Key computational findings reveal proton transfer as driving the functional cycle in the phosphate transporter PiPT
Yu Liu, Chenghan Li, Meghna Gupta, Nidhi Verma, Atul Kumar Johri, Robert M. Stroud, Gregory A. Voth
Abstract
Significance Proton- or sodium-coupled transporters within the major facilitator superfamily are essential for nutrient uptake in all forms of life. We focus on a high-affinity eukaryotic proton-coupled phosphate symporter using extensive classical molecular dynamics (MD) simulations and quantum mechanics/molecular mechanics MD combined with free energy sampling and confirm the role of a key residue, D324, at the molecular level. The results explain the exit direction of the proton once dissociated from D324 and reveal a perspective on proton-coupled transporters, showing that titratable substrates can be involved in the proton transport process as a key aspect of the functional cycle. Mutagenesis and phosphate transport confirm the essential nature of the key D45 residue in the protonation pathway.