Exhaustive conformational search for transition states: the case of catechol O-methyltransferase active site

A combination of the common quantum mechanics based transition state theory and exhaustive conformational search for the modeling of difficult reactions with hundreds of competing transition states is proposed. This approach is applied to study all transition state conformations of a reaction occurring in the catechol O-methyltransferase (COMT) active site in the absence of a major part of the enzyme, and the results are compared to the recent QM/MM modeling of this reaction within the enzyme. The main points of the method are (i) constraining of forming bonds upon conformer generation and (ii) preliminary constrained optimizations of located conformations to minima using a quantum mechanical method. Importantly, this methodology is applicable to the quantum mechanical part in QM/MM calculations and can reduce demand for large sampling in difficult cases.