Review Question - QID 4453

Aggrecan molecules bind to hyaluronic acid molecules via link proteins to form a macromolecule complex, known as a proteoglycan aggregate, which attracts water.

Proteoglycans are composed of subunits known as glycosaminoglycans. Glycosaminoglycans include two subtypes: chondroitin sulfate and keratin sulfate. These glycosaminoglycans link to a protein core by sugar bonds to form an aggrecan molecule. Link proteins then stabilize many of these aggrecan molecules to hyaluronic acid to form the proteoglycan aggregate. Cartilage also contains ancillary proteoglycans that are much smaller than the aggregating proteoglycans. These small proteoglycans include decorin, biglycan, and fibromodulin. They bind to other molecules (eg, type II collagen) and assist in matrix stabilization.

Ulrich-Vinther et al. authored a Level 5 review on cartilage structure. The negative charge present within the hyaline cartilage extracellular matrix attracts cations and results in an increase in tissue osmolality. This then attracts water, which decreases the osmolality. Thus, articular cartilage has a high tissue pressure, but the presence of type II collagen matrix prevents it from swelling.

Nap et al. present a basic science review article on aggrecans. They discus that the main function of aggrecan in cartilage is to resist compressive forces. They note that the negative charge of the aggrecan molecule disaccharides create the high osmotic swelling pressure of cartilage.

Illustration A depicts the molecular organization of an aggregated proteoglycan molecule.

Incorrect Answers:
Answer 2,3,4: These are small proteoglycans that bind to other molecules (eg, type II collagen) and assist in matrix stabilization.
Answer 5: Osteocalcin is the most prevalent noncollagenous protein in bone.