Last modified 19th May '95 © Birkbeck College 1995

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Glycosidases

These enzymes cleave oligosaccharides by hydrolysis. Some examples:

A number of enzymes based on the alpha/beta barrel fold

Cellulases

Various species of fungi produce enzymes which hydrolyze cellulose. Trichoderma reesei produces two endoglucanases, endoglucanase I (EG1) and endoglucanase II (EGII) and two exoglucanases, cellobiohydrolase I (CBHI) and cellobiohydrolase II (CBHII). All four of these enzymes contain a highly homologous 36-residue region called the A domain (here is a picture from The Swiss-3DImage Collection), connected to the (enzymatically active) core domain by a threonine- and serine-rich linking sequence. The A domain has no catalytic activity in CBHI and CBHII but is thought to have a cellulose-binding function, as the core protein alone does not have full cellulose-hydrolyzing activity (but has normal activity on small synthetic substrates). In EGI and CBHI the A domain is at the C-terminus, while it is at the N-terminus in EGII and CBHII.

CBHI and CBHII produce cellobiose (a disaccharide). The substrate of CBHII requires at least three consecutive ß(1-4)-bonded glycosidic units for the enzyme to cleave a glycosidic bond. The proposed active site has four subsites (Rouvinen et al., 1990).

Two extensive loops (residues 172-189, 394-429) occur at the C-terminal end of the alpha/beta barrel, which form an enclosed tunnel approximately 20Å in length, in which the substrate is believed to be bound.
Here is a view down this tunnel (the loop residues mentioned above are coloured green) and a view down the axis of the alpha/beta barrel, in which the positions of some of the more important residues of the active site are coloured yellow. Examine the pdb structure file (C-alpha atoms only).

Alpha-amylase (Taka Amylase A)

pdb file diagram

Acid alpha-amylase

pdb file diagram

Beta-amylase

pdb file

Glycosidases with other folds

Pectate Lyase

pdb file diagram

Glucoamylase [check this- what fold is it?]

pdb file diagram

Neuraminidase

pdb file
refer to this section in the Chapter on Protein Folds

Xylanase

pdb file
diagram 1 diagram 2

Lysozyme

pdb file
Click here for the page on lysozyme.

References

Refer to the page on glycosidases in the Enzyme Structures Database at UCL, and also the glyosidases page of the Enzyme nomenclature database at ExPASy .

Kinemages

Here is the index to the Protein Science Kinemages on Glycosidases .


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J. Walshaw