The sulphated-galactan hydrolases, agarases and carrageenases: structural biology and molecular evolution

The carrageenans and agars are major cell-wall polysaccharides from red alg ae. These sulphated galactans are degraded by enzymes, called carrageenases and agarases that display strict substrate specifities and recognize the p attern of galactan sulphation. From a set of various marine bacteria enzyme s, we have investigated the influence of ester-sulphate groups, of D/L isom ery and of linkage anomery on the structure-function relationships of the s pecific galactan hydrolases that degrade sulphated polysaccharides. With th is aim, we have cloned a representative set of sulphated-galactan hydrolase genes. The sequence analysis methods indicate that the beta -agarases and kappa -carrageenases display secondary structure similarities with members of family 16 of glycoside hydrolases. In contrast, the iota -carrageenases have no structural relationships with the family-16 beta -agarases and kapp a -carrageenases and they constitute a novel structural family of glycan hy drolases. As a preliminary step towards the functional analysis of these tw o structural families, we have overexpressed the iota- and kappa -carrageen ase genes in Escherichia coli and crystals from these enzymes have been obt ained. Finally, an alpha -agarase, the only one galactanase known to cleave the alpha -1,3 linkage in agarose has no similarity with other glycoside h ydrolases or proteins and display some interesting characteristics. To date , this enzyme is an unclassified glycoside hydrolase.