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Nature of the Aggregate Linkage

The linkage of proteoglycan subunit, aggrecan, to hyaluronan is noncovalent, which contrasts with the covalent linkage of glycosaminoglycans to core protein in aggrecan. The aggregate linkage is facilitated and strengthened by low-molecular-weight proteins termed link proteins .

The linkage can occur without the presence of a link protein, but such proteins have been found in all cartilages so far examined. The noncovalent linkage of aggrecan and hyaluronan can be dissociated by concentrated solutions of guanidinium hydrochloride, calcium chloride, or magnesium chloride.

The dissociated components can be reassociated by a reduction in the concentration of the dissociative solvents. Under conditions of approximately 0.5 M guanidinium hydrochloride, the three elements hyaluronan, link protein, and aggrecan reassociate to form the aggregate once again.

This process has been the key technique in unraveling the chemical structure of aggrecan and aggregate, in understanding the nature of their association, and in deciphering the role of link proteins. Cartilages from different sources possess different percentages of aggregation of proteoglycan, but the factors controlling this process are not yet fully understood.

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