An Introduction To Glycoproteins

Glycoproteins are proteins with covalently linked oligosaccharide chains (glycans) to amino acids.

Sugars are attached to many eukaryotic proteins, making glycosylation (the enzymic binding of sugars) the most common post translational alteration of proteins. 

An Introduction To Glycoproteins

Glycoproteins play a role in fertilisation and inflammation, and abnormalities in glycoprotein synthesis and catabolism are linked to a variety of illnesses such as influenza, AIDS, peptic ulcers, and anaemia to name a few.

Glycoproteins are present on cell membranes’ lipid bilayer surfaces.

They can operate in the aquatic environment because of their hydrophilic character, which permits them to act through cell-cell recognition and molecule binding. 

Cross-linking proteins and proteins and cells which provide stability and strength to a tissue is also facilitated by cell surface glycoproteins.

Plant cells contain glycoproteins that allow them to resist gravity and stand upright.

This article will provide an informative introduction to glycoproteins and explain why they are important for propagating bodily processes and keeping us healthy.

Why Are Glycoproteins Important?

Glycoproteins are found in all living things, from bacteria to humans. 

Reversible glycosylation occurs when a particular sugar (N-acetylglucosamine) is attached to a serine and threonine residue which is also a reversible phosphorylation hotspot in many proteins.

This is a key metabolic control mechanism. 

Glycation is the term used to describe the nonenzymatic binding of sugars to proteins.

This process has the potential to cause major health problems such as diabetes. 

Glycoproteins are a type of glycoconjugate as well as complex carbohydrate—equivalent terminology for molecules with carbohydrate chains covalently attached to protein (to produce glycoproteins and proteoglycans) or lipid (to form lipidoglycans) (to form glycolipids). 

With the significant exception of albumin, almost all human plasma proteins are glycoproteins.

Many membrane proteins include significant carbohydrates, and several membrane proteins are attached to the cell membrane by a glycan chain. 

Glycoproteins make up some of the human blood constituents, whereas glycosphingolipids make up others. 

Glycoproteins make up a large portion of peptide hormones. 

Metastasis is a serious problem in cancer, and evidence is mounting that changes in the architecture of glycoproteins as well as other glycoconjugates upon the surface of cancer cells play a role in metastasis.

Glycoproteins are found in many viruses, and some of them serve critical functions in attachment to host cells.

Glycoproteins serve a variety of purposes. 

Their carbohydrate content varies from 1% to more than 85% by weight.

Processes involved in cell development, regular physiology, as well as neoplastic transformation affect the glycan profiles of glycoproteins.

This is the outcome of distinct glycosyltransferase expression patterns under different situations.

How Do Glycoproteins Work?

In one significant way, the physiological content in the sequencing and connections of glucose in glycans varies from that in RNA, DNA, and proteins: it is second rather than primary. 

The sequence of glycosylation of a particular protein is determined by the expression pattern of the numerous glycosyltransferases associated with glycoprotein synthesis in the cell, as well as the affinity of the numerous glycosyltransferases for carbohydrate content and the availability of various carbohydrate substrates. 

Glycoproteins have microheterogeneity as a result of this.

Some of a glycoprotein’s glycan chains are incomplete; others are truncated.

Interactions between glycans with proteins like lectins and other molecules express the content from the sugars. 

Changes in cellular activity result from these interactions.

Decoding the sugar code (one of glycomics’ main goals) includes figuring out all of the exchanges that sugars or sugar-containing molecules are involved in, as well as the consequences of these connections on cellular behaviour.

Techniques Used To Analyse And Synthesize Glycoproteins

The same procedures that are used to clean proteins and enzymes can also be used to purify glycoproteins. 

When a glycoprotein is purified, the architecture of its glycan chains can often be identified using mass spectrometry, high-resolution NMR spectroscopy, and glycosyl micro-arrays. 

The fact that glycoproteins frequently occur as glycoforms—proteins with similar amino acid sequences but micro-heterogeneity in the glycan chains—complicates analysis. 

The precise nature of glycoprotein connections between sugars is critical in shaping their forms and functions.

Complex glycans can now be synthesised and studied for biological and pharmacological activities because of advances in synthetic chemistry. 

In addition, methods for synthesising and secreting glycoproteins of medical relevance into their extracellular environment have been created using organisms such as yeasts.

The physiological functions of glycoproteins can be determined using a variety of glycosidases.  

Endoglycosidases break oligosaccharide chains inside the polypeptide backbone at certain N-acetylglucosamine residues.

They can be used to break up huge oligosaccharide sequences for structural analysis.

Glycoproteins And Fertilization

Fertilization is highly dependent on glycoproteins.

A sperm must pass through the zona pellucida which is a dense, translucent, noncellular membrane that surrounds the egg, in order to reach the oocyte’s plasma membrane. 

The glycoprotein ZP3 works as a sperm receptor and is an O-linked glycoprotein. ZP3 oligosaccharide chains interact with a protein upon the sperm surface. 

This connection causes the acrosomal response, in which enzymes like proteases and hyaluronidase, as well as other contents of the sperm’s acrosome, are liberated. 

These enzymes are released, allowing sperm to penetrate the zona pellucida to access the oocyte’s plasma membrane.

PH-30, another glycoprotein, is involved in both the attachment of the sperm plasma membrane to the oocyte plasma membrane and the ensuing fusing of the two membranes. 

These interactions allow the sperm to penetrate the oocyte and fertilise it.

It may be able to prevent fertilisation by producing medications that interfere with ZP3 and PH-30’s normal actions, so acting as contraceptives.

Selectins Play A Key Role In Lymphocyte Homing And Inflammation

Leukocytes are involved in a variety of inflammatory and immunological processes.

Connections involving migrating leukocytes or endothelial cells precede the passage of the latter out from the circulation in many of these events. 

Selectins, which are cell surface lectins found on leukocytes and endothelial cells, have a role in intercellular adhesion.

Selectins are solitary Ca2+-binding transmembrane proteins with a lectin domain that binds to certain carbohydrate ligands at their amino termini.

Selectins upon the neutrophil cell membrane interact with glycoproteins at the endothelial cell surface to momentarily retain neutrophils, causing them to roll along the endothelium surface.

The neutrophils become activated, alter form, and firmly cling to the endothelium during this process.

Interactions between integrins on endothelial cells cause this adherence. 

Following attachment, neutrophils inject pseudopodia into endothelial cell connections, push through such junctions, pass the basal layer, and are free to travel in the interstitial spaces.

Selectins bind to oligosaccharides that have been sialylated or fucosylated. Sulfated lipids can also act as ligands.

Inhibition of inflammatory reactions may be aided by the development of drugs like monoclonal antibodies which block selectin-ligand interactions.

Selectin ligands are found on the surfaces of many cancer cells, and they may play a role in cancer cell invasion and metastasis.

An Introduction To Glycoproteins (1)

Glycoprotein Roles In Parasites, Viruses, And Bacteria

The fact that glycans attach precisely to proteins as well as other glycans underpins several of their biological activities.

Their capacity to bind viruses, germs, and parasites is one example of this.

Influenza virus A uses the hemagglutinin protein to bind to cell membrane glycoprotein receptors containing N-acetylneuraminic acid.

The virus also possesses a neuraminidase enzyme that allows newly generated progeny to be eluted from infected cells. 

The spread of viruses is significantly reduced if this procedure is stopped.

Blockers of this enzyme (such as zanamivir and oseltamivir) have become available to treat influenza sufferers. 

The AIDS-causing HIV-1 virus binds to cells through  aglycoprotein and fuses with the host cell membrane through a glycoprotein. 

Antibodies are created through HIV-1 infection, and the protein has been considered for use as a vaccine.

One major flaw with this strategy is that the composition can change quickly due to a mutation, enabling the virus to evade neutralisation by antibodies.

Peptic ulcers are caused by the bacteria Helicobacter pylori.

It attaches to two or more distinct glycans found on the surface of stomach epithelial cells, allowing it to form a stable attaching location to the lining of the stomach. 

Furthermore, several bacteria that cause diarrhoea connect to intestinal mucosa surface cells via glycans found within glycoproteins.

A GPI found on the surface of Plasmodium falciparum (a malarial parasite) mediates the pathogen’s adhesion to human cells.

Other Glycoprotein Disease Roles

Rheumatoid arthritis is linked to a change inside the glycosylation of circulation immunoglobulin G (IgG) molecules, causing them to lack galactose in their Fc sections and end in GlcNAc.

Mannose-binding protein is a lectin that links mannose, N-acetylglucosamine, and a few other sugars.

It is produced by the liver and secreted into the bloodstream. 

It can therefore engage agalactosyl IgG molecules, causing the complement system to activate and contribute to persistent inflammation in joint synovial membranes.

Some muscular dystrophies are caused by problems with glycan synthesis in the protein -dystroglycan. 

This protein extends from muscle cells’ surface membranes and interacts with the basal lamina’s laminin-2 (merosin). 

If the glycans of -dystroglycan are not produced correctly (due to mutations in genes producing some glycosyltransferases), the interaction of -DG with laminin is disrupted.

The presence of haemoglobin in urine owing to hemolysis of red cells, especially during sleep, is characterised by paroxysmal nocturnal hemoglobinuria, an acquired mild anaemia characterised by the presence of haemoglobin in urine which may represent a slight decrease in plasma pH throughout sleep, that increases susceptibility to lysis.  

Somatic gene mutations encoding for enzymes that connect glucosamine with phosphatidylinositol inside the GPI structure cause the disease in hematopoietic cells.

As a result, proteins that are GPI-linked with the red cell membrane become deficient.

Which Sugars Are Prominent In Glycoproteins?

There are about 200 monosaccharides in nature, but only eight are usually found in glycoprotein oligosaccharide chains. 

N-acetylneuraminic acids (NeuAc) are frequently found linked to subterminal galactose residues at the ends of oligosaccharide chains. 

The remaining sugars are typically found in more interior locations.

Sulfate is commonly found in glycoproteins, where it is frequently bound to Gal, or GlcNAc, or GalNAc.

The majority of nucleotide sugars are produced in the cytosol, usually through processes involving the nucleoside triphosphate.

In the nucleus, CMP-sialic acids are produced. In mammalian tissues, two processes are required to produce uridine diphosphate galactose (UDP-Gal).

Because many glycosylation processes take place within the Golgi apparatus’s lumen, nucleotide sugars must be transported across the membrane via carriers (permeases and transporters). 

They are antiporter systems in which the outflow of one molecule of the matching nucleotide (UMP, GMP, or CMP) generated from the sugar nucleotide balances the input among one sugar molecule. 

This method guarantees that each nucleotide sugar has a sufficient concentration within the Golgi apparatus.

Galactosyl transferase or nucleoside diphosphate phosphatase catalyse processes that produce UMP from UDP-Gal.

Glycoproteins make up the majority of peptide hormones and plasma proteins.

A terminal N-acetylneuraminic acid component is removed by neuraminidase treatment, revealing the subterminal galactose residues. 

This asialoglycoprotein is removed from the bloodstream significantly more quickly than the undamaged glycoprotein.

An asialoglycoprotein receptor is found in liver cells, and it detects the galactose moiety of several desialylated plasma proteins, causing their endocytosis and degradation.

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Types Of Glycoprotein

Glycoproteins are classified into three groups based on the type of the connection in between oligosaccharide and polypetide chains.

There are further smaller classes of glycoproteins:

  1. O-glycosidic – Those with an O-glycosidic bond (O-linked), which involves the hydroxyl side chain of serine and threonine (and occasionally also tyrosine) and a sugar like N-acetylgalactosamine (GalNAc-Ser[Thr])
  2. N-glycosidic – Those with an N-glycosidic connection (N-linked) between the asparagine amide nitrogen and N-acetylglucosamine (GlcNAc-Asn)
  3. GPI-anchored – Those which are connected to a protein’s carboxyl terminal amino acid by a phosphoryl-ethanolamine moiety attached with an oligosaccharide, which is coupled to phosphatidylinositol via glucosamine (PI). 

They drive glycoproteins towards the apex or basolateral portions of the plasma membrane of polarised epithelial cells, among other things.

The quantity of oligosaccharide chains linked to a single protein can range from one to thirty or more, with sugar chains ranging in length from one to two residues to considerably longer structures.

Glycan chains can be straight or branched. 

Many proteins include multiple sugar chains; for example, glycophorin, an essential red cell membrane glycoprotein, includes O-glycosidic and N-glycosidic oligosaccharide chains.

Human glycoproteins contain at least four types of O-glycosidic linkages:

  • The GalNAc-Ser(Thr) connection is the most common. The N-acetylgalactosamine is usually connected to a galactose or a N-acetylneuraminic acid residue, but there are various variations with sugar contents and lengths of these oligosaccharide chains. Mucins also include this form of connection.
  • A Gal-Gal-Xyl-Ser trisaccharide is found in proteoglycans.
  • Collagens have a Gal-Hydroxylysine (Hyl) connection.
  • Several nuclear and cytosolic proteins have a single N-acetylglucosamine linked to threonine or serine residues as a side chain.

Factors Involved In The Glycosylation Of Glycoproteins

Glycosylation of Glycoproteins is controlled by a number of factors.

Glycoprotein glycosylation is a difficult process involving a lot of enzymes; around 1% of a human genome encodes for genes involved in protein glycosylation. 

At least 10 different GlcNAc transferases exist. Other glycosyltransferases (such as sialyltransferases) come in a variety of species. 

The availability of sugar nucleotides, as well as the existence of appropriate acceptor locations in proteins, the tissue potency of dolichol phosphate, as well as the function of a oligosaccharide: protein transferase, are all controlling variables in the first phase of N-linked glycoprotein biosynthetic pathways (assembly and exchange of a dolichol pyrophosphate oligosaccharide).

When it comes to the manufacture of therapeutic glycoproteins using recombinant DNA technology, species differences among handling enzymes are crucial.

Recombinant erythropoietin (EPO), for example, is used to accelerate erythropoiesis in individuals with certain kinds of chronic anaemia. 

The glycosylation pattern of erythropoietin in plasma affects its half-life; some factors have been associated with short half-life, which limits its therapeutic effectiveness.

It’s crucial to get EPO via host cells that have a glycosylation pattern that corresponds to a normal plasma half-life.

Analyzing the activity of enzymes in diverse cancer cell types is also a hot topic.

These cells have been observed to create oligosaccharide chains that differ from those produced by healthy cells.  

This could be related to cancer cells having distinct glycosyltransferas patterns than normal cells due to particular regulation of gene expression or repression.

Changes in oligosaccharide chains may change the adhesive connections among cancer cells with their parent tissue cells, allowing cancer to spread.

Many proteins undergo glycosylation using a single sugar moiety, N-acetylglucosamine, including nuclear pore proteins, cytoskeleton proteins, transcription factors and proteins involved with chromatin, and also nuclear oncogene proteins and tumour suppressor proteins. 

This is a glycosylation that can be reversed quickly.

Glycosylation and phosphorylation of these proteins occur reciprocally in response to biological signalling, and phosphorylation and glycosylation happen at the same serine and threonine sites.

This glycosylation is catalysed by the O-linked N-acetylglucosamine transferase, which employs UDP-N-acetylglucosamine as the sugar donor and contains phosphatase activity, allowing it to effectively exchange a serine or threonine phosphate using N-acetylglucosamine. 

Although there is no exact typical sequence for the reaction, Pro-Val-Ser is found in almost half of the sites that are prone to reciprocating glycosylation and phosphorylation. 

The enzyme is triggered by insulin activity, and N-acetylglucosamine is eliminated by N-acetylglucosaminidase (leaving the site open for phosphorylation).

The quantity of UDP-N-acetylglucosamine affects both the activity and the peptide selectivity of O-linked N-acetylglucosamine transferase.

Depending on the cell type, the hexosamine route leading to N-acetylglucosamine production accounts for up to 5% of glucose metabolism, providing O-linked N-acetylglucosamine a vital function in nutrition sensing in the cell.

Insulin resistance and glucose poisoning in diabetes mellitus, as well as neurodegenerative disorders, are related to greater O-glycosylation via N-acetylglucosamine.

Summary

Glycoproteins are proteins containing carbohydrate sequences that have a variety of functions.

Many biological processes, such as fertilisation and inflammation, are mediated by glycoproteins. 

A glycoprotein’s carbohydrate content can range from 1%  85% of its weight, and its structure might be simple or quite complex.

O-linked N-linked, and GPI-linked glycoproteins are the three main types.

At least some glycoprotein oligosaccharide chains transmit biological information; they also play a role in altering glycoprotein solubility and viscosity, shielding them from proteolysis, and regulating their biological functions.

A number of illnesses are associated with anomalies in glycoprotein synthesis and breakdown.

Many other disorders, such as AIDS, anaemia, peptic ulcers, and influenza. 

Jennifer Dawkins

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