Abstract
BackgroundChitin is a polysaccharide that forms the hard, outer shell of arthropods and the cell walls of fungi and some algae. Peptidoglycan is a polymer of sugars and amino acids constituting the cell walls of most bacteria. Enzymes that are able to hydrolyze these cell membrane polymers generally play important roles for protecting plants and animals against infection with insects and pathogens. A particular group of such glycoside hydrolase enzymes share some common features in their three-dimensional structure and in their molecular mechanism, forming the lysozyme superfamily.ResultsBesides having a similar fold, all known catalytic domains of glycoside hydrolase proteins of lysozyme superfamily (families and subfamilies GH19, GH22, GH23, GH24 and GH46) share in common two structural elements: the central helix of the all-α domain, which invariably contains the catalytic glutamate residue acting as general-acid catalyst, and a β-hairpin pointed towards the substrate binding cleft. The invariant β-hairpin structure is interestingly found to display the highest amino acid conservation in aligned sequences of a given family, thereby allowing to define signature motifs for each GH family. Most of such signature motifs are found to have promising performances for searching sequence databases. Our structural analysis further indicates that the GH motifs participate in enzymatic catalysis essentially by containing the catalytic water positioning residue of inverting mechanism.ConclusionsThe seven families and subfamilies of the lysozyme superfamily all have in common a β-hairpin structure which displays a family-specific sequence motif. These GH β-hairpin motifs contain potentially important residues for the catalytic activity, thereby suggesting the participation of the GH motif to catalysis and also revealing a common catalytic scheme utilized by enzymes of the lysozyme superfamily.
Highlights
Due to a worldwide effort of structural genomics projects, the number of known three-dimensional protein structures rapidly increases [1]
Structural Relationships in the Lysozyme Superfamily As protein families of the lysozyme superfamily do not share any sequence similarity, in order to highlight the relationships among these proteins, we compared their structures by computing pairwise structural similarity scores using the DaliLite program
32 X-ray structures were selected from the protein structure databank
Summary
Due to a worldwide effort of structural genomics projects, the number of known three-dimensional protein structures rapidly increases [1]. Many of them are based on the occurrence of particular clusters of residues, in protein sequence or in protein 3D structure that could give a functional role to the unknown protein [9,10,11,12,13,14]. Such clusters can be called patterns, motifs, signatures or fingerprints, and were accumulated from various protein families in freely accessible databases, such as PROSITE [15], PRINTS [16], BLOCKS [17], MSDmotif [18] or FunClust [19]. A particular group of such glycoside hydrolase enzymes share some common features in their three-dimensional structure and in their molecular mechanism, forming the lysozyme superfamily
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