This domain has a fibronectin type III-like structure (PUBMED:10368285). It is often found in association with Glycoside hydrolase family 3. Its function is unknown.
Structural and functional analyses of beta-glucosidase 3B from Thermotoganeapolitana: a thermostable three-domain representative of glycoside hydrolase 3.
J Mol Biol. 2010; 397: 724-39
Display abstract
Based on sequence and phylogenetic analyses, glycoside hydrolase (GH) family 3can be divided into several clusters that differ in the length of their primarysequences. However, structural data on representatives of GH3 are still scarce,since only three of their structures are known and only one of them has beenthoroughly characterized-that of an exohydrolase from barley. To allow a deeperstructural understanding of the GH3 family, we have determined the crystalstructure of the thermostable beta-glucosidase from Thermotoga neapolitana, whichhas potentially important applications in environmentally friendly industrialbiosynthesis at a resolution of 2.05 A. Selected active-site mutants have beencharacterized kinetically, and the structure of the mutant D242A is presented at 2.1 A resolution. Bgl3B from Th. neapolitana is the first example of a GH3glucosidase with a three-domain structure. It is composed of an (alpha/beta)(8)domain similar to a triose phosphate isomerase barrel, a five-stranded alpha/betasandwich domain (both of which are important for active-site organization), and aC-terminal fibronectin type III domain of unknown function. Remarkably, thedirection of the second beta-strand of the triose phosphate isomerase barreldomain is reversed, which has implications for the active-site shape. The active site, at the interface of domains 1 and 2, is much more open to solvent than the corresponding site in the structurally homologous enzyme from barley, and onlythe -1 site is well defined. The structures, in combination with kinetic studies of active-site variants, allow the identification of essential catalytic residues(the nucleophile D242 and the acid/base E458), as well as other residues at the-1 subsite, including D58 and W243, which, by mutagenesis, are shown to beimportant for substrate accommodation/interaction. The position of thefibronectin type III domain excludes a direct participation of this domain in therecognition of small substrates, although it may be involved in the anchoring of the enzyme on large polymeric substrates and in thermostability.
