Charge Translocation During Cosubstrate Binding in the Na+/Proline Transporter of E. coli

Charge translocation associated with the activity of the Na+/proline cotransporter PutP of Escherichia coli was analyzed for the first time. Using a rapid solution exchange technique combined with a solid-supported membrane (SSM), it was demonstrated that Na+and/or proline individually or together induce a displacement of charge. This was assigned to an electrogenic Na+and/or proline binding process at the cytoplasmic face of the enzyme with a rate constant of k>50 s−1 which preceeds the rate-limiting step. Based on the kinetic analysis of our electrical signals, the following characteristics are proposed for substrate binding in PutP. (1) Substrate binding is electrogenic not only for Na+, but also for the uncharged cosubstrate proline. The charge displacement associated with the binding of both substrates is of comparable size and independent of the presence of the respective cosubstrate. (2) Both substrates can bind individually to the transporter. Under physiological conditions, an ordered binding mechanism prevails, while at sufficiently high concentrations, each substrate can bind in the absence of the other. (3) Both substrate binding sites interact cooperatively with each other by increasing the affinity and/or the speed of binding of the respective cosubstrate. (4) Proline binding proceeds in a two-step process: low affinity (∼1 mM) electroneutral substrate binding followed by a nearly irreversible electrogenic conformational transition.