The iota-carrageenase of Alteromonas fortis - A beta-helix fold-containingenzyme for the degradation of a highly polyanionic polysaccharide

Carrageenans are gel-forming hydrocolloids extracted from the cell walls of marine red algae. They consist Of D-galactose residues bound by alternate alpha (1 -->3) and beta (1 -->4) linkages and substituted by one (kappa -ca rrageenan), two (iota -carrageenan), or three (lambda -carrageenan) sulfate -ester groups per disaccharide repeating unit. Both the kappa- and iota -ca rrageenan chains adopt ordered conformations leading to the formation of hi ghly ordered aggregates of double-stranded helices. Several kappa -carragee nases and iota -carrageenases have been cloned from marine bacteria. kappa -Carrageenases belong to family 16 of the glycoside hydrolases, which essen tially encompasses polysaccharidases specialized in the hydrolysis of the n eutral polysaccharides such as agarose, laminarin, lichenan, and xyloglucan . In contrast, iota -carrageenases constitute a novel glycoside hydrolase s tructural family. We report here the crystal structure of Alteromonas forti s iota -carrageenase at 1.6 Angstrom resolution. The enzyme folds into a ri ght-handed parallel beta -helix of 10 complete turns with two additional C- terminal domains. Glu(245), Asp(247), or Glu(310), in the cleft of the enzy me, 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.