Structural Basis for the Carbohydrate Specificities of Artocarpin: Variation in the Length of a Loop as a Strategy for Generating Ligand Specificity

Abstract

Artocarpin, a tetrameric lectin of molecular mass 65 kDa, is one of the two lectins extracted from the seeds of jackfruit. The structures of the complexes of artocarpin with mannotriose and mannopentose reported here, together with the structures of artocarpin and its complex with Me-amannose reported earlier, show that the lectin possesses a deep-seated binding site formed by three loops. The binding site can be considered as composed of two subsites; the primary site and the secondary site. Interactions at the primary site composed of two of the loops involve mainly hydrogen bonds, while those at the secondary site comprising the third loop are primarily van der Waals in nature. Mannotriose in its complex with the lectin interacts through all the three mannopyranosyl residues; mannopentose interacts with the protein using at least three of the five mannose residues. The complexes provide a structural explanation for the carbohydrate specificities of artocarpin. A detailed comparison with the sugar complexes of heltuba, the only other mannose-specific jacalinlike lectin with known three-dimensional structure in sugar-bound form, establishes the role of the sugar-binding loop constituting the secondary site, in conferring different specificities at the oligosaccharide level. This loop is four residues longer in artocarpin than in heltuba, providing an instance where variation in loop length is used as a strategy for generating carbohydrate specificity.

Multiplicity of carbohydrate-binding sites in β-prism fold lectins: occurrence and possible evolutionary implications

“…The stereo-specifi c selectivity of plant lectins has been exploited in a wide variety of applications, such as purifi cation of glycoproteins, markers for cancer cells, antimicrobial agents and drug delivery (Lehr and Gabor 2004). Studies on plant lectins have also contributed substantially to the understanding of the structure and assembly of proteins and strategies for generating ligand specifi city Delbaere et al 1993;Banerjee et al 1994;Rini 1995;Elgavish and Shaanan 1998;Jeyaprakash et al 2004;Jeyaprakash et al 2005).…”

Real-time monitoring and kinetic parameter estimation of the affinity interaction of jArtinM and rArtinM with peroxidase glycoprotein by the electrogravimetric technique

“…The QCM transducer was strategically functionalized to use a simple model of protein binding kinetics. This approach allowed for the determination of the binding/dissociation kinetics rate and affinity equilibrium constant of both forms of ArtinM with horseradish peroxidase glycoprotein (HRP), a N-glycosylated protein that contains the trimannoside Man␣1-3[Man␣1-6]Man, which is a known ligand for jArtinM (Jeyaprakash et al, 2004). Monitoring of the real-time binding of rArtinM shows that it was able to bind HRP, leading to an analytical curve similar to that of jArtinM, with statistically equivalent kinetic rates and affinity equilibrium constants for both forms of ArtinM.…”

Cloning and Expression of a Mannose-Binding Jacalin-Related Lectin from Leaves of Japanese Cycad (Cycas revolutaThunb.)

“…The three-dimensional structures of some mJRLs (Heltuba, 19) Artocarpin, 20,21) Calsepa, 22) MornigaM, 23) and banana lectin 24,25) ) have been reported. In Heltuba, six amino acid residues (G18, M92, G135, D136, V137, and D139) were involved in the binding of mannose residue.…”