A -HELIX FOLD-CONTAINING ENZYME FOR THE DEGRADATION OF A HIGHLY POLYANIONIC

Carrageenans are gel-forming hydrocolloids extracted from the cell walls of marine red algae. They consist of D-galactose residues bound by alternate (133) and (134) linkages and substituted by one (carrageenan), two (-carrageenan), or three (-carrageenan) sulfate-ester groups per disaccharide repeating unit. Both the - and -carrageenan chains adopt ordered conformations leading to the formation of highly ordered aggregates of double-stranded helices. Several -carrageenases and -carrageenases have been cloned from marine bacteria. -Carrageenases belong to family 16 of the glycoside hydrolases, which essentially encompasses polysaccharidases specialized in the hydrolysis of the neutral polysaccharides such as agarose, laminarin, lichenan, and xyloglucan. In contrast, -carrageenases constitute a novel glycoside hydrolase structural family. We report here the crystal structure of Alteromonas fortis -carrageenase at 1.6 A resolution. The enzyme folds into a right-handed parallel -helix of 10 complete turns with two additional C-terminal domains. Glu 245 , Asp 247 ,o r Glu 310 , in the cleft of the enzyme, are proposed as candidate catalytic residues. The protein contains one sodium and one chloride binding site and three calcium binding sites shown to be involved in stabilizing the enzyme structure.