Is arginine charged in a membrane?

Abstract

"Charged" amino acids play countless important roles in protein structure and function. Yet when these side chains come into contact with membranes we do not fully understand their behavior. This is highlighted by a recent model of voltage-gated ion channel activity and translocon-based experiments that suggest small penalties to expose these side chains to lipids, opposing the prevailing view in membrane biophysics. Here we employ a side chain analog as well as a transmembrane helix model to determine the free energy as a function of protonation state and position for a lipid-exposed arginine (Arg) residue across a membrane. We observe high free energy barriers for both the charged and neutral states. Due to the stabilizing influence of membrane deformations for the protonated form, the Arg side chain experiences a pK(a) shift of <or=4.5 units and remains mostly protonated. The cost for exposing Arg to lipid hydrocarbon is prohibitively high with implications for many membrane translocating processes and the activation mechanisms of voltage-gated ion channels.

FIGURE 1

PMFs for: ArgH⁺ (solid red); MguanH⁺ (dashed red); Arg⁰ (solid blue); and Mguan⁰ (dashed blue). Insets show equilibrated systems for z = 0 Å. Arg (blue/gray balls), helix (green ribbon), DPPC (gray C atoms, orange P atoms), and water (red/white). Atoms in the bilayer core drawn as balls.

FIGURE 2

(A) Thermodynamic cycle for calculation of deprotonation free energy at position, z, relative to bulk, z′. (B) ΔpK_a profile for the Arg side chain (solid) and analog (dashed).