugars (carbohydrates) and other molecules, which can be other monosaccharides or other molecules of a diverse nature.
These links make possible the existence of multiple fundamental components for life, not only in the formation of reserve fuels and structural elements, but also of information-carrying molecules essential for cellular communication.
The formation of polysaccharides depends fundamentally on the establishment of glycosidic bonds between the free alcohol or hydroxyl groups of the individual monosaccharide units.
Glycosidic bonds occur in multiple cellular contexts, including the attachment of the polar headgroup of some sphingolipids, essential constituents of cell membranes in many organisms, and the formation of glycoproteins and proteoglycans.
Important polysaccharides such as cellulose, chitin, agar, glycogen, and starch would not be possible without glycosidic bonds. Likewise, protein glycosylation, which occurs in the endoplasmic reticulum and in the Golgi complex, is of paramount importance for the activity of many proteins.
Characteristics
Glycosidic bonds are much more varied than their analogues in proteins and nucleic acids, since in principle any two sugar molecules can be linked together in many ways, since they have multiple -OH groups that can participate in the formation. of the link.
Furthermore, isomers of monosaccharides, that is, one of the two orientations that the hydroxyl group can have in the cyclic structure with respect to the anomeric carbon, provide an additional level of diversity.
The isomers have different three-dimensional structures, as well as different biological activities. Cellulose and glycogen consist of repeating D-glucose units but differ in the type of glycosidic bond (α1-4 for glycogen and β1-4 for cellulose), and therefore have different properties and functions.
Just as polypeptides have a polarity with an N- and C-terminus, and polynucleotides have 5′ and 3′ ends, oligo- or polysaccharides have a polarity defined by reducing and non-reducing ends.
The reducing end has a free anomeric center that does not form a glycosidic bond with another molecule, thus retaining the chemical reactivity of the aldehyde.
The glycosidic bond is the most flexible region of an oligo- or polysaccharide moiety, since the structural chair conformation of individual monosaccharides is relatively rigid.
Glycosidic bond formation
The glycosidic bond can join two monosaccharide molecules through the anomeric carbon of one and the hydroxyl group of the other. That is, the hemiacetal group of one sugar reacts with the alcohol group of another to form an acetal.
In general, the formation of these bonds occurs by condensation reactions, where a water molecule is released with each bond that is formed.
However, in some reactions the oxygen does not leave the sugar molecule as water, but as part of the diphosphate group of a uridine diphosphate nucleotide.
The reactions that give rise to glycosidic bonds are catalyzed by a class of enzymes known as glycosyltransferases. They are formed between a covalently modified sugar by the addition of a phosphate group or a nucleotide (Glucose 6-phosphate, UDP-galactose, for example) that is attached to the growing polymer chain.
hydrolysis of the glycosidic bond
Glycosidic bonds can be easily hydrolyzed in slightly acidic environments, but are quite resistant to alkaline environments.
The enzymatic hydrolysis of glycosidic bonds is mediated by enzymes known as glycosidases. Many mammals do not have these enzymes for the degradation of cellulose, so they are not able to extract energy from this polysaccharide, despite being an essential source of fiber.
Ruminants such as cows, for example, have bacteria associated with their intestines that produce enzymes capable of degrading the cellulose they ingest, making them capable of taking advantage of the energy conserved in plant tissues.
The enzyme lysozyme, produced in the tears of the eye and by some bacterial viruses, is capable of destroying bacteria thanks to its hydrolytic activity, which breaks the glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid in the cell wall of bacteria. .
Diversity
Oligosaccharides, polysaccharides or glycans are highly diverse molecules and this is due to the multiple ways in which monosaccharides can be linked together to form higher order structures.
This diversity stems from the fact, as mentioned above, that sugars have hydroxyl groups that allow for different binding regions, and that bonds can occur between the two possible stereoisomers with respect to the anomeric carbon of the sugar (α or β).
Glycosidic bonds can be formed between a sugar and any hydroxylated compound such as alcohols or amino acids.
In addition, a monosaccharide can form two glycosidic bonds, thus serving as a branch point, introducing potential complexity into the structure of glycans or polysaccharides in cells.
Guys
Regarding types of glycosidic bonds, two categories can be distinguished: glycosidic bonds between monosaccharides that constitute oligo- and polysaccharides, and glycosidic bonds that occur in glycoproteins or glycolipids, which are proteins or lipids with carbohydrate portions. .
O-glycosidic bonds
O-glycosidic bonds occur between monosaccharides, they are formed by the reaction between the hydroxyl group of one sugar molecule and the anomeric carbon of another.
Disaccharides are among the most common oligosaccharides. Polysaccharides have more than 20 monosaccharide units linked together in a linear fashion and sometimes have multiple branches.
In disaccharides such as maltose, lactose, and sucrose, the most common glycosidic bond is the O-glycosidic type. These bonds can occur between the carbons and -OH of the α or β isomeric forms.
The formation of glycosidic bonds in oligo- and polysaccharides will depend on the stereochemical nature of the sugars that are attached, as well as their number of carbon atoms. Typically, for sugars with 6 carbons, linear bonds occur between carbons 1 and 4 or 1 and 6.
There are two main types of O–glycosides which, depending on the nomenclature, are defined as α and β or 1,2-cis and 1,2-trans-glycosides.
Residues 1,2-cis glycosylated, α-glycosides for D-glucose, D-galactose, L-fucose, D-xylose or β-glycosides for D-mannose, L-arabinose; as well as the 1,2-trans (β-glycosides for D-glucose, D-galactose and α-glycosides for D-mannose, etc.), are of great importance for many natural components.
O-glycosylation
One of the most common post-translational modifications is glycosylation, which consists of the addition of a carbohydrate moiety to a growing peptide or protein. Mucins, secretory proteins, can contain large amounts of oligosaccharide chains linked by O-glycosidic bonds.
The O-glycosylation process occurs in the Golgi complex of eukaryotes and consists of the binding of proteins to the carbohydrate portion through a glycosidic bond between the -OH group of a serine or threonine amino acid residue and the anomeric carbon. of sugar.
The formation of these links between carbohydrates and hydroxyproline and hydroxylysine residues and with the phenolic group of tyrosine residues has also been observed.
N-glycosidic bonds
N-glycosidic bonds are the most common among glycosylated proteins. N-glycosylation occurs primarily in the eukaryotic endoplasmic reticulum, with further modifications that may occur in the Golgi complex.
N-glycosylation depends on the presence of the Asn-Xxx-Ser/Thr consensus sequence. The glycosidic bond occurs between the amide nitrogen of the side chain of the asparagine residues and the anomeric carbon of the sugar that is attached to the peptide chain.
The formation of these linkages during glycosylation is dependent on an enzyme known as oligosaccharyltransferase, which transfers oligosaccharides from a dolichol phosphate to the amide nitrogen of asparagine residues.
Other types of glycosidic bonds
S-glycosidic bonds
They also occur between proteins and carbohydrates, they have been observed between peptides with N-terminal cysteines and oligosaccharides. Peptides with these types of links were initially isolated from proteins in human urine and erythrocytes attached to glucose oligosaccharides.
C-glycosidic bonds
They were first observed as a post-translational modification (glycosylation) at a tryptophan residue in RNase 2 present in human urine and in RNase 2 of erythrocytes. A mannose is attached to the carbon at position 2 of the indole nucleus of the amino acid by a C-glycosidic bond.
Nomenclature
The term glycoside is used to describe any sugar whose anomeric group is replaced by a group -OR (O-glycosides), -SR (thioglycosides), -SeR (selenoglucosides), -NR (N-glycosides or glucosamines) or even -CR (C-glycosides).
They can be named in three different ways:
(1) replacing the terminal “-o” of the name of the corresponding cyclic form of the monosaccharide with “-ido” and writing before it, as a different word, the name of the substituent R group.
(2) using the term «glycosiloxy» as a prefix to the name of the monosaccharide.
(3) using the term EITHER-glycosyl, N-glycosyl, S-glycosyl or C-glycosyl as a prefix to the name of the hydroxy compound.
References
Nelson, DL, & Cox, MM (2009). Lehninger Principles of Biochemistry. Omega Editions (5th ed.).
Nomenclature of Carbohydrates (Recommendations 1996). (nineteen ninety six). Retrieved from www.qmul.ac.uk