Glycoside Hydrolase Family 43

This page has been approved by the Responsible Curator as essentially complete. CAZypedia is a living document, so further improvement of this page is still possible. If you would like to suggest an addition or correction, please contact the page's Responsible Curator directly by e-mail.

• Author: Harry Gilbert
• Responsible Curator: Harry Gilbert

---

Glycoside Hydrolase Family GH43
Clan	GH-F
Mechanism	inverting
Active site residues	not known
CAZy DB link
http://www.cazy.org/fam/GH43.html

Substrate specificities

The major activities reported for this family of glycoside hydrolases are alpha-L-arabinofuranosidases [1], endo-alpha-L-arabinanases (or endo-processive arabinanases) [2, 3] and beta-D-xylosidases [4]. An enzyme with exo alpha-1,3-galactanase has also been described [5]. A significant number of enzymes in this family display both alpha-L-arabinofuranosidase and beta-D-xylosidase activity using aryl-glycosides as substrates. It is likely that the natural activity of these enzymes is conferred by the aglycone component of the substrate. Indeed, the arabionofuranosidase activities already reported target very different glycans. Thus, the Bacillus subtilis enzyme arabinoxylan alpha-L-arabinofuranohydrolase specifically removes arabinofuranose side chains that are linked either alpha-1,2 or alpha-1,3 to backbone xylose residues [6], while the arabinoxylan arabinofuranohydrolase-D3 (AXHd3) from Bifidobacterium adolescentis will remove an alpha-1,3-linked arabinofuranose from xylans where the xylose residue is substituted at both alpha-1,2 and alpha-1,3 with arabinose [7]. By contrast some arabinofuranosidases are exo-alpha-1,5-L-arabinanases [8]. It should be noted that in several plant cell wall degrading organisms there has been a dramatic expansion in GH43 family enzymes, which may reflect a more extensive range of specificities than described to date.

Kinetics and Mechanism

NMR, deploying arabinan as the substrate, showed that an endo-alpha1,5-arabinanase uses an inverting mechanism [9].

Catalytic Residues

The catalytic general base, an aspartate, the catalytic general acid, a glutamate, and an aspartate that modules the pK_a of the general base were identified through the crystal structure of Cellvibrio japonicus CjAbn43A, and confirmed by site-directed mutagenesis [10]. Further biochemical proof for the catalytic function of the equivalent residues in a beta-xylosidase were obtained by demonstrating a relationship between the activity of the catalytic acid and the pK_a of the leaving group of the substrate. The identity of the catalytic base was achieved by azide rescue of a mutant of this residue [4]. In contrast to many inverting glycoside hydrolases there appears to be a single candidate catalytic general base.

Three-dimensional structures

The GH43 enzymes display a 'non-velcroed' five-bladed-beta-propeller. The propeller is based upon a five-fold repeat of blades composed of four-stranded beta-sheets [10]. The substrate-binding surface of Arb43A is in a long surface depression, with the catalytic constellation of carboxylates at its center. The exo-processive activity of the enzyme is conferred by a subtle steric block at the +3 subsite explaining why the enzyme releases, exclusively, arabinotriose [11]. In the arabinofuranosidases and xylosidases the active site comprises a deep pocket and the orientation of the substrate is very different between the enzymes, which contributes to the varied specificities observed across the GH43 landscape [12, 13].

Family Firsts

First sterochemistry determination

First general base residue identification

First general acid residue identification

First 3-D structure

alpha-L-arabinanase from Cellvibrio japonicus [1].

References

Error fetching PMID 7764056:
Error fetching PMID 9163351:
Error fetching PMID 7764386:
Error fetching PMID 15628881:
Error fetching PMID 15866877:
Error fetching PMID 17426966:
Error fetching PMID 15650848:
Error fetching PMID 10657233:
Error fetching PMID 8946944:
Error fetching PMID 12198486:
Error fetching PMID 15708971:
Error fetching PMID 18980579:
Error fetching PMID 16631196: