Simulation studies of the mechanism of membrane transporters

Membrane transporters facilitate active transport of their specific substrates, often against their electrochemical gradients across the membrane, through coupling the process to various sources of cellular energy, for example, ATP binding and hydrolysis in primary transporters, and pre-established electrochemical gradient of molecular species other than the substrate in the case of secondary transporters. In order to provide efficient energy-coupling mechanisms, membrane transporters have evolved into molecular machines in which stepwise binding, translocation, and transformation of various molecular species are closely coupled to protein conformational changes that take the transporter from one functional state to another during the transport cycle. Furthermore, in order to prevent the formation of leaky states and to be able to pump the substrate against its electrochemical gradient, all membrane transporters use the widely-accepted "alternating access mechanism," which ensures that the substrate is only accessible from one side of the membrane at a given time, but relies on complex and usually global protein conformational changes that differ for each family of membrane transporters. Describing the protein conformational changes of different natures and magnitudes is therefore at the heart of mechanistic studies of membrane transporters. Here, using a number of membrane transporters from diverse families, we present common protocols used in setting up and performing molecular dynamics simulations of membrane transporters and in analyzing the results, in order to characterize relevant motions of the system. The emphasis will be on highlighting how optimal design of molecular dynamics simulations combined with mechanistically oriented analysis can shed light onto key functionally relevant protein conformational changes in this family of membrane proteins.