Polylactosamine glycosylation on human fetal placental fibronectin weakens the binding affinity of fibronectin to gelatin
Abstract
Gelatin-binding chymotryptic fragments from placental fibronectin contain polylactosamine carbohydrates. We have separated polylactosamine-containing gelatin-binding fragments of placental fibronectin from their counterparts containing smaller 'complex' N-linked saccharides using Sephadex G-200 gel permeation chromatography. The peptide portions of both fragments have similar amino acid composition and N-terminal sequence. The strength of binding of these two glycosylation types of chymotryptic fragments to gelatin differs as shown by the following experiments. 1) Upon urea gradient elution of affinity-bound fibronectin fragments from gelatin-Sepharose chromatography, the apex of the elution peak for polylactosamine-containing fragments occurs at 2.0 M urea while the peak for complex N-linked carbohydrate-containing fragments maximized at 2.5 M urea indicating a tighter binding. Removal of polylactosamine sequences from the former glycopeptide by endo-β-galactosidase digestion caused the elution peak for this fragment to change from 2.0 to 2.5 M, the same as for the complex N-linked carbohydrate-containing glycopeptide. 2) Competitive displacement experiments give an apparent dissociation constant of polylactosamine-containing fragments at 3 x 10-9 M whereas this constant for complex carbohydrate-containing fragments is 1 x 10-9 M. These results indicate that the binding of placental fibronectin to gelatin is weakened by the presence of high molecular weight polylactosamine carbohydrate. To our knowledge this is the first report that the type and extent of glycosylation of a glycoprotein can affect its binding affinity to a proteinacious ligand. Thus, fetal placental fibronectin may have different biological properties than fibronectins containing only the smaller N-linked complex carbohydrate.