How important are quantum mechanical effects in controlling biological functions: Enzymes, electron transfer and bird navigation

In light of the centennial of the Schrödinger equation, this article addresses the popular idea that quantum mechanical effects play an important role in biological systems. We start by defining what is qualified as a quantum effect. We then clarify the idea that quantum mechanical tunneling is a crucial factor in enzyme catalysis. Here we show that quantum tunneling is in fact anticatalytic and that there is no consistent theoretical and experimental evidence that the tunneling effects are enhanced significantly by enzymes relative to the corresponding reference solution reactions. We next turn to electron transfer reactions, arguing that in most cases the efficiency of the reaction is controlled by classical effects. We also consider the low barrier hydrogen bond idea and clarify its anticatalytic nature. In addition, we present a specific case study of electron transfer in a protein system potentially responsible for bird navigation, providing a concrete example for evaluating the role of quantum and classical effects under biologically relevant conditions. We argue that for the electron transfer step in this system, the most important control is associated with the classical control of the relevant potential surfaces and the fluctuations of the key energy gaps.