Catalytic Mechanism of S-Acyltransferases: Acylation is Triggered on by a Loose Transition State; Deacylation is Turned off by a Tight Transition State
Dynamics protein S-Palmitoylation catalyzed by S-acyltransferases is one of the fundamental post-translational modifications, involved not only in a wide range of vital cellular processes, but also in a series of human health and diseases related issues (such as brain development and behavior, immune response regulation, tumor suppressor, and cancer). Interestingly, the human S-acyltransferase was recognized as a promising drug-target for cancer treatment. Despite of the prominent importance, the fundamental catalytic mechanism of S-acyltransferases remains elusive. In this study, we performed extensive simulations and calculations to describe the fundamental catalytic mechanism of autoacylation catalyzed by human S-acyltransferase, revealing a single-step reaction pathway characterized by a loose transition state. Deacylation process with a tight transition state is avoided due to substantially high free energy barrier. This specific catalytic mechanism is necessary to protect palmitoylated-enzymes from unwanted hydrolysis by water molecules, since accumulated negative charge of tight transition state cannot be effectively stabilized by a weak hydrogen bond of oxyanion hole. The activation free energy barrier of autoacylation catalyzed by human S-acyltransferase was predicted to be 18.2 kcal/mol, in good agreement with experimental derived activation free energy barrier (18.5 kcal/mol) based on the conventional transition state theory, suggesting the validity of the computational results. The mechanistic insights (e.g., detailed catalytic mechasnism and the nature of transition state) are expected to do good not only to rational drug discovery (e.g., design of transition state analogue) toward cancer treatment and design of engineered S-acyltransferases with desired acyl-CoA specificity, but also to future studies on catalytic mechanism of other S-acyltransferases.