Rerun of essentials of week one

From Rotamers to Models and back via the Entropy of Water

Protein structure bioinformatics

Predict mutations Analyse mutationsUnderstand biologyDesign medicines (etc)

Homology modelling for the above

Mutations and rotamers

‘Rotamer’ is an abused word. It both means any side chain conformation and preferred side chain conformation.

Improbable things

©CMBI 2006

β-branched prefers β-strand

Common sense and tau…

Valine, Isoleucine, and Threonine are β-branched. Common sense dictates to treat their tau angles special. Their γ-atoms bulldozer into their own backbone, and that is why β -branched residues prefer β -strands

Tau angle

Rotamers

So, there is much we don’t understand

RotamersPredict mutations Analyse mutationsHelp with dockingHomology modellingStructure validation

Model!

1: Template recognition and initial alignment

2: Alignment correction

3: Backbone generation

4: Loop modeling

5: Sidechain modeling

6: Model optimization

7: Model validation

8: Iteration

8: Iteration

8: Iteration

8: Iteration

MUTANT DESIGN

BIO-INFORMATICS

QUESTION

‘MOLECULARBIOLOGY’

BIOPHYSICS

MutationsProtein stability Enzyme activityEnzyme specificityAntigenicityValidate/falsify hypothesesEtcetera

PROTEIN STABILITY

ΔG = ΔH - TΔS ΔG = -RT ln(K)

K = [Folded] / [Unfolded]

So, you can interfere either with the folded, or with the unfolded form.

Choosing between ΔH and ΔS will be much more difficult, because ΔG is a property of the complete system, including H2O….

There is a natural tendency for all things (even atoms & molecules) to roll downhill - to fall to lower energy.

H wants to be negative

This is opposed (at the molecular level) by the equally natural tendency for thermal/Brownian motion (otherwise known as “entropy”) to make things go the other way…

…and this effect gets bigger as the temperature increases.

T.S wants to be positive

A bluffer’s guide to Thermodynamic Equilibrium, by Alan Cooper

Thermodynamic Equilibrium, expressed in terms of the Gibbs Free Energy change, reflects just the balance between these opposing tendencies…

G = H - TS

Equilibrium is reached when these two forces just balance (G = 0).

The standard free energy change, G, is just another way of expressing the equilibrium constant, or affinity (K) for any process, on a logarithmic scale…

G = -RTlnK

H(T) = H(Tref) + Tref

T

Cp .dT

S(T) = S(Tref) + Tref

T

(Cp /T).dT

Both enthalpy and entropy are integral functions of heat capacity...

….from which DG = DH - T.DS

So DCp is the key - if we can understand heat capacity effects, then we can understand everything else.

So, what is the role of water?

So DCp is the key - if we can understand heat capacity effects, then we can understand everything else. And DCp is largely determined by the interactions between water and the macromolecule(s).

In figure b many more waters are free than in a. And free waters are happy waters!

Stability engineeringEntropic versus enthalpicFolded versus unfolded formThermodynamic versus kineticAlways compensatory