ABF and Kinetic Barriers

From: McGuire, Kelly (mcg05004_at_byui.edu)
Date: Sun Aug 11 2019 - 19:26:00 CDT

How well does ABF handle kinetic barriers? I have on and off rate constants for a ligand binding to a protein from electrophysiology. The on rate constant is ~370 - 400 M(-1)s(-1). This is a relatively low rate constant, suggesting that the ligand does not pass the entry barrier very frequently. The off rate constant however is very small (2x10^-5 s(-1)), suggesting that when the ligand gets passed the entry barrier, it remains in the binding site for a long time. The Kd is therefore in the low micromolar range. The barrier to entry in this protein is believed to be a cluster of valines, 4 of them to be exact. The protein is a homotetramer.

I ran an ABF with the protein and the ligand, and the binding site well is right in the free energy range we expect relative to bulk water, ~ -13 kcal/mol. However, the energy barrier at the valines in only 2.5-3 kcal/mol. Eyring rate theory predicts the barrier to be ~10-12 kcal/mol with the on rate constant obtained from electrophsyiology.

I used 17 windows for my ABF, each window is 3 angstroms wide. I ran each ABF window for 100 nanoseconds. We are concerned that running one of the windows with the ligand near the entry barrier (valines) and the some of the other windows near the valines for too long might over sample the energy at that region and capture only the thermodynamic free energy and not the free energy including the kinetic barrier. It is unlikely that the ligand sits at the valines for 100 ns. It probably enters/collides for a short time then leaves and comes back or a new ligand collides with the barrier, but having an ABF window with the ligand right at the valines for 100 ns is probably too much sampling.

So is it possible to over sample some regions of the ABF reaction coordinate and get a lower energy than expected because the valines were allowed to relax around the ligand, or does ABF still capture the thermodynamic and kinetic energy barrier effects correctly?

Kelly L. McGuire

PhD Candidate

Biophysics

Department of Physiology and Developmental Biology

Brigham Young University

LSB 3050

Provo, UT 84602

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