Glycoprotein


Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. Secreted extracellular proteins are often glycosylated.
In proteins that have segments extending extracellularly, the extracellular segments are also often glycosylated. Glycoproteins are also often important integral membrane proteins, where they play a role in cell–cell interactions. It is important to distinguish endoplasmic reticulum-based glycosylation of the secretory system from reversible cytosolic-nuclear glycosylation. Glycoproteins of the cytosol and nucleus can be modified through the reversible addition of a single GlcNAc residue that is considered reciprocal to phosphorylation and the functions of these are likely to be additional regulatory mechanism that controls phosphorylation-based signalling. In contrast, classical secretory glycosylation can be structurally essential. For example, inhibition of asparagine-linked, i.e. N-linked, glycosylation can prevent proper glycoprotein folding and full inhibition can be toxic to an individual cell. In contrast, perturbation of glycan processing, which occurs in both the endoplasmic reticulum and Golgi apparatus, is dispensable for isolated cells but can lead to human disease and can be lethal in animal models. It is therefore likely that the fine processing of glycans is important for endogenous functionality, such as cell trafficking, but that this is likely to have been secondary to its role in host-pathogen interactions. A famous example of this latter effect is the ABO blood group system.
Glycosylation is also known to occur on nucleocytoplasmic proteins in the form of O-GlcNAc.

Types of glycosylation

There are several types of glycosylation, although the first two are the most common.
Monosaccharides commonly found in eukaryotic glycoproteins include:
SugarTypeAbbreviation
β-D-GlucoseHexoseGlc
β-D-GalactoseHexoseGal
β-D-MannoseHexoseMan
α-L-FucoseDeoxyhexoseFuc
N-AcetylgalactosamineAminohexoseGalNAc
N-AcetylglucosamineAminohexoseGlcNAc
N-Acetylneuraminic acidAminononulosonic acid
NeuNAc
XylosePentoseXyl

The sugar group can assist in protein folding, improve proteins' stability and are involved in cell signalling.

Examples

One example of glycoproteins found in the body is mucins, which are secreted in the mucus of the respiratory and digestive tracts. The sugars when attached to mucins give them considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes.
Glycoproteins are important for white blood cell recognition. Examples of glycoproteins in the immune system are:
H antigen of the ABO blood compatibility antigens.
Other examples of glycoproteins include:
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.
Variable surface glycoproteins allow the sleeping sickness Trypanosoma parasite to escape the immune response of the host.
The viral spike of the human immunodeficiency virus is heavily glycosylated. Approximately half the mass of the spike is glycosylation and the glycans act to limit antibody recognition as the glycans are assembled by the host cell and so are largely 'self'. Over time, some patients can evolve antibodies to recognise the HIV glycans and almost all so-called 'broadly neutralising antibodies recognise some glycans. This is possible mainly because the unusually high density of glycans hinders normal glycan maturation and they are therefore trapped in the premature, high-mannose, state. This provides a window for immune recognition. In addition, as these glycans are much less variable than the underlying protein, they have emerged as promising targets for vaccine design.

Hormones

s that are glycoproteins include:

Analysis

A variety of methods used in detection, purification, and structural analysis of glycoproteins are
MethodUse
Periodic acid-Schiff stainDetects glycoproteins as pink bands after electrophoretic separation.
Incubation of cultured cells with glycoproteins as radioactive decay bandsLeads to detection of a radioactive sugar after electrophoretic separation.
Treatment with appropriate endo- or exoglycosidase or phospholipasesResultant shifts in electrophoretic migration help distinguish among proteins with N-glycan, O-glycan, or GPI linkages and also between high mannose and complex N-glycans.
Agarose-lectin column chromatography, lectin affinity chromatographyTo purify glycoproteins or glycopeptides that bind the particular lectin used.
Lectin affinity electrophoresisResultant shifts in electrophoretic migration help distinguish and characterize glycoforms, i.e. variants of a glycoprotein differing in carbohydrate.
Compositional analysis following acid hydrolysisIdentifies sugars that the glycoprotein contains and their stoichiometry.
Mass spectrometryProvides information on molecular mass, composition, sequence, and sometimes branching of a glycan chain. It can also be used for site-specific glycosylation profiling.
NMR spectroscopyTo identify specific sugars, their sequence, linkages, and the anomeric nature of glycosidic chain.
Multi-angle light scatteringIn conjunction with size-exclusion chromatography, UV/Vis absorption and differential refractometry, provides information on molecular mass, protein-carbohydrate ratio, aggregation state, size, and sometimes branching of a glycan chain. In conjunction with composition-gradient analysis, analyzes self- and hetero-association to determine binding affinity and stoichiometry with proteins or carbohydrates in solution without labeling.
Dual Polarisation InterferometryMeasures the mechanisms underlying the biomolecular interactions, including reaction rates, affinities and associated conformational changes.
Methylation analysisTo determine linkage between sugars.
Amino acid or cDNA sequencingDetermination of amino acid sequence.