Protein Ionizability Estimated

Long-awaited experimental data on the effect of membrane microenvironment on the protonation states of ionizable protein side chains have come from a new study (Nature 2005, 438, 975). Membrane proteins depend on ionizable side chains for structure and function, notes Claudio Grosman of the University of Illinois, Urbana-Champaign. But "their ionization states under physiological conditions largely elude experimental estimation," he points out. To address this question, Grosman's team engineered individual lysines, histidines, and arginines along the α-helical lining of the transmembrane pore of the nicotinic acetylcholine receptor (AChR). The researchers then used electrophysiology to detect the binding and release of individual protons. Kinetic analysis of the data yielded ionization constants (pK_a values) for residues at each position. Grosman says the experimental data may be used to predict the pK_a values of some of the native ionizable groups in AChRs. And as models of AChRs improve, he adds, the data could be used as benchmarks to validate electrostatic models for ion-channel proteins.