General molecular structure overview
Proteoglycans differ fundamentally from most conventional glycoproteins in their chemical composition: the protein portion accounts for about 5% of the mass, while the remaining 95% consists of long carbohydrate chains — glycosaminoglycans (GAGs).
The core protein acts as the central element of the molecule, representing a single polypeptide chain (its molecular mass can vary greatly). Glycosaminoglycans are covalently attached to this protein core. Important exception: hyaluronic acid never binds covalently to a protein.
Carbohydrate chains attach to the protein at strictly defined sites:
- Via the OH group of serine or threonine amino acids (an O-glycosidic bond is formed).
- Via the $NH_2$ group of asparagine (an N-glycosidic bond).
A typical attachment region (using serine as an example) includes a specific linking trisaccharide: Xylose — Galactose — Galactose. A long polysaccharide chain consisting of repeating disaccharide units (hexuronic acid and amino sugar) is then built onto it.
Aggrecan — the basis of cartilage matrix
The most famous representative of proteoglycans is aggrecan. It is a giant molecule that serves as the main structural component of the cartilage tissue matrix.
The aggrecan core protein has a mass of about 220 kDa and is structurally divided into three globular domains: G1, G2 (at the N-terminus) and G3 (at the C-terminus). Carbohydrate chains attach predominantly in the region between the G2 and G3 domains. An impressive amount of GAGs attaches to a single such protein:
- About 100 chains of long chondroitin sulfate (make up the bulk of the mass).
- About 30 chains of keratan sulfate (located closer to the N-terminus, between the G2 domain and the chondroitin sulfate zone).
Supramolecular aggregates
In the intercellular substance, aggrecan monomers do not float individually. They combine into colossal supramolecular complexes.
The central axis of such a complex is formed by a long strand of hyaluronic acid. About 100 aggrecan molecules are non-covalently attached to it (via the G1 domain). To prevent this massive structure from falling apart, it is additionally stabilized by about 100 molecules of a small link protein (link protein), which anchors aggrecan to the hyaluronic acid.
As a result, a giant complex with a molecular mass of over $200 \cdot 10^6$ Daltons is formed, resembling a dishwashing brush, where the wire is hyaluronic acid and the bristles are aggrecan monomers.
Functions and physiological role
The main localization of proteoglycans is the intercellular substance of tissues that are constantly subjected to mechanical deformations and high loads. These are articular cartilage, intervertebral discs, menisci, ligaments, tendons, and skin.
Due to a huge number of negatively charged groups (sulfate and carboxyl), glycosaminoglycans possess extremely high hydrophilicity — they attract and firmly retain water. This ensures the cushioning function. Under mechanical pressure, proteoglycans compress, releasing part of the water, and upon load removal, instantly return it back, absorbing impacts.
Basement membrane proteoglycans
A separate important group consists of heparan sulfate proteoglycans (HSPGs), which are a key element of basement membranes.
According to their structure, they are divided into two varieties:
- Low-density HSPGs (have a large multi-domain protein core and three very long heparan sulfate chains).
- High-density HSPGs (distinguished by a small core and four short carbohydrate chains).
HSPG molecules are strong polyanions (carry a pronounced negative charge). Due to this, in the renal glomeruli, they create a powerful electrostatic filtration barrier that repels other negatively charged plasma molecules, preventing them from penetrating into the primary urine.
Briefly: answers to topical questions
What pathological conditions are associated with genetic defects in glycosaminoglycan degradation?
Mucopolysaccharidoses are hereditary enzymopathies involving connective tissue metabolism disorders. For hereditary enzymopathies, the mechanism is described as follows: a defect in one of the enzymes of the metabolic pathway leads to the accumulation of intermediate products and a deficiency of final products. The list of nosologies under Government Resolution No. 1416 mentions types I, II, and IV mucopolysaccharidoses.
To easily remember the trisaccharide in the GAG-protein attachment region, use the phrase: "Xyphon Gali and Gali" (Xylose — Galactose — Galactose).
Class and exam questions
Why is human height a couple of centimeters taller in the morning than in the evening?
During the day, under the action of gravity and walking, water is mechanically squeezed out of the intervertebral discs, they flatten, and height decreases. At night, in a horizontal position, the load disappears, and due to the high hydrophilicity of proteoglycans, water returns to the discs, restoring their height.
Which amino acid of the core protein does not participate in GAG attachment: serine, threonine, asparagine, or alanine?
Alanine. Glycosaminoglycans attach covalently only via the hydroxyl groups of serine and threonine (O-glycosidic bond) or via the amino group of asparagine (N-glycosidic bond).
What is the main functional significance of HSPGs in the kidneys?
Heparan sulfate proteoglycans of the glomerular basement membrane work as an electrostatic filter. Being polyanions, they prevent the loss of plasma proteins (which are also negatively charged) in the urine.
What else to study on this topic
- Structural organization of the G1, G2, and G3 globular domains of the aggrecan core protein.
- Biochemical mechanism of interaction of the G1 domain with the link protein and hyaluronic acid.
- Differences in the structure of high- and low-density heparan sulfate proteoglycans.
- Structure of repeating disaccharide units (hexuronic acid + amino sugar) of various GAGs.
- The role of proteoglycans in mediating cell interactions with the extracellular matrix.
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