Multifunctional xylooligosaccharide/cephalosporin C deacetylase revealed by the hexameric structure of the Bacillus subtilis enzyme at 1.9A resolution.
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.
Vincent
Florence
F
Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York YO10 5YW, UK.
Charnock
Simon J
SJ
Verschueren
Koen H G
KH
Turkenburg
Johan P
JP
Scott
David J
DJ
Offen
Wendy A
WA
Roberts
Shirley
S
Pell
Gavin
G
Gilbert
Harry J
HJ
Davies
Gideon J
GJ
Brannigan
James A
JA
D000115:Acetylesterase / Q000737:chemistry / Q000378:metabolism; D000595:Amino Acid Sequence; D001412:Bacillus subtilis / Q000201:enzymology*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002265:Carboxylic Ester Hydrolases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D009844:Oligosaccharides / Q000378:metabolism; D010648:Phenylacetates / Q000378:metabolism; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid
D001426:Bacterial Proteins; D009844:Oligosaccharides; D010648:Phenylacetates; D002265:Carboxylic Ester Hydrolases; C019861:cephalosporin-C deacetylase; D000115:Acetylesterase; C081851:acetylxylan esterase; C118008:4-nitrophenylacetic acid