MBE Advance Access originally published online on July 22, 2009
Molecular Biology and Evolution 2009 26(11):2487-2497; doi:10.1093/molbev/msp162
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Research Articles |
Evolution and Biochemistry of Family 4 Glycosidases: Implications for Assigning Enzyme Function in Sequence Annotations
* Bellingham Research Institute, Bellingham, WA
Microbial Biochemistry and Genetics Unit, Oral Infection and Immunity Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD
E-mail: barryhall{at}zeninternet.com.
Accepted for publication July 17, 2009.
Glycosyl hydrolase Family 4 (GH4) is exceptional among the 114 families in this enzyme superfamily. Members of GH4 exhibit unusual cofactor requirements for activity, and an essential cysteine residue is present at the active site. Of greatest significance is the fact that members of GH4 employ a unique catalytic mechanism for cleavage of the glycosidic bond. By phylogenetic analysis, and from available substrate specificities, we have assigned a majority of the enzymes of GH4 to five subgroups. Our classification revealed an unexpected relationship between substrate specificity and the presence, in each subgroup, of a motif of four amino acids that includes the active-site Cys residue: 
-glucosidase, CHE(I/V); -galactosidase, CHSV; -glucuronidase, CHGx; 6-phospho--glucosidase, CDMP; and 6-phospho-β-glucosidase, CN(V/I)P. The question arises: Does the presence of a particular motif sufficiently predict the catalytic function of an unassigned GH4 protein? To test this hypothesis, we have purified and characterized the -glucoside–specific GH4 enzyme (PalH) from the phytopathogen, Erwinia rhapontici. The CHEI motif in this protein has been changed by site-directed mutagenesis, and the effects upon substrate specificity have been determined. The change to CHSV caused the loss of all -glucosidase activity, but the mutant protein exhibited none of the anticipated -galactosidase activity. The Cys-containing motif may be suggestive of enzyme specificity, but phylogenetic placement is required for confidence in that specificity. The Acholeplasma laidlawii GH4 protein is phylogenetically a phospho-β-glucosidase but has a unique SSSP motif. Lacking the initial Cys in that motif it cannot hydrolyze glycosides by the normal GH4 mechanism because the Cys is required to position the metal ion for hydrolysis, nor can it use the more common single or double-displacement mechanism of Koshland. Several considerations suggest that the protein has acquired a new function as the consequence of positive selection. This study emphasizes the importance of automatic annotation systems that by integrating phylogenetic analysis, functional motifs, and bioinformatics data, may lead to innovative experiments that further our understanding of biological systems.
Key Words: glycosyl hydrolases • family GH4 • phylogenetics • genomic annotation • enzyme specificity
Michele Vendruscolo, Associate Editor