Fig. 3

The catalytic mechanism of CCHs. a In an apo CCH, the three loops (loop A, B, and C) next to the catalytic pocket are very flexible and the size of the pocket can be fluctuated to a large extent. E157 is protonated, whereas E185 is deprotonated. b The entry of substrate into the CCH leads to the outward shift of loop C, resulting in a large space in the binding pocket. This facilitates the substrate molecule entering the catalytic region. c After substrate situating at the catalytic site and in an activated binding mode. E185 acts as a nucleophile, attacking the methylene group in the cyclopropyl ring. This leads to transformation of the quinone moiety into a deprotonated phenol. d The deprotonated phenol moiety of the transitional substrate acquires a proton from a water molecule. e With the assistance of E157, the transitional state is hydrolyzed by a water molecule. f Finally, with the outward fluctuations of loop B and loop C, the product leaves catalytic pocket. The E185 residue returns to its deprotonated state