RIBONUCLEASE-A CATALYZED TRANSPHOSPHORYLATION - AN AB-INITIO THEORETICAL-STUDY

The ribonuclease A transphosphorylation mechanism is studied using ab initio quantum chemical methods, incorporating for the first time deta iled all-electron components which mimic important amino acid residues within the enzyme active site. The 2-hydroxyethyl methylphosphate mon oanion is chosen as a model substrate, methyleneimines (CH2NH) are use d in place of the imidazole rings for His-12 and His-119, and methylam ine (CH3NH2) is used in place of Lys-41. Each pseudoresidue is held fi xed in its appropriate relative position found crystallographically. W ithin this model, structures and relative energetics for the stationar y points along the transphosphorylation reaction pathway are determine d at the RHF level using a 3-21G+ basis set, The data reveal several low-barrier proton transfer steps between the substrate and the active site residues which allow transphosphorylation to occur with modest a ctivation, consistent with experimental results for the actual enzyme. Two distinct aspects of the active site are identified: a general aci d to help protonate the substrate and an acid/base pair which cooperat ively facilitates proton transfer as transphosphorylation takes place.