How family 26 glycoside hydrolases orchestrate catalysis on different polysaccharides: structure and activity of a Clostridium thermocellum lichenase, CtLic26A.
One of the most intriguing features of the 90 glycoside hydrolase families (GHs) is the range of specificities displayed by different members of the same family, whereas the catalytic apparatus and mechanism are often invariant. Family GH26 predominantly comprises beta-1,4 mannanases; however, a bifunctional Clostridium thermocellum GH26 member (hereafter CtLic26A) displays a markedly different specificity. We show that CtLic26A is a lichenase, specific for mixed (Glcbeta1,4Glcbeta1,4Glcbeta1,3)n oligo- and polysaccharides, and displays no activity on manno-configured substrates or beta-1,4-linked homopolymers of glucose or xylose. The three-dimensional structure of the native form of CtLic26A has been solved at 1.50-A resolution, revealing a characteristic (beta/alpha)8 barrel with Glu-109 and Glu-222 acting as the catalytic acid/base and nucleophile in a double-displacement mechanism. The complex with the competitive inhibitor, Glc-beta-1,3-isofagomine (Ki 1 microm), at 1.60 A sheds light on substrate recognition in the -2 and -1 subsites and illuminates why the enzyme is specific for lichenan-based substrates. Hydrolysis of beta-mannosides by GH26 members is thought to proceed through transition states in the B2,5 (boat) conformation in which structural distinction of glucosides versus mannosides reflects not the configuration at C2 but the recognition of the pseudoaxial O3 of the B2,5 conformation. We suggest a different conformational itinerary for the GH26 enzymes active on gluco-configured substrates.
York Structural Biology Laboratory, Department of Chemistry, University of York, York, YO10 5YW, United Kingdom.
Edward J
EJ
Taylor
Arun
A
Goyal
Catarina I P D
CI
Guerreiro
José A M
JA
Prates
Victoria A
VA
Money
Natalie
N
Ferry
Carl
C
Morland
Antoni
A
Planas
James A
JA
Macdonald
Robert V
RV
Stick
Harry J
HJ
Gilbert
Carlos M G A
CM
Fontes
Gideon J
GJ
Davies
D001667:Binding, Competitive; D002384:Catalysis; D003001:Cloning, Molecular; D048013:Acetivibrio thermocellus / Q000201:enzymology*; D018360:Crystallography, X-Ray; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology / Q000378:metabolism; D005947:Glucose / Q000737:chemistry; D006026:Glycoside Hydrolases / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008358:Mannose / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D011108:Polymers / Q000737:chemistry; D011134:Polysaccharides / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry; D013379:Substrate Specificity; D014994:Xylose / Q000737:chemistry
D011108:Polymers; D011134:Polysaccharides; D011994:Recombinant Proteins; D014994:Xylose; D006026:Glycoside Hydrolases; C028020:licheninase; D005947:Glucose; D008358:Mannose