spectroscopic probes of protein dynamics: hemoglobin and...
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Spectroscopic Probes of Protein Dynamics:
Hemoglobin and Myoglobin
NC State University
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N N
NN
O O-O O-
Fe
The iron in heme is the binding site for oxygen and peroxide
Heme is iron protoporphyrin IX.
Functional aspects in Mb
O|||O
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N N
NN
O O-O O-
Fe
The iron in heme is the binding site for oxygen and peroxide
Heme is iron protoporphyrin IX.
Functional aspects in Mb
1. Discrimination againstCO binding.
O|||C
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N N
NN
O O-O O-
Fe
The iron in heme is the binding site for oxygen and peroxide
Heme is iron protoporphyrin IX.
Functional aspects in Mb
1. Discrimination againstCO binding.
2. O2 is the physiologicallyrelevant ligand, but it canoxidize iron (autooxidation).
3+
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Myoglobin
PDB: 1A6G
SCOPClass: All α proteinsSuperfamily:
Globin-likeFamily: Globins
Vojetchovsky,Berendzen,Schlicting
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The substrate of DHP binding site
Lebioda et al., J.Biol.Chem. (2000) 275, 18712
4-iodophenol
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Catalytic activity in a hemoglobin
Tribromophenol
DibromoQuinone
DHP + TPB + H2O2
DHP + DBQ + H2O
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Conformational SubstatesProteins can assume a huge number of slightly different structures (conformational substates), which can be represented by nearly isoenergetic minima in an energy landscape with a typical barrier height of 10 kJ/mol.
What is the functional role of these substates?
kBT
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Enzymatic catalysis involves lowering of the energy barrier
∆H*
Protein fluctuations are certainly required for catalysis.However, the important question is:How is specificity built into the catalytic mechanism?
∆H*
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Non-equilibrium relaxationProtein-substrate, protein-protein and protein-DNA interactions usually do not occur under equilibrium conditions in living systems.
Non-equilibrium relaxations from one state to another occur in many situations in biology:
Ligand binding and transportSignalingElectron TransferEnzymatic CatalysisProtein Folding
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MbCO
The peptide backboneis shown as a ribbonthat follows the a-helicalstructure of myoglobin.
The structure shown isat equilibrium.
Conformational substatesare called A states.
Teng, Srajer, MoffatNature Struct. Biol. (1994), 1, 701
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Mb:COThe photoproduct.
Iron moves out of the heme planewhen CO is photolyzed.
CO moves to a dockingsite and is parallel to the heme plane.
Conformational substatesare called B states.
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Deoxy Mb
The deoxy structurehas no CO ligand.
The protein backbonehas shifted to permita water to enter thedistal pocket.
This form is often referred to an S state.
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MbCO
Mb:CO
Deoxy Mb
A comparison ofthree structures showsthe changes in bothproximal and distalhistidines.
k12
k23
Protein Data Bank 1A6N 1AJG 1AJH
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Deoxy Mb
Mb:CO
MbCO
These structuresrepresent the three states of a kinetic scheme.
k21
k32
Teng, Srajer, MoffatNature Struct. Biol. (1994), 1, 701
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Absorption Spectroscopy:Introduction to
Vibronic Coupling
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Porphine orbitals
eg eg
a2u a1u
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The four orbital model is used to represent the highest occupied and
lowest unoccupied MOs of porphyrins
eg π∗
a1u πa2u π
The two highest occupiedorbitals (a1u,a2u) are nearly equal in energy. The egorbitals are equal in energy.Transitions occur from:a1u→ eg and a2u → eg.
M1
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The transitions from ground state π orbitalsa1u and a2u to excited state π* orbitals eg
can mix by configuration interaction
eg π∗
a1u πa2u π
Two electronic transitionsare observed. One is verystrong (B or Soret) and the other is weak (Q).The transition moments are:MB = M1 + M2MQ = M1 - M2 ≈ 0
M1 M2
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The porphine ring is an aromatic ring that has a fourfold symmetry axis
The ring and metal can beconsidered separately.The ring has been succesfullymodeled using the Goutermanfour orbital model.In globins the iron is Fe(II)and can be either high spin or low spin.MbCO ------ low spinDeoxy Mb - high spin
N N
NN
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Four orbital model of metalloporphyrin spectra
|By0⟩ = 1
2 a2uegy + a1uegx
|Qy0⟩ = 1
2 a2uegy – a1uegx
|Bx0⟩ = 1
2 a2uegx + a1uegy
|Qx0⟩ = 1
2 a2uegx – a1uegy
There are four excited state configurations possible inD4h symmetry. These are denoted B (strong) and Q(weak).
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The absorption cross sectionThe absorption cross section for a Franck-Condon active transition is proportional to:
Here i and f represent individual vibrationallevels in each electronic manifold. The polarization can be σ = x, y, or z.
<i|eσ|f>2
Ef – Ei – hω – iΓ f
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The Herzberg-Teller expansionThe Herzberg-Teller expansion of a particularvibronic level is given by:
This expansion describes how electronic statesn and r can mix by virtue of distortions of themolecular geometry.
|nv⟩ = |n⟩ 0|v⟩ +r| ∂H∂QK
|n u|QK|v
En,v – Er,u|r⟩|u⟩Σ
KΣr
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Vibronic matrix elements
By0| ∂H∂Q |Qy
0 = a2u| ∂H∂Q |a2u
– a1u| ∂H∂Q |a1u + egy| ∂H∂Q |egy – egx| ∂H∂Q |egx
Qy0| ∂H∂Q |Qy
0 = a1u| ∂H∂Q |a1u
+ a2u| ∂H∂Q |a2u + egy| ∂H∂Q |egy + egx| ∂H∂Q |egx
Interstate Herzberg-Teller coupling:
Intrastate Jahn-Teller coupling:
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Vibronic theory of absorptionThe extinction coefficient is proportional to the square of the transition moment:
FC part
Vibronicpart
∈ = <i|eσ|f>2<0|v>2
Efv – Ei0 – hω – iΓ fΣv +
<r| ∂H∂QK
|f><u|QK|v><i|eσ|r>Ef ,v – Er,u
2
<0|u>2
Efv – Ei0 – hω – iΓ fΣK
Σr
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Transition moments in the four orbital model with configuration interaction
The mixing of the B and Q states is representedby the angle α:
rσ0 ≡ G|eσ|Qσ0
Rσ0 ≡ G|eσ|Bσ
0
rσ = G|eσ|Qσ = cos α G|eσ|Qσ0 + sin α G|eσ|Bσ
0
= cos α rσ0 +sin α Rσ0
Rσ = G|eσ|Bσ = cos α G|eσ|Bσ0 + sin α G|eσ|Qσ
0
= cos α Rσ0 +sin α rσ0
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The B and Q bands for B1g and B2gsymmetry vibronic matrix elements
Franck-Condonpart
Herzberg-Teller part
∈ = Rσ0 2 sin2 α
EQ0 – hω 2 + ΓQ2
+cos2 α
EB0 – hω 2 + ΓB2
+ Rσ0 2b g
2
sin α sin 2αhωK
+cos α cos 2αEQ
0 – EB0 + hω
EQ1 – hω 2 + ΓQ2
+
cos α sin 2αhωK
+sin α cos 2αEQ
0 – EB0 – hω
EB1 – hω 2 + ΓB2
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Vibronic lineshapes as a function of angle α for B1g vibronic modes
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Vibronic lineshapes for B1g modes as a function of vibronic strength (α =1o)
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Vibronic lineshapes for B1g modes as a function of vibronic strength (α =15o)
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The B and Q bands for A2gsymmetry vibronic matrix elements
Franck-Condonpart
Herzberg-Teller part
∈ = Rσ0 2 sin2 α
EQ0 – hω 2 + ΓQ2
+cos2 α
EB0 – hω 2 + ΓB2
+ Rσ0 2a2g
2
cos2 α
EQ0 – EB
0 + hω2
EQ1 – hω 2 + ΓQ2
+
sin2 α
EQ0 – EB
0 – hω2
EB1 – hω 2 + ΓB2
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Vibronic lineshapes for A2g modes as a function of vibronic strength (α =15o)
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Ligand recombination is a sum of single exponential processes at room temperature
Difference spectrum from nanosecond transient absorption spectroscopy.
kescape
kbimolecular
His - FeP::CO
His-FeP + CO
His-FeP-CO
kgeminatehν
S(t) = Φgee– (k gem + k esc)t + Φbie– k bit
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The heme iron center moves out of the heme plane and the porphyrin macrocycle
domes upon deligation of CO
CO is photolyzed Fe displacement
PlanarHeme
DomedHeme
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The ligation of CO changes the spin state of the heme iron
dz2
dx2-y2
dxz,dyz
dxy
dz2
dx2-y2
dyz
dxz
dxy
Low spin Fe(II) High spin Fe(II)
S = 0 S = 2
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Absorption spectra for Absorption spectra for MbCO MbCO and and deoxy deoxy MbMb
Soret Band Q Band
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Infrared Spectroscopy
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Structural heterogeneity in myoglobin has been proposed based on the observation
of multiple CO stretching bands• The bands are observed at 1966 cm-1,
1945 cm -1, and 1927 cm -1.• The heterogeneity has been attributed to
conformational substates.• Rebinding to each substate is also
observed to be non-exponential.• Are there many relevant tiers of
substates?
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The origin of the A states is the hydrogen bonding conformations to CO
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L29
H64F43
V68
Distal
Proximal
Protein Data Bank 2MGK
Infrared spectra of myoglobin
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L29
H64F43
V68
Distal
ProximalQuillin, Phillips et al.JMB 1993, 234, 140-155
Wild type Mb IR spectra show multiple bands
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Mutants at the V68 position also show multiple bands
L29
H64F43
V68Distal
Proximal
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The H64V mutant shows a single IR band
L29
H64F43
V68Distal
Proximal
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DFT calculation of νCO frequencies
Multiple hydrogen bonding interactions
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Single hydrogen bonding interaction
DFT calculation of νCO frequencies
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No hydrogen bonding interaction
DFT calculation of νCO frequencies
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DFT calculation in an applied electric field+ + + + + + + + + + + +
− − − − − − − − − − − − − − − −
F
Franzen JACS (2002) 124, 13271
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DFT calculation of νCO frequencies
Park and BoxerJPC 1999, 103, 9013
Stark tuning rate is 2.4 cm-1/(MV/cm).This is value predictedfrom correlations shownon right.
Franzen JACS (2002) 124, 13271
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Cryogenic Spectroscopy
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At less than 10K photolyzed Mb*CO recombines by nuclear tunneling
k = Ae– HBA/RT
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Current hypothesisThe iron coordinate controls the barrier to CO rebinding and explains the distribution of energy barriers.
Austin et al. Biochemistry 1975, 14, 5355
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Experimental observation:
Mb*CO at t1
Steinbach et al. Biochemistry1991, 30, 3988
Iron relaxation hypothesis rests on the assignment of charge transfer band III
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Experimental observation:
Mb*CO at t2
Steinbach et al. Biochemistry1991, 30, 3988
Iron relaxation hypothesis rests on the assignment of charge transfer band III
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Experimental observation: Kinetic holeburning
Mb*CO t2 - t1
Steinbach et al. Biochemistry1991, 30, 3988
Iron relaxation hypothesis rests on the assignment of charge transfer band III
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Assignment of band IIIThe interpretation of the data depends critically on the assignment of band III.
Band III has been assigned based on single crystal absorption data.
However, there have been problems with the interpretation from the very earliest experiments.
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Interpretation of kinetic hole burning
Conformational substatesare correlated with theband III maximumand theenthalpy of rebindingandthe rate constant distribution.
Steinbach et al. Biochemistry1991, 30, 3988
ν (cm-1)
H (kJ/mol)
log(t/s)-6 -5 -4 -3 -2
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Ultrafast Spectroscopy
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Ligand dynamics, protein dynamics, and fast recombination can be
monitored by ultrafast spectroscopy
Mode lockedTi:sapphire Laser
ZAP!MbCOSample
Time resolution from100 fs to nanoseconds.
Nd:YLF Laser
Regenerativeamplifier
BBO ContinuumDelay line
Frequency doubled 100 fs pulses 1000 Hz
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Ultrafast near-infrared spectroscopy shows that a protein relaxation
follows photolysis
Jackson, Lim, Anfinrud Chem. Phys. 1994, 180, 131-140
Band III at 10 ps, 10 ns,100 ns, and 10 µs
Non-exponential band IIIdecay in buffer solution η = 1cp
Mb:CO
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Ultrafast mid-infrared shows CO trapping in the distal pocket
C O
O C
Two CO orientationsin infrared bands
B1 and B2
Spectral bands arerotamers of CO in Mb
Nature structural BiologyLim, Jackson, Anfinrud, 1997, 4, 209
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What is the structure origin of the viscosity dependent protein relaxation?
The heme iron out-of-plane motion could change heme spectra
- band III a1u,a2u → dπ charge transfer band and therefore its frequency maximum is thought to depend on the heme iron position.
- rebinding enthalpy H is thought to depend on the heme iron position and there is a connection between H and the position of band III (kinetic hole burning).
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The iron hypothesis“The red shift of the Soret band and band III by the same amount ∆ν = 140 cm-1 , suggests that changes in a single coordinate affect both the Soret band and band III. We expect that the primary coordinate associated with the heme relaxation from Mb* to Mb involves heme doming, characterized by the iron out-of-plane displacement”
Srajer and Champion Biochemistry (1991), 30, 7390
“The similarity of the deoxyheme (Soret) spectral changes to those observed for hemoglobin suggests that they correspond to a displacement of the iron relative to the heme plane that is coupled to a protein conformational change on the proximal side of the heme.”
Ansari, Hofrichter, Eaton et al. Science (1992), 256, 1976
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The iron hypothesis“As the effective coordinate involved in the time-dependent barrier (responsible for non-exponential NO rebinding kinetics), we focus on the Fe-heme distance. It is, in turn, modulated by the protein relaxation. The latter is the major factor that regulates the distribution of geminate rebinding rates over a picosecond time scale at room temperature.”
Petrich, Karplus, and Martin Biochemistry (1991), 30, 3986
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Iron
Dockingsite
What structural feature is responsible for the relaxation?
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An alternative view of relaxationProtein relaxation involves CO dockingnot the iron out-of-plane motion.1. Iron out-of-plane motion occurs in < 2 ps. 2. NO recombination is biexponential. The two components are due to the rotamers of NO.
3. The Soret band and band III shift couple to distal mutations, and are not coupled to proximal mutations.
4. Band III is a vibronically coupled charge transfer band! Band III is coupled exclusively to non-totally symmetric modes.
5. MCD spectra are consistent with vibronic coupling model.6. Temperature dependent νFe-L mode is consistent withanharmonic coupling and not conformational substates indeoxy Mb.
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Ultrafast NO recombination kinetics show an acceleration in rate with
increasing ηMaximum entropy method1. Two populations2. Tends towards a single
population at high η
Picosecond kinetics of photolyzed MbNO
Shreve, Franzen, and Dyer JPC 1999, 103,7969
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FTIR data show that there are two populations of NO
Miller, Chance et al. Biochemistry 1997, 40, 12199
Photoproduct statescan interconverteven at 10 K
The lower wavenumberband corresponds to alower barrier
B1 B2
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Protein relaxation can be monitored by Soret band and Band III
frequency shifts
Franzen and Boxer JBC 1997, 272, 9655
Band III shift Soret band shift
Comparison oftime-dependentfrequency shifts
290 K250 K
250 K
250 K
Observed in 75%glycerol/buffer
η = 30 cP at 290 Kη = 300 cP at 250 K
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Time-resolved absorption spectroscopy shows protein relaxation following photolysis does
not depend on the proximal ligand
CO rebinding progressis monitored by ∆A.
∆A
Protein relaxation monitoredby ∆ν of the absorption band.
MbCO in 75% glycerol/buffer solution.
Franzen and Boxer JBC 1997, 272, 9655
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Near-infrared bands of heme in deoxy myoglobin
MagneticCircularDichroism
Absorption
CircularDichroism
Single crystalpolarizedabsorptionspectra
c axis
a axis
a,c axis
Frequency (cm-1)
Band III
IIII
IVII
Eaton et al. JACS 1978, 100, 4991
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Near-infrared charge transfer bands of the heme in deoxy myoglobin
Eaton et al. JACS 1978, 100, 4991
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Raman Spectroscopy
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A resonance Raman spectrum is obtained by laser light scattering
Laser Spectrograph
Detector
Sample
Inelastic light scattering produces a frequency shift. There is exchange of energy between the vibrations of the molecule and the incident photon.
Lens
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Resonance Raman spectrum for Resonance Raman spectrum for excitation of hemeexcitation of heme SoretSoret bandband
Soret BandB Band Excitation Laser Q Band
Raman spectrum
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Soret (B) band Resonance Raman spectra of MbCO and Deoxy Mb
ν8
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B band Resonance Raman spectra of MbCO and Deoxy Mb
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The cooperative R - T switch relies on iron displacement to communicate
between α and β subunits
Hemoglobin is composed of two α and two βsubunits whose structures resemble myoglobin.
Eaton et al. Nature Struct. Biol. 1999, 6, 351
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Ultrafast resonance Raman spectroscopy shows that heme doming occurs in ≈1 ps
Equilibrium HbCO
Difference spectra obtainedby subtraction of the redspectrum from spectra obtained at the timedelays shown.
Franzen et al. Nature Struct. Biol. (1994) 1, 230
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The frequency of the iron-histidine vibration shows strain in T state
The comparison of photolyzedHbCO in the R state andthe equilibrium T state.Hb*CO at 10 ns Fe-His = 230 cm-1
Deoxy HbFe-His = 216 cm -1The lower frequency indicatesweaker bonding interactionand coupling to bending modes.
λexc = 435 nmFe-His
Deoxy HbT-state
Hb*CO10 nsR-state
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The motion of the F-helix tugs on the proximal histidine and introduces strain
The frequency lowering in the T state arisesfrom weaker Fe-His ligation and from anharmonic coupling introduced by thebent conformation of the proximal histidine.
FeN
NH
FeN
NH
R state T state
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Time-resolved resonance Raman can follow the R - T structure change
Strain is introduced instages as intersubunitcontacts are made.Based on the x-ray datait was proposed that theiron displacement fromthe heme plane is a triggerfor the conformational changes.
Hb*CO
10 ns
100 ns400 ns1 µs
8 µs
15 µs40 µs60 µs
120 µs
Deoxy Hb
Time evolution
200 210 220 230 240Raman Shift (cm-1)
Scott and Friedman JACS 1984, 106, 5877
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Many Peroxidases belong to the Cytochrome c Peroxidase family
PDB: 1A2FCytochrome c Peroxidase (CCP)Class: All α proteinsSuperfamily:
Heme peroxidasesFamily: CCP-like Goodin and McCree
Scripps Institute
PDB: 2ATJHorseradish Peroxidase (HRP)Class: All α proteinsSuperfamily:
Heme peroxidasesFamily: CCP-like Hendrickson et al.
Biochemistry (1998)37, 8054
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Dehaloperoxidase is a peroxidase that belongs to the globin family
PDB: 1A6GMyoglobin (Mb)Class: All α proteinsSuperfamily:
Globin-likeFamily: Globins
Vojetchovsky,Berendzen,Schlichting
PDB: 1EW6Dehaloperoxidase (DHP)Class: All α proteinsSuperfamily:
Globin-likeFamily: Globins
Lebioda et al. J.Biol.Chem. 27518712 (2000)
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MbDHP
Comparison of DHP and Mb Structures比起DHP和Mb 结构
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Resonance Raman spectroscopy of iron-histidine stretching mode of
deoxy dehaloperoxidase
The frequency of the Fe-His mode is intermediate betweenthat of myoglobin (HHMb) and horseradish peroxidase (HRP).
Franzen et al., JACS (1998), 120, 4658-4661
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Model for proximal hydrogen bonding
Peroxidase Catalytic TriadAsp-His-Fe
Franzen JACS 2001,123, 12578
Asp
HisFe
This is the “push”In peroxidase mechanism
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Model for proximal hydrogen bonding
Dehaloperoxidase Catalytic TriadC=O-His-Fe?
Franzen JACS 2001,123, 12578
Backbone C=O
HisFe
Hydrogen bond strengthis intermediate betweenMb and HRP/CcP.
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General expression for Raman scattering transition polarizability
The transition polarizability involves an electricfield interaction for the incident wave σ and thescattered wave ρ:
αρσ if=
i|eσ|n n|eρ| fEn – E f – hω – iΓn
Σn
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Raman scattering transition polarizability for vibronic bands
Substituting in the H-T expansion we have for σ:
αρσ 01
′ =i0|eσ|r0
r0| ∂H /∂Q |n1
Er0 – En11|Q|0 n1|eρ|i1
En1 – Ei0 – hω – iΓnΣv
αρσ 01
′′ =i0|eσ|n0 0|Q|1
r1| ∂H /∂Q |n0
Er1 – En0r1|eρ|i1
En0 – Ei0 – hω – iΓnΣv
and for ρ:
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Vibronic matrix elements for B1g and B2g vibronic modes
αxybg = – bgRσ02
sin2 α sin 2αhωΚ
+sin α cos α cos 2α
EB0 – EQ0 + hωΚ
EQ0 – hω 2 + ΓQ2
–
cos2 α sin 2αhωΚ
+sin α cos α sin 2α
EB0 – EQ0 – hωΚ
EB0 – hω 2 + ΓB2
– bgRσ0 2
sin α cos α sin 2α
EB0 – EQ0 – hωΚ
–sin2 α sin 2α
hωΚ
EQ1 – hω 2 + ΓQ2
+
cos2 α sin 2αhωΚ
–sin α cos α sin 2α
EB0 – EQ0 + hωΚ
EB1 – hω2
+ ΓB2
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Vibronic Raman excitation profile as a function of angle α for B1g modes
Predicted Jahn-Tellerenhancement
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Vibronic Raman excitation profile for the Q band for B1g modes
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Vibronic Raman excitation profile as a function of vibronic strength of B1g
modes for α = 1o
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Vibronic Raman excitation profile as a function of vibronic strength of B1g
modes for α = 15o
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Vibronic matrix element for A2gvibronic modes
αxya2g = – a2gRσ
0 2cos α sin α
1/ EB0 – EQ0 + hωΚ
EQ0 – hω – iΓQ+
1/ EB0 – EQ0 – hωΚ
EB0 – hω – iΓB–
1/ EB0 – EQ0 – hωΚ
EQ1 – hω – iΓQ–
1/ EB0 – EQ0 + hωΚ
EB1 – hω – iΓB
The A2g modes can be easily assigned from the depolarization ratio. These modes are prediected to have ρ = ∞. Experimentally, large ρ >> 0.75are observed.
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Vibronic Raman excitation profile as a function of vibronic strength of A2g
modes for α = 15o
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Raman spectra for the Soret, Q, and III
Franzen et al. JACS (2002) 124, 7146
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Q band Resonance Raman spectra
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REPs of B1g modes
ν11
ν10
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ν19
ν20 ν21
REPs of A2g modes
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Band III resonance Raman spectra
Franzen et al. JACS (2002) 124, 7146
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REP of high frequency band III modes
ν10
ν13
ν11
ν19ν28
Franzen et al. JACS (2002) 124, 7146
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B1g modes
REP of high frequency band III modes
Franzen et al. JACS (2002) 124, 7146
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Significance of Mb substatesConformational substates must include functional tests forspecific functional states.
The biological role of myoglobin is consistent with relaxationand conformational states of the distal pocket.
There are specific conformational states that are important1. The docking site (transient)2. The distal histidine interaction (closed form)3. The open form
Inhomogeneous broadening is a background that is always present.
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Deoxy Mb MbCO
The 150 cm-1 B1g mode is ν18 in MbCOand a mixed ν18, γ16 mode in deoxy Mb
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The side view reveals significant out-of-plane character for ν18 in deoxy Mb
Deoxy Mb MbCO
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Magnetic Circular Dichroism
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The Perimeter Model
The porphine ring has D4h symmetry.The aromatic ring has 18 electrons.The p system approximates circular electron path.
N N
NN
m = 01
2
3
4
5
-1-2
-3
-4
-5
Φ = 12πeimφ
∆m=1∆m=9
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MCD spectra
Franzen, JPC Accepted
∆ε ν = A1 –∂ f ν∂ν + B0 + C0
k BTf ν
∆εm ν
ε ν= A1µBD0
–∂ f ν∂νf
∆Lz = 2A1
D0
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MbCO MCD spectra follow the PM
The spectra are A-term MCD as shown by the derivatives of the absorption spectrum (red). The (Q MCD) = 9 x (B MCD).
B Q
Franzen, JPC Accepted
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Deoxy MCD spectra are anomalous
C-term 4 time larger than MbCO!
B Q
A-term but with vibronic structure
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MCD spectra: Vibronic couplingin the Perimeter Model
Franzen, JPC Accepted
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MCD spectra: Vibronic couplingin the Perimeter Model
Metal Porphyrin Vibronic Distortions
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Conclusions for band IIIAlthough band III behaves as a ring-to-metalcharge transfer band it is vibronically coupledto the Soret band. It is not the iron-histidinemode that is coupled to the transition, but ratherin-plane non-totally symmetric modes. 1. The polarization arises from vibronic coupling
to in-plane non-totally symmetric modes.
2. The small charge displacement results fromstrong mixing of III with the Soret band.
3. The temperature dependence arises from thermalpopulation of the B1g mode ν18 at 150 cm-1.
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Anharmonic Coupling
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Temperature dependence of the H93G(Im) axial mode
Franzen, Fritsch, Brewer JPC Accepted
80 K
293 K
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Temperature dependence of the H93G(2-Me Im) axial mode
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Temperature dependence of the H93G(H2O) axial mode
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Differences in anharmonicity
for axial ligands
Experimental approachshows that axial ligands differ.
Horse heart is closest toH93G(4-Me Im).
H93G(2-Me Im) is the most anharmonic.
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DFT calculated mode-mode anharmonic
coupling
Comparison with experiment
Frequency shift of axial-ligand mode calculated for displacement of iron doming mode.
Only H93G(1-Me Im) excluded because of Fermi resonance.
Im4-Me Im
2-Me Im
4-Br Im
2,4-diMe Im
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Iron-ligand stretching and doming modes
νFe-L νFe-doming
4-Me Im
1-Me Im
143 cm-1 77 cm-1
161 cm-1 81 cm-1
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Iron-ligand stretching and doming modes
νFe-L νFe-doming
Im
4-Br Im
70 cm-1
87 cm-1
143 cm-1
170 cm-1
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Significance of substatesFunctional test is needed.
In spectroscopy solvation states give rise toinhomogeneous broadening.
Propose the same distinction for kinetics.Non-exponential kinetics in myoglobin is dueto an inhomogeneous distribution.
The inhomogeneous linewidth depends on the environment and hence is not the samefor all proteins.
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Myoglobin function depends on specific distal dynamics
Prevent autooxidation: docking site
Discriminate against CO binding: histidine 64
Trigger for O2 release: low pH form?
These functional criteria are all consistent withthe alternative view. They all involve dynamicson the distal side.
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A picture of the open and closed states of myoglobin
H64pH 7 νCO = 1943 cm-1
pH 5 νCO = 1967 cm-1
Protein Data Bank 1VXC, 2MGKYang and Phillips JMB 1996, 256, 762
1920 1950 1980 1920 1950 1980