It behooves drug chemists to know as much as they can about protein allostery, since so many of their drugs attempt to manipulate it. An earlier post discussed dynamic allostery which is essentially change in ligand binding affinity without structural change in the protein binding the ligand. A new paper challenges the concept.
First here’s the old post, and then the new stuff
Remember entropy? — Take II
Organic chemists have a far better intuitive feel for entropy than most chemists. Condensations such as the Diels Alder reaction decrease it, as does ring closure. However, when you get to small ligands binding proteins, everything seems to be about enthalpy. Although binding energy is always talked about, mentally it appears to be enthalpy (H) rather than Gibbs free energy (F).
A recent fascinating editorial and paper [ Proc. Natl. Acad. Sci. vol. 114 pp. 4278 – 4280, 4424 – 4429 ’17 ]shows how the evolution has used entropy to determine when a protein (CzrA) binds to DNA and when it doesn’t. As usual, advances in technology permit us to see this (e.g. multidimensional heteronuclear nuclear magnetic resonance). This allows us to determine the motion of side chains (methyl groups), backbones etc. etc. When CzrA binds to DNA methyl side chains on the protein move more, increasing entropy (deltaS) as well. We all know the Gibbs free energy of reaction (deltaF) isn’t just enthalpy (deltaH) but deltaH – TdeltaS, so an increase in deltaS pushes deltaF lower meaning the reaction proceeds in that direction.
Binding of Zinc redistributes these side chain motion so that entropy decreases, and the protein moves off DNA. The authors call this dynamics driven allostery. The fascinating thing, is that this may happen without any conformational change of CzrA.
I’m not sure that molecular dynamics simulations are good enough to pick this up. Fortunately newer NMR techniques can measure it. Just another complication for the hapless drug chemist thinking about protein ligand interactions.
A recent paper [ Proc. Natl. Acad. Sci. vol. 114 pp. 6563 – 6568 ’17 ] went into more detail about measuring side chain motions as a surrogate for conformational entropy. It can now be measured by NMR. They define complete restriction of the methyl group symmetry axis as 1, and complete disorder, and state that ‘a variety of models’ imply that the value is LINEARLY related to conformational entropy making it an ‘entropy meter’. They state that measurement of fast internal side chain motion is largely restricted to the methyl group — this makes me worry that other side chains (which they can’t measure) are moving as well and contributing to entropy.
The authors studied some 28 protein/ligand systems, and found that the contribution of conformational entropy to ligand binding can be favorable, negligible or unfavorable.
What is bothersome to the authors (and to me) is that there were no obvious structural correlates between the degree of conformation entropy and protein structure. So it’s something you measure not something you predict, making life even more difficult for the computational chemist studying protein ligand interactions.
Now the new stuff [ Proc. Natl. Acad. Sci. vol. 114 pp. 7480 – 7482, E5825 – E5834 ’17 ]. It’s worth considering what ‘no structural change’ means. Proteins are moving all the time. Bonds are vibrating at rates up to 10^15 times a second. Methyl groups are rotating, hydrogen bonds are being made and broken. I think we can assume that no structural change means no change in the protein backbone.
The work studied a protein of interest to neurological function, the PDZ3 domain — found on the receiving side of a synapse (post-synaptic side). Ligand binding produced no change in the backbone, but there were significant changes in the distribution of electrons — which the authors describe as an enthalpic rather than an entropic effect. Hydrogen bonds and salt bridges changed. Certainly any change in the charge distribution would affect the pKa’s of acids and bases. The changes in charge distribution the ligand would see due to hydrogen ionization from acids and binding to bases would certainly hange ligand binding — even forgetting van der Waals effects.
Chemiotics II 