Efficient Factors in Protein Modification:Adenosine Deaminase Esterification by Woodward Reagent K

Chemical modification of Adenosine Deaminase (ADA) with N-ethyl-5-phenyl isoxazoliom-3-sulfonate (Woodward’s reagent K) (WR-K) was studied using experimental and theoretical techniques. Reaction concentration ranges were 0.8-6 mM WR-K at pH 7.8 and 27 °C. It was observed that the maximum number of moles of esterified residues per mol of enzyme (ν ) in this concentration ranges is 4. However, esterification of ADA does not affect the activity of ADA, suggesting that the active site residues are not esterified. Similar results were obtained when the active site was blocked with 0.1 mM erythro-9-(2-hydroxy-3- nonyl) adenine (EHNA), followed by esterification, as measured by enol ester formation using absorbance at 340 nm. A theoretical approach was employed to study the modification process using molecular dynamic simulation, MM and QM/MM minimization. A full ASA empirical model and B3LYP method were used to evaluate the relative stability of some species which may arise from the reaction of ADA with WR-K. Theoretical results have shown that five residues (Glu 244, Glu 121, Glu 337, Asp 127, Asp 338) can be possible cases for modification in reaction 1:1 between ADA and WR-K at ν = 1. Glu 121 was possible initially modified in this process. Besides, it is specified that atomic accessible surface area cannot be an appropriate criterion in determination of primary sites which are modified by WR-K. Ultimately, it is clarified that among effective factors in modification of enzyme surface such as atomic accessible surface, stability of modified segment and local residues changes of ADA, latter factor plays a basic role in this process from kinetics and thermodynamics point of view