Two types of rat mannose-binding proteins (MBPs), MBP-A (serum type) and MBP-C (liver type), have similar binding specificity for monosaccharide and similar binding site construct according to the X-ray structure, but exhibit different affinity toward natural oligosaccharides and glycoproteins. To understand the basis for this phenomenon, we used cloned fragment of MBP-A and -C (entire carbohydrate-recognition domain and a short connecting piece) that exists as stable trimers in various binding studies. Binding of a number of mannose-containing di- and tri-saccharides and high-mannose type oligosaccharides indicated that MBP-C has an extended binding area of weak interaction with the second and the third mannose residues, whereas MBP-A recognizes just a single mannose residue. In addition, MBP-C has a weak secondary binding site some 25 Å away from the primary site. These findings explain the higher affinity of MBP-C for natural high-mannose type oligosaccharides as compared to MBP-A. A huge affinity differential manifested by natural glycoproteins (e.g., inhibitory potency of thyroglobulin is ∼200 fold higher for MBP-C than for MBP-A in a solid-phase assay) may be due to steric hindrance experienced by MBP-A in the competition assay, and suggests different arrangement of subunit in the MBP trimers.

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