# 20 amino acids Deceptive---packing is critical Solution crystal 20 amino acids Deceptive---packing is critical Solution crystal structure of the prion monomer to Discovery Studio

monomer Dimer (domain swapped) Expression and crystallization.Human prion protein (90231)was expressed in E. coli,

refolded and purified as described.Crystals, obtained at 20 C by vapor diffusion with protein at 5 mg/ml and reservoir consisting of 0.1 M Tris-HCl, pH 8, 3 M NaCl and 5 mM CdCl2, were stabilized with 2,4-dimethyl-2,4-pentanediol prior to data collection at 180 C. Energy and Stability

A B If A is favored over B then A is at lower energyhow much lower?

= RT is thermal energythe temperature converted into energy units by R = RT is thermal energythe temperature converted into energy units by R

Our body temperature: 37C or 310 Kelvin or 2600 Joules = RT is thermal energythe temperature converted into energy units by R Our body temperature: 37C or 310 Kelvin or 2600 Joules If the energy difference is zero, =1 so both are in equal amountsat equilibrium

If the energy difference is 2600 Joules then B is 2.7 times greater than A = RT is thermal energythe temperature converted into energy units by R Our body temperature: 37C or 310 Kelvin or 2600 Joules/mole If the energy difference is zero, =1 so both are in equal amounts at equilibrium

If the energy difference is 2600 J/mole then B is 2.7 times greater than A If the energy difference is 10 times 2600 J/mole then B is 20,000 times greater than A = RT is thermal energythe temperature converted into energy units by R Our body temperature: 37C or 310 Kelvin or 2600 Joules

If the energy difference is zero, =1 so both are in equal amounts at equilibrium If the energy difference is 2600 Joules then B is 2.7 times greater than A If the energy difference is 10 times 2600 Joules then B is 20,000 times greater than A If the energy difference is very big then it seems that all of A goes to B How big is 2600 Joules/mole? 2.6 kJ/mole The heat needed to melt water molecules: 6 kJ/mole

To break a hydrogen bond: 18-24 kJ/mole Events happen spontaneously when energy needed is close to thermal energy Energy barrier:

Events happen spontaneously when energy needed is close to thermal energy Energy barrier: A

C B C Energy

A Energy to get change Energy change

Activation energy B For a rate of 1 in 109 sec:

energy of 132kJ/mole For probability of 1 in a million: energy=166kJ/mole uncatalyzed

catalyzed catalyzed plus bound changes energy Different species must be stable for multiple animal lifetimes

Thus very low energy statesdeep wells or high energy barriers protein must pack very well (low energy) in many different forms or transition structures are highly unstable