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glycoside hydrolase family 64 (beta-1,3-glucanases which produce specific pentasaccharide oligomers) and thaumatin-like proteins This superfamily includes glycoside hydrolases of family 64 (GH64), these are mostly bacterial beta-1,3-glucanases which cleave long-chain polysaccharide beta-1,3-glucans, into specific pentasaccharide oligomers and are implicated in fungal cell wall degradation. Also included in this superfamily are thaumatin, the sweet-tasting protein from the African berry Thaumatococcus daniellii, and thaumatin-like proteins (TLPs) which are involved in host defense and a wide range of developmental processes in fungi, plants, and animals. Like GH64s, some TLPs also hydrolyze the beta-1,3-glucans of the type commonly found in fungal walls. Plant TLPs are classified as pathogenesis-related (PR) protein family 5 (PR5), their expression is induced by environmental stresses such as pathogen/pest attack, drought and cold. Several members of the plant TLP family have been reported as food allergens from fruits, and pollen allergens from conifers. Streptomyces matensis laminaripentaose-producing, beta-1,3-glucanase (GH64-LPHase), and TLPs have in common, a core N-terminal barrel domain (domain I) composed of 10 beta-strands, two coming from the C-terminal region of the protein. In TLPs, this core domain is flanked by two shorter domains (domains II and III). Small TLPs, such as Triticum aestivum thaumatin-like xylanase inhibitor, have a deletion in the third domain (domain II). GH64-LPHase has a second C-terminal domain which corresponds positional to, but is much larger than, domain III of TLP. GH64-LPHase and TLPs are described as crescent-fold structures. Critical functional residues, common to GH64-LPHase and TLPs are a Glu and an Asp residue. LPHase has an electronegative, substrate-binding cleft and the afore mentioned conserved Glu and Asp residues are the catalytic residues essential for beta-1,3-glucan cleavage. In TLPs, these residues are two of the four conserved residues which contribute to the strong electronegative character of the cleft which is associated with the antifungal activity of TLPs. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",