Time Domain EPR: Membrane-binding Proteins

Using R1 from EPR as a probe of the Structure-function and the Dynamics-function relation in

biologyGraduate Students:

Tamara Okonogi

Robert Nielsen

Faculty:

Michael Gelb

Kate Pratt

Post Docs:

Andy Ball

Ying Lin

Stephane Canaan

Kepeng CheSupported by NSF and NIH

Time Domain EPR: Outline

• Time Domain: Saturation Recovery and Pulsed Electron Double Resonance Methods– Comparison with CW methods– Spectrometer, experiment, data

• Theory of relaxation Rates

• Application to the Spin Relaxant Method

• Using site directed mutagenesis– Orienting a Membrane-binding Protein– Determine an Oxygen gradient in a membrane

CW Power Saturation

The method to obtain spin-lattice relaxation rates using CW methods. Plot the Peak to Peak height as a function of microwave power (or really amplitude).

Details of CW Power Saturation

Peak-to-Peak height

1

21

2

Y

1P

hC

h

1 3

2 2

2 1 2P = R R

A product of spin-spin and spin-lattice relaxation rates.

CW

Cr

O

O O

O

O

O

CC

O

O

CC

O

O

CC

O

O

3

Field (Gauss)

+Crox -Crox2 2 2R R R CROX

15N

O

C

O

NH2

TD ESR Spectrometer

AB

C

D

G

90

0

90

0 0

90

observe osc.

pump osc.

E

Amplifier/Digitizer

F

Sig.2Sig.1

Balancecontrol

Phasecontrol

0

90 RFIF

RFIF

0

90 LO

LO

H

Pulsed Bridge

Free Induction Decay

Z'

X'

Y'

Z'

X'

Y'

Measures spin-spin dephasing

Pulsed Saturation Recovery

Z'

X'

Y'

Z'

X'

Y'

Measures relaxation to equilibrium

Pulsed Electron-Electron Double Resonance

pSR; the effect of a relaxant

+Crox -Crox1 1 1R R R CROX

Collision with Oxygen

14N

OH

O

Redfield Theory (or BWRT)

Relaxation rate theory began with Bloch and Wangness, and was amplified by Alfred Redfield to be a complete theory for the effects of dynamics and stochastic processes on spins in condensed matter. Relaxation theory is often called BWRT (Bloch Wangness and Redfield Theory).

BWRT Predicts Spin-Lattice (R1) and Spin-Spin (R2) relaxation rates.

Relaxation Rates are related to Relaxation times:

1 21 2

1 1 and R RT T

What Redfield Theory (BWRT) Uses

• There is a system Hamiltonian • BWRT requires a bilinear operator, which couples

the spin system (S) to the lattice (or bath), F – The Hamiltonian is:

• F is the fluctuating variable that causes the spins to have a fluctuating environment

• The fluctuation of the lattice, coupled to the spin system, then causes the spins to relax or dissipate the absorbed (microwave or r.f.) energy, non-radiatively.

H S F

sH

A problem: R2 diverges

The coupling Hamiltonian (e.n.d.) is:

The orientation variable, , is a stochastic function of time.

The correlation function (at high temperature) is:

This shows the statistical origin of the rotational correlation time. Exponential decay of the correlation function with time is typical of such functions.

212 where 1 3cosz zH S I F F a

2 214

11 3cos 1 3cos 0

5

t

ct e

Nielsen, R. D. and Robinson, B. H. "A Novel Relaxation Equation of Motion". J. Physical Chemistry 2004; 108: 1589-1600.

CW Spectra

CTPO

14N

O

C

O

NH2

R2 from BWRT, diverging R2 rates from

Kubo Theory

R2 rates from modified BWRT

log c

Rel

axat

ion

Rat

es (

MH

z)D.A.Haas, C. Mailer, and B.H. Robinson, Biophysical J. (1993) 64, 594

R1 does not diverge

• R1 for the electron and R1 for the nitrogen nucleus in a nitroxide spin label as a function of rotational correlation times can be computed from BWRT.

• If R2 diverges for correlation times longer than a few nanoseconds how can we rely on the theory to give us R1 values out to milliseconds and beyond?

• The Problem: Why does the theory fail for R2

rates but not for R1 rates?

• It is important to understand why R1 works and to understand why R2 fails.

Nitroxide Nitrogen Spin Lattice Relaxation Rates

Electron spin-lattice relaxation rates:

With O2

and Without O2

Correlation Time (sec)

Rel

axat

ion

Rat

es (

sec-1

)

B.H. Robinson, D. A. Haas and C. Mailer (1994). Science 263(5146): 490-3.

Mechanisms of R1

• Sum the rates from statistically independent processes– Spin-Rotation, rate goes as – Electron-Nuclear Dipolar Coupling

• Electron rate peaks at the spectrometer frequency

• Nuclear rate Peaks at coupling

– Oxygen relaxation (used later)– Empirical “Spin-Diffusion” process

• Just a catch-all effect, goes at

• Partially due to spins local to the nitroxide

1c

2a

1

8c

2

0x x t

tRS S e

2 2

0x x t

R t R tS S a e a e

The ideal form of the solution is:

The actual form of the solution is a bit more complicated:

2

222

1 1 11 1 2

2 2 2o c oc c c

R f f

The two rates are:

The slower rate dominates.

Time (sec)

Solution (Signal)Black: two rate solution

Blue: rate Green: BWRT rate

0

0.1

2 1

10

c f

4 10 5 10 secf

2eal R

Dominant Rate in all limits

2

2c ofR

In the fast motion limit: In the slow motion limit:

2

12 cR

BWRT gives only the fast motion limit, which predicts that the rate goes to infinity as the correlation time goes to infinity. The new theory avoids this and correctly predicts coherent oscillations of (at frequency ) as the interaction becomes coherent, in the no motion limit.

2

2 21 2

co

c o

R ff

The rates in the two limits may be “combined” into a rate that does cover both motional regimes (for both R1 and R2):

2

1 2 21

c o

c s c o

fR

f

New Term in both R2 and R1 rates

ofxS

Spin Labeled-Fatty Acids in DOPC

• Different spectrometer frequencies (from 2 to 35 GHz) with the best possible single effective correlation time.

• A poor fit. The frequency dependence of simple isotropic rotational motion is incomplete.

Spin-Lattice Relaxation rates for varied Doxyl-Steric Acids in DOPC.

Data from Jin and Hyde

SL at 5 position

SL at 12 position

SL at 16 position

Same Data Different Model

• Improving the model to include anisotropic dynamics.

• For simplicity the anisotropy ratio was kept constant.

• Improved agreement indicates the need to improve the model, and the frequency dependence of the relaxation rates can rule-out some incorrect models.

Relaxation rates from 60 different experimentsCorrelation among all

the data and the model. Model has 1 adjustable parameter (the mean rotational correlation time) for each sample at all 5 different frequencies and two different isotopic forms.

Bee venom phospholipase

Oriented on a membrane surface by

Site Directed Mutagenesis

EPR spin relaxant method

Lin, Y., Nielsen, R., Murray, D.,

Hubbell, W. L., Mailer, C., Robinson, B. H. and Gelb, M. H. Science 1998; 279 (5358): 1925-9

Membrane Binding Proteins

Labeling a protein (PLA2) with a Spin Probe

Use site directed mutagenesis techniques to prepare proteins with a single

properly placed cytsteine. General Reaction for adding

relaxants

H3C S S CH2

N OO

OS CH2

N O

PLA2 C SHPLA2 C S

+

..

The protein should contain only one cysteine for labeling.

Protein labeled at only one site at a time per experiment.

Relaxant Method: Nitroxide Spectra depend on concentration of relaxants

Spin-Lattice: T1-1 or R1

Spin-Spin: T2-1 or R2

1 1

2 2

o

o

R R Rlxnt

R R Rlxnt

Rates are increased by the same amount due to additional relaxing agents (relaxants).

2 1 2 1 2

2 1 2

02 2 2 1 2

o o

o o o

o o

P R R R Rlxnt R Rlxnt

R R R Rlxnt

P P P R R Rlxnt

Human (HGIIA) Secretory Phospholipase sPLA2

A highly charged (+20 residues) lipase, 14kDa protein

And a highly charged (-70 mV) membrane

All exposure data was determined by SR and pELDOR directly measuring spin-lattice relaxation rates.

CW Spectra of hGIIA on Micelles

hGIIACTPO14N

O

C

O

NH2

CW Spectrum of site N70C with CROX

Probing the hGIIA protein surface potential using CW and TD EPR

8 [ ]

[ ] [ ] o

B

B

r NaClo k Tcrox oz ek T

CROX CROX e

rates from pSR and pELDOR for CTPO

solvent accessibility

Spin Lattice Relaxation Rates for sl-sPLA2

Power Saturation Curves site S120C

O2 Relaxant: hGIIA on LUV

TD

CW

Compare O2 Relaxant Effects from TD-SR and CW

Summary of Vesicle data

• Large protein surface charge determined by CW and TD data

• Complete protection from Crox for all EPR data

• Oxygen effect reduced relative to solution

• Light scattering occurs

Aggregation model

~50 enzymes (36 Angstrom diameter)

LUV of DOPM (100 nm diameter)

TD data, Vesicles vs. Mixed Micelles

Vesicle

(DTPM)

Mixed

Micelles

hGIIA-sPLA2 on mixed micelles

Ni2+

OC NH

O

CH2

OC NH

O

CH2

Cr

O

O O

O

O

O

CC

O

O

CC

O

O

CC

O

O

3

NiEDDA

Crox

sPLA2 on MembraneView from membrane

Yellow: Hydrophobic Residues

Blue: Charged (pos) residues

Orientation perpendicular to that predicted by M. Jain.

Anchored by hydrophobic residues. Charges not essential

sPLA2 Conclusions

• sPLA2 causes the vesicles to aggregate.

Explains much other data and misconceptions about the kinetics and processive nature of sPLA2 action.

• sPLA2 was oriented on micelles (instead) using spin-lattice relaxation rates alone.

Orientation different from that of another model.

• Hydrophobic residues are the main points of contact.

• Charges provide a general, non-specific

attraction.•Substrate binding site identified by orientation on the mixed micelles

The WALP Proteins

WALP proteins are single alpha helical membrane-spanning proteins.

The sequence is 23 residues long:

HCO-NH-G-WW-L-(AL)8-WW-A-CO-NH2

Leucine and Alanine are both hydrophobic. In a membrane this forms a single turn alpha helix.

The membrane, di-oleic (DO) PC, is about 28-30 Ang thick. The two outer Tryptophans (W) are about 30 Angs apart. The membrane will stretch (or shrink) to accommodate the protein.

Demmers et al: J. Biol. Chem., 276, 34501-34508, 2001

WALP23The sequence is 23 residues long: HCO-NH-G-WW-L-(AL)8-WW-A-CO-NH2

Subcyznski et al. Biochemistry, 2003, 42, 3939

WALP23-sl CW spectraCW EPR spectra of spin labeled WALP23 at various

positions.

Oxygen Transport Parameter

The Oxygen transport parameter is the change in the spin-lattice relaxation rate due to oxygen collision-relaxation

,

where

1 2 1 2 1 2 2 21 1

1 1R O R O R O O OT T

1 2 2 R O O

Depends on transport

properties (e.g. Diffusion) of Oxygen in the local environment of the spin label

Typical WALP/DOPC Saturation Recovery EPR

CW

With Oxygen

Without Oxygen

Walp23 in DOPM: Oxygen Transport Parameter

From SR

11

1e

e

RT

2eREstimated from the CW line width

Estimated from the CW line width2eR11

1e

e

RT

From SR

Walp23 in DOPC: Oxygen Transport Parameter

Ratio Parameter*

1 2

1

lnR O

R Ni

*Altenbach, C. et al. PNAS (1994) 91 (5), 1667-71.

Conclusions

• The gradient of the oxygen transport parameter, measured on WALP 23, is ideal as a ruler for determination of spin label position in membranes.

• The spin-lattice and spin-spin relaxation rates show dependence on local mobility of the spin label in the bi-layer.

• The oxygen transport parameter cannot be separated into its two

components: the oxygen concentration and transport-dependent coefficient.

• The ratio parameter, designed to cancel out transport effects, provides a profile of relative relaxant concentration.

• Ratio parameter can be used to position nitroxide in the membrane.