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Biophys. J. BioFAST: First Published November 2, 2007. doi:10.1529/biophysj.107.121566
© 2007 by the Biophysical Society.

A more recent version of this article appeared on January 15, 2008.
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BIOPHYSICAL LETTERS

Is arginine charged in a membrane?

Libo Li 1, Igor Vorobyov 1, Alexander D. MacKerell, Jr. 2 and Toby W. Allen 3*

1 University of Claifornia, Davis
2 University of Maryland
3 University of California, Davis

* To whom correspondence should be addressed. E-mail: twallen{at}ucdavis.edu.

Submitted on September 7, 2007
Revised on October 12, 2007
Accepted on 25 October 2007


  Abstract
So-called '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 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 pKa shift of ≤ 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.

Key Words: arginine, charged amino acid, free energy, membrane, molecular dynamics, pKa




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Copyright © 2007 by the Biophysical Society.