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Protein Dynamics from NMRProtein Dynamics from NMR
03/19/02
Protein and Peptide Drug Analysis, pages 714-716
Amide proton exchange
Heteronuclear relaxation
Application to determine the mechanism of cooperativity in binding of Ca2+ by calbindin D9k
Why The Interest In Dynamics? Why The Interest In Dynamics?
• Function requires motion/kinetic energy
• Entropic contributions to binding events
• Protein Folding/Unfolding
• Uncertainty in NMR and crystal structures
• Effect on NMR experiments- spin relaxation is dependent on rate of motions know dynamics to predict outcomes and design new experiments
• Quantum mechanics/prediction (masochism)
Characterizing Protein Dynamics: Characterizing Protein Dynamics: Parameters/TimescalesParameters/Timescales
Relaxation
NMR Parameters That Report On NMR Parameters That Report On Dynamics of MoleculesDynamics of Molecules
• Number of signals per atom: multiple signals
for slow exchange between conformational states
• Linewidths: narrow = faster motion, wide = slower; dependent on MW and conformational states
• Exchange of NH with solvent: requires local and/or global unfolding events slow timescales
• Heteronuclear relaxation measurementsR1 (1/T1) spin-lattice- reports on fast motionsR2 (1/T2) spin-spin- reports on fast & slowHeteronuclear NOE- reports on fast & some slow
Relaxation- Return to EquilibriumRelaxation- Return to Equilibrium
t
z axisx,y plane
0
1
2
t
0
1
2
8 8
E-t/T2
t
1-e-t/T1
t
LongitudinalTransverse
Transverse always faster!
Longitudinal (TLongitudinal (T11) Relaxation) RelaxationMECHANISMMolecular motions cause the nuclear magnets to
fluctuate relative to a fixed point in space
Fluctuating magnetic fields promote spins to flip
between states [Induced by the lattice!!]
Over time, spin flips cause a return to equilibrium
Slow motions make effect more efficient
t
dMz/dt = Meq – Mz/T1
Mz(t) = Meq (1-e-t/T1) Mz(t) Meq
FastSlow
Transverse (TTransverse (T22) Relaxation) Relaxation
t
MECHANISMMagnetic field is not homogenous to an infinite
degree
Each spin comprising the bulk magnetization will
feel a slightly different field
Over time, the spin fan out (lose coherence)
Slow motions make effect more efficient
dMx,y/dt = Mx,y/T2
Linewidthtime
FastSlow
Linewidth is Dependent on MW
A B A B
1H
1H
15N
15N
1H
15N Linewidth determined by size of particle
Fragments have narrower linewidths
Small(Fast)
Big(Slow)
Amide Proton Exchange(secminhoursdaysmonths)
•Peptides/unfolded proteins exchange rapidly
•Folded proteins protected: solvent accesibility, H-bonds
-N-H
D-O-D
OH-N
H-bonded amides: exchange occurs via local or global unfolding events
Heteronuclear Relaxation(psecnsec & secmsec)
•15N relaxation dominated by 1H
•N-H distance fixed, variation in relaxation due to differences in motional properties
•Overall tumbling, internal motions
Must fit relaxation parameters to a motional model: Lipari-Szabo “order parameter” (S2) most common
-15N-
H-1
5 N-
H -15N-
H
Dynamics To Probe The OriginDynamics To Probe The OriginOf Structural UncertaintyOf Structural Uncertainty
Measurements show if high RMSD is due to high flexibility (low S2)
Strong correlation
Weak correlation
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