The plant cytotoxin ricin is a heterodimer. The A chain, known to be a specific N-glycosidase, has a prominent active site cleft. The B chain is a two-domain lectin, which arose from the replication of a primitive sugar binding peptide. The B chain subunit of ricin (RTB)1 binds to mammalian cell membranes by recognizing galactose-containing receptors. RTB has two domains each with three subdomains; tripeptide kinks in the loops from subdomains 1alpha, 1beta, 2alpha, and 2gamma may interact with galactosides. Each of these subdomains has aromatic residues that can interact with the nonpolar face of galactose, and three of the four subdomain folds (1alpha, 1beta, and 2gamma) have polar residues for hydrogen bond formation to the sugar hydroxyls .
The family 10 xylanase from Streptomyces olivaceoviridis E-86 contains a (beta/alpha)(8)-barrel as a catalytic domain, a family 13 carbohydrate binding module as a xylan binding domain (XBD) and a Gly/Pro-rich linker between them. The crystal structure of this enzyme showed that XBD has three similar subdomains, as indicated by the presence of a triple-repeated sequence, forming a galactose binding lectin fold similar to that found in the ricin toxin B-chain .