Members of this family are found in both fungi, bacteria and wood-eating arthropods. The domain is found at the N-terminus of rhamnogalacturonase B, a member of the polysaccharide lyase family 4. The domain adopts a structure consisting of a beta super-sandwich, with eighteen strands in two beta-sheets [1]. The three domains of the whole protein rhamnogalacturonan lyase (RGL4), are involved in the degradation of rhamnogalacturonan-I, RG-I, an important pectic plant cell-wall polysaccharide. The active-site residues are a lysine at position 169 in UniProtKB:Q00019 and a histidine at 229, Lys169 is likely to be a proton abstractor, His229 a proton donor in the mechanism. The substrate is a disaccharide, and RGL4, in contrast to other rhamnogalacturonan hydrolases, cleaves the alpha-1,4 linkages of RG-I between Rha and GalUA through a beta-elimination resulting in a double bond in the nonreducing GalUA residue, and is thus classified as a polysaccharide lyase (PL) [2].

1: Rhamnogalacturonan lyase reveals a unique three-domain modular structure for polysaccharide lyase family 4. McDonough MA, Kadirvelraj R, Harris P, Poulsen JC, Larsen S; FEBS Lett. 2004;565:188-194. PMID:15135077

2: Structural and biochemical studies elucidate the mechanism of rhamnogalacturonan lyase from Aspergillus aculeatus. Jensen MH, Otten H, Christensen U, Borchert TV, Christensen LL, Larsen S, Leggio LL; J Mol Biol. 2010;404:100-111. PMID:20851126