4.2.3.42: aphidicolan-16beta-ol synthase

enzyme ACS belongs to the diterpene synthase family

malfunction

aphidicolan-16beta-ol synthase (EC 4.2.3.42) enzyme mutants Y658L and D661A show a premature termination of the cyclization reaction cascade en-route from syn-copalyl diphosphate to aphidicolan-16-beta-ol, the mutants generates the diterpene macrocycle syn-copalol and a minor, non-hydroxylated labdane related diterpene. The structural changes D661A and Y658L still allow syn-CDP binding in the active site with subsequent hydrolyses of the diphosphate group. The syn-copalyl carbocation is then quenched either by water (release of syn-copalol) or an amino acid side chain (release of non-hydroxylated diterpene). No other substitution is found that stops cyclization at the proposed transitional states nor changes in the byproduct formation of the active mutants. The ACS cyclization occurs in a spatially restricted area and the diphosphate group remains in the active site

physiological function

the bifunctional enzyme aphidicolan-16beta-ol synthase (ACS) catalyzes the complex conversion of GGDP via syn-copalyl diphosphate to the tetracyclic aphidicolan-16beta-ol which is the core structure of the cytostatic compound aphidicolin, cyclization reaction cascade en-route from syn-copalyl diphosphate to aphidicolan-16-beta-ol, overview

additional information

modeling of the protein-ligand complex structure of fungal, bifunctional aphidicolan-16-beta-ol synthase, molecular dynamic studies, only the ACS alpha-domain sequence is used for homology prediction. The catalytically relevant ACS amino acid network includes residues I626, T657, Y658, A786, F789, and Y923. The initial conversion from the universal diterpene precursor GGDP to syn-CDP occurs in class II active site, located between the ACS beta- and gamma-domain. The subsequent syn-CDP cyclization to aphidicolan-16beta-ol is then conducted in the class I active site that is positioned in the middle of an alpha-helical bundle forming the ACS alpha-domain