Interaction of carbohydrate and protein in thyroxine binding globulin.

The fluorescence properties of human thyroxine binding globulin were evaluated during enzymatic deglycosylation by using both neuraminidase and a mixture of glycosidases. Three fluorescent chromophores, one intrinsic and two extrinsic, were monitored, and all showed changes in fluorescent parameters that have been interpreted in terms of a loss of interactions between the carbohydrate and amino acid residues during deglycosylation. The loss of carbohydrates also results in a decrease in stability of the protein to both acid and guanidinium chloride inactivation. Since deglycosylation decreases the frictional ratio of thyroxine binding globulin, it is concluded that, although sialic acid and other sugar residues are in contact with the protein surface, the hydrated carbohydrate chains protrude partially into the solvent.