PDBsum entry 1odt

Hydrolase

PDB id: 1odt

Contents

Protein chains

317 a.a. *

Ligands

ACT ×2

Waters ×563

* Residue conservation analysis

References listed in PDB file

Key reference

Title

Multifunctional xylooligosaccharide/cephalosporin c deacetylase revealed by the hexameric structure of the bacillus subtilis enzyme at 1.9a resolution.

Authors

F.Vincent, S.J.Charnock, K.H.Verschueren, J.P.Turkenburg, D.J.Scott, W.A.Offen, S.Roberts, G.Pell, H.J.Gilbert, G.J.Davies, J.A.Brannigan.

Ref.

J Mol Biol, 2003, 330, 593-606. [DOI no: 10.1016/S0022-2836(03)00632-6]

PubMed id

12842474

Abstract

Esterases and deacetylases active on carbohydrate ligands have been classified into 14 families based upon amino acid sequence similarities. Enzymes from carbohydrate esterase family seven (CE-7) are unusual in that they display activity towards both acetylated xylooligosaccharides and the antibiotic, cephalosporin C. The 1.9A structure of the multifunctional CE-7 esterase (hereinafter CAH) from Bacillus subtilis 168 reveals a classical alpha/beta hydrolase fold encased within a 32 hexamer. This is the first example of a hexameric alpha/beta hydrolase and is further evidence of the versatility of this particular fold, which is used in a wide variety of biological contexts. A narrow entrance tunnel leads to the centre of the molecule, where the six active-centre catalytic triads point towards the tunnel interior and thus are sequestered away from cytoplasmic contents. By analogy to self-compartmentalising proteases, the tunnel entrance may function to hinder access of large substrates to the poly-specific active centre. This would explain the observation that the enzyme is active on a variety of small, acetylated molecules. The structure of an active site mutant in complex with the reaction product, acetate, reveals details of the putative oxyanion binding site, and suggests that substrates bind predominantly through non-specific contacts with protein hydrophobic residues. Protein residues involved in catalysis are tethered by interactions with protein excursions from the canonical alpha/beta hydrolase fold. These excursions also mediate quaternary structure maintenance, so it would appear that catalytic competence is only achieved on protein multimerisation. We suggest that the acetyl xylan esterase (EC 3.1.1.72) and cephalosporin C deacetylase (EC 3.1.1.41) enzymes of the CE-7 family represent a single class of proteins with a multifunctional deacetylase activity against a range of small substrates.

Figure 4.
Figure 4. The fold of a CAH monomer. (a) Ribbon diagram, with secondary structure elements from the N to the C terminus colour ramped blue to red. The active site catalytic triad residues are depicted as ball-and-stick models, with oxygen atoms coloured red and nitrogen atoms blue. The extended portion of the sequence on the top right of the molecule in this orientation forms the b-sheet-like interface between adjacent subunits. This and subsequent Figures were drawn with BOBSCRIPT[72.]/MOLSCRIPT [73.] and raster3D. [74.] (b) Topology diagram. The a-helices and b-strands are coloured and labelled as for Figure 3. The b-sheet-like interface-region is shaded in black. The catalytic triad of Ser181, Asp269 and His298 is indicated.

Figure 8.
Figure 8. Comparison of the oxyanion hole of CAH and acetyl-xylan esterase AXEII. Divergent stereographic view of the superposition of the residues involved in the interaction of CAH with acetate and AXEII[6.] with sulphate. AXEII residues Thr13 and Gln91, which interact with the sulphate ion are in cyan whilst the equivalent CAH residues (Tyr91 and Gln182) interacting with the acetate are in white. This Figure was drawn using PyMOL [76.] (http://www.pymol.org).

The above figures are reprinted by permission from Elsevier: J Mol Biol (2003, 330, 593-606) copyright 2003.