Structural Biochemistry/Enzyme Catalytic Mechanism/Phillips mechanism
Glu35 and Asp52 are catalytic residues
Enzyme binds hexasaccharide unit, residue D distorted towards half-chair to minimize CH2OH interactions.
Glu35 transfers H+ to O1 of D ring (general acid), C1-O1 bond cleaved generating resonance-stabilized oxonium ion at C1.
Asp52 stabilizes planar (transition state binding catalysis) oxonium ion through charge-charge interactions (electrostatic catalysis), SN1 mechanism.
Enzyme releases hydrolyzed E ring with attached polysaccharide, yielding glycosyl-enzyme intermediate, H2O adds to oxonium ion to form product and reprotonated Glu35, retention of configuration is result of enzyme cleft shielding one face of oxonium ion.
Catalytic residues were identified by chemical (group specific reagents) and molecular mutagenesis.