title: Stability, subunit interactions and carbohydrate-binding of the seed lectin from Pterocarpus angolensis.
abstract: From 1 kg of defatted Pterocarpus angolensis (mukwa tree) seed meal, 21.6 grams of an alpha,D-mannose/glucose-specific lectin can be purified on mannose-Sepharose. Relative affinities for several (oligo)saccharides and glycoproteins were studied by haemagglutination-inhibition. Gel filtration shows that the lectin exists as a dimer above pH 5 and as a monomer below pH 3.5. This is confirmed by studies on the release of lectin subunits that were adsorbed from solution to lectin monomers immobilized onto Eupergit-c. From the gel filtration patterns it is calculated that a residue with pK(a) of about 4.4 is involved in dimer dissociation. Titration of glutamic acids (E60, E209) is postulated to be involved. CD spectroscopy shows that the secondary structure of the lectin is unchanged between pH 1 and 12.5, and that the tertiary structure remains unchanged between pH 5 and 12. In the acid pH region, reversible spectral changes occur that may be due to the titration of one or more amino acids with a pK(a) value of 3.9-4.2, probably aspartic acid. These residues are implicated in sugar-binding but not in dimerization of the lectin. Only at pH 12.5, irreversible denaturation occurs. Mukwa lectin displays full carbohydrate-binding capacity between pH 4 and 12, as is concluded from ELLA (Enzyme Linked Lectin Assay) using ovalbumin and fetuin, and from binding of the same glycoproteins to immobilized lectin monomers. The lectin is rapidly and fully reversibly demetallized at pH 2.5 with 5 mM EDTA. The demetallized lectin is completely devoid of sugar-binding activity. Mukwa lectin is a very thermostable molecule (at least till 85 degrees C). However, addition of non-ionic detergents substantially lowers its thermostability.
authors.affiliation: Laboratory of Protein Chemistry, Institute of Molecular Biology and Biotechnology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium.
authors.forename: Dannele
authors.initials: D
authors.lastname: Echemendia-Blanco
authors.forename: Edilbert
authors.initials: E
authors.lastname: Van Driessche
authors.forename: Ignatious
authors.initials: I
authors.lastname: Ncube
authors.forename: John S
authors.initials: JS
authors.lastname: Read
authors.forename: Sonia
authors.initials: S
authors.lastname: Beeckmans
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry; D002241:Carbohydrates / Q000737:chemistry; D002850:Chromatography, Gel; D002942:Circular Dichroism; D006168:Guinea Pigs; D006385:Hemagglutination Inhibition Tests; D006863:Hydrogen-Ion Concentration; D055767:Immobilized Proteins / Q000737:chemistry; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D037121:Plant Lectins / Q000737:chemistry* / Q000302:isolation & purification; D010940:Plant Proteins / Q000737:chemistry*; D011489:Protein Denaturation; D055503:Protein Multimerization; D055550:Protein Stability; D020836:Protein Structure, Quaternary / Q000187:drug effects; D017433:Protein Structure, Secondary / Q000187:drug effects; D017434:Protein Structure, Tertiary / Q000187:drug effects; D035721:Pterocarpus / Q000737:chemistry; D011817:Rabbits
chemical_list: D002241:Carbohydrates; D055767:Immobilized Proteins; D037121:Plant Lectins; D010940:Plant Proteins; D008345:Manganese; D002118:Calcium