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Publication Abstract from PubMed

The mechanism by which polysaccharide-hydrolysing enzymes manifest specificity towards heterogeneous substrates, in which the sequence of sugars is variable, is unclear. An excellent example of such heterogeneity is provided by the plant structural polysaccharide glucomannan, which comprises a backbone of 1,4-linked glucose and mannose units. -Mannanases, located in glycoside hydrolase (GH) families 5 and 26, hydrolyse glucomannan by cleaving the glycosidic bond of mannosides at the -1 subsite. The mechanism by which these enzymes select for glucose or mannose at distal subsites, which is critical to defining their substrate specificity on heterogeneous polymers, is currently unclear. Here we report the biochemical properties and crystal structures of both a GH5 and GH26 mannanase and describe the contributions to substrate specificity in these enzymes. The GH5 enzyme, BaMan5A, derived from Bacillus agaradhaerens, can accommodate glucose or mannose at both its -2 and +1 subsites, while the GH26 Bacillus subtilis mannanase, BsMan26A, displays tight specificity for mannose at its glycone binding sites. The crystal structure of BaMan5A reveals that a polar residue at the -2 subsite can make productive contact with -substrate 2-OH in either its axial (as in mannose) or equatorial (as in glucose) configuration, while other distal subsites do not exploit the 2-OH as a specificity determinant. Thus BaMan5A is able to hydrolyse glucomannan in which the sequence of glucose and mannose residues is highly variable. The crystal structure of BsMan26A in light of previous studies on the Cellvibrio japonicus GH26 mannanases CjMan26A and CjMan26C, reveals that the tighter mannose recognition at the -2 subsite is mediated by polar interactions with the axial 2-OH of a 4C1 ground state mannoside. Mutagenesis studies showed that variants of CjMan26A, in which these polar residues are removed, do not distinguish between Man and Glc at the -2 subsite, while one of these residues, Arg 361, confers the elevated activity displayed by the enzyme against mannooligosaccharides. The biological rationale for the variable recognition of Man and Glc configured sugars by -mannanases is discussed.

Understanding how diverse -mannanases recognise heterogeneous substrates.,Tailford LE, Ducros VM, Flint JE, Roberts SM, Morland C, Zechel DL, Smith N, Bjornvad ME, Borchert TV, Wilson KS, Davies GJ, Gilbert HJ Biochemistry. 2009 May 14. PMID:19441796^[2]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.