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All characterised members of glycoside hydrolase family 82 enzymes cleave the β-1,4 galactosidic bond of the marine algal polysaccharide iota-carrageenan [1, 2, 3] yielding products of the neocarrabiose series.
Kinetics and Mechanism
Family 82 enzymes operate with an inverting mechanism, as first shown by NMR [1] on the iota-carrageenase from Alteromonas fortis. The reaction seems to involve a chloride ion which through the Glu222 participated in the polarisation of the catalytic water molecule [2].
Catalytic Residues
From structural analysis the catalytic residues have been predicted to be two out of the three candidate amino acids Glu245, Asp247 or Glu310 in the A. fortis iota-carrageenase [4]. A study in 2010, which utilized site directed mutagenesis, has confirmed that Glu245 plays the role of the general acid residue in this inverting enzyme, and Asp247 is the general base activating the nucleophilic water molecule [2]. However, intriguingly the position of equivalent residues to Asp247 in other iota-carrageenase sequences are not strictly conserved [2, 3].
Three-dimensional structures
Figure 1: Movement of domain A induced by substrate binding. The β-helix is in blue, domain A is in red and domain B is in green.
A crystal structure has only been determined for the iota-carrageenase from A. fortis and the protein folds into a right-handed parallel β-helix of 10 complete turns with two additional C-terminal domains (A and B) [4]. The crystal structure of a product complex has shed light on the existence of domain movement of domain A that is closed around the oligo-carrageenan in the complexed form and open in the uncomplexed enzyme (Figure 1) [5]. Recent discovery of shorter sequences indicates the existence of enzymes devoid of the C-terminal domain A or both domains A and B [2].
Family Firsts
First sequence identification and family creation
iota-carrageenase sequences have been first reported for enzymes from A. fortis and Z. galactanivorans [1].
