Proteoglycans are high molecular-weight polymers of sulfated sugars bound to proteins. Proteoglycans consist of a core protein (collagen) with multiplesulfated carbohydrates (glycosaminoglycans) bound. The sulfate groups provide the negative charge that attracts and binds water.
Proteoglycans are categorized by size and the type of glycosaminoglycan chain that it contains (chondroitin, keratin or heparin).
Proteoglycans are the filler material between cells. They are bound to each other, to the protein of collagen and in the case of articular cartilage, they are bound to hyaluronan, the major component of synovial fluid and the extracellular coat of cartilage cells.
Proteoglycan production is greatly accelerated during fractures, injuries, wound healing and arthritis. Proteoglycans are necessary for proper joint function.
These protein sugar complexes are not only integral to ligaments and tendons but heart valves as well.
Proteoglycans are found in high concentration in lubricating fluids such as the synovial fluid of joints or the aqueous and vitreous humors of the eye.
Proteoglycans are also found in the protective mucus of the body. Its versatility is based on the presence of the protein collagen and the need for collagen to possess tensile strength.
Dietary supplements that contain cartilage powders such as shark skeleton and bovine trachea are recommended because they strengthen collagen and also enhance wound healing.
The active components of these powders, which provide the increase in tensile strength, are proteoglycans.
These proteoglycans contain chondroitin sulfates, compounds which inhibit degradation and stimulate new glycosaminoglycan synthesis
Chondroitin sulfate is a sulfated glycosaminoglycan, composed of alternating sugars of n-acetylgalactosamine and glucuronic acid. Chondroitin chains are fairly long (up to 100
sugars), which are sulfated at a number of positions Chondroitin sulfate is normally attached to the amino acid serine to form proteoglycans and as such, is an important structural component of cartilage that provides strength and resistance to compression.
Chondroitin sulfates also provide a protective effect on chondrocytes (cartilage cells) and helps regulate the production of its connective tissue matrix.
Hyaluronan or hyaluronic acid is a very long polymer of repeating units of glucuronic acid and N-acetyl-glucosamine.
This chain forms a slimy, viscous gel that lubricates the joints and provides an excellent resistance to friction.
Hyaluronan is essential for connective tissue repair. During inflammation and repair hyaluronan is incorporated into the
matrix of joints to improve cushioning. Due to the large molecular weight of hyaluronan it is not absorbed in whole
from the intestine and thereby probably has no value as a dietary supplement. Glucosamine is recommended instead.
Doctors now give intra-articular (within joints) injections of hyaluronan in knees that no longer have cushioning in its
cartilage. These intra-articular injections are classified as pharmaceutical drugs and does improve cartilage cushioning.
Repair of cartilage is the core of many of the cures of arthritis. These include the unconventional use of magnets and
leeches to stimulate healing as well as the pharmaceutical approach, which relies on drugs.
Chondroitin sulfate has been coupled to glucosamine and marketed to babyboomers as treatments for osteoarthritis. In
this program glucosamine and chondroitin sulfate are recommended early in life to prevent arthritis.
Proteoglycans are also found in vascular smooth muscle cells and contribute to an anticoagulant, non-stick surface. The nonstick
surface prevents or minimizes the adhesion of oxidized LDL-cholesterol to blood vessel linings. Less proteoglycans
increase the stickiness in blood vessels.
Due to its high concentration of sulfates, proteoglycans carry a negative charge. This negative charge attracts and holds water
molecules. Collagen is thus a hydrated gel whose stiffness contributes to the resiliency and shock-absorbing strength of
joints. Collagen also has important lubricating properties as demonstrated in the eye.
The matrix of cartilage contains 60-80% water. During joint movement, when the joint is subject to pressure the movement
of water occurs. Its high degree of hydration and flow of water allows cartilage to respond to the increased demands and varying
pressure loads that cartilage is often subjected to.
The ability of damaged cartilage to repair itself is extremely limited. As pointed out, the main biochemical change in
osteoarthritis is a decrease in proteoglycan concentration. This decrease in proteoglycans results in articular cartilage softening
and a loss of elasticity.
The cells of cartilage or the chondrocytes, unable to keep up with the loss of proteoglycans, are forced to synthesize two
enzymes, proteoglycanase and collagenase. These enzymes breakdown their respective macromolecules, further degrading
the collagen matrix and damaging the joint that it is part of.
Collagen strength and elasticity is improved by an increase in proteoglycans, the matrix that bathes collagen.
Proteoglycans are produced in higher amounts when glucosamine is available. Half of the structure of proteoglycans
is made up of glucosamine complexed to keratin and chondroitin sulfates.