Schematic representation of the four reactions catalysed by CGTase
A saccharide binds to the substrate-binding subsites of CGTase followed by bond cleavage to yield an enzyme-glycosyl intermediate. The second step of the reaction involves the transfer of the covalently bound saccharide to an acceptor molecule. (A) In the primary cyclization reaction, the OH-4 group of the non-reducing end saccharide is used as the acceptor. (B) A short saccharide acts as the acceptor for disproportionation. (C) For the side reaction of hydrolysis, a water molecule replaces the saccharide as acceptor for shorter saccharide production. (D) Coupling, the reverse reaction of cyclization, involves binding of cyclodextrin, breakdown of ring structure and linearization of saccharide before subsequent breakdown of enzyme-glycosyl intermediate by attacking of an acceptor saccharide.