Model compounds for the T state of hemoglobin.

Collman, James P.; Brauman, John I.; Doxsee, Kenneth M.; Halbert, Thomas R.; Suslick, Kenneth S.

doi:10.1073/pnas.75.2.564

Cited by 98 publications

(68 citation statements)

References 33 publications

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“…The third postulate, of a distal side steric effect, has been the subject of some controversy in model compound studies (7,9).In hemoproteins studied by x-ray crystallography, the Fell-CO has the ligand "pushed" off the axis by a nearby histidine or other distal side residue (12)(13)(14), in contrast to the observations in simple complexes (15). This effect was postulated to decrease the affinity of CO (16).…”

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confidence: 64%

Section: Introductionmentioning

confidence: 99%

“…Structural and dynamic studies of hemoproteins have led to three distinct but not necessarily exclusive proposals for the effects of the protein on the reactivity of heme toward 02 and CO. Two of these postulates, the proximal base tension effect (1,2) and the proximal imidazole deprotonation effect (3)(4)(5), have recently been documented by studies of model compounds (6)(7)(8)(9)(10)(11). The third postulate, of a distal side steric effect, has been the subject of some controversy in model compound studies (7,9).…”

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confidence: 99%

“…This effect was postulated to decrease the affinity of CO (16). A corollary suggestion is that Fel-OO, being bent already, should not suffer this steric effect, and CO binding in the hemoproteins should be impaired relative to that in simple model complexes (7,16).In contrast to these predictions, the model compounds chelated protoheme and chelated mesoheme were reported to bind both 02 and CO in aqueous suspension at 20°C with affinities and kinetics similar to those of the high-affinity proteins R-state hemoglobin and hemoglobin chains (8,17,18). A number of other simple model systems have CO affinities similar to those of the chelated hemes (6,9,19,20) although the corresponding studies with 02 have not been made.…”

mentioning

confidence: 99%

“…The third postulate, of a distal side steric effect, has been the subject of some controversy in model compound studies (7,9).…”

mentioning

confidence: 99%

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Binding of O2 and CO to hemes and hemoproteins.

Traylor¹,

Berzinis²

1980

Proc. Natl. Acad. Sci. U.S.A.

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Enthalpies and entropies have been determined for the reversible binding of 02 Structural and dynamic studies of hemoproteins have led to three distinct but not necessarily exclusive proposals for the effects of the protein on the reactivity of heme toward 02 and CO. Two of these postulates, the proximal base tension effect (1, 2) and the proximal imidazole deprotonation effect (3-5), have recently been documented by studies of model compounds (6-11). The third postulate, of a distal side steric effect, has been the subject of some controversy in model compound studies (7,9).In hemoproteins studied by x-ray crystallography, the Fell-CO has the ligand "pushed" off the axis by a nearby histidine or other distal side residue (12)(13)(14), in contrast to the observations in simple complexes (15). This effect was postulated to decrease the affinity of CO (16). A corollary suggestion is that Fel-OO, being bent already, should not suffer this steric effect, and CO binding in the hemoproteins should be impaired relative to that in simple model complexes (7,16).In contrast to these predictions, the model compounds chelated protoheme and chelated mesoheme were reported to bind both 02 and CO in aqueous suspension at 20°C with affinities and kinetics similar to those of the high-affinity proteins R-state hemoglobin and hemoglobin chains (8,17,18). A number of other simple model systems have CO affinities similar to those of the chelated hemes (6,9,19,20) although the corresponding studies with 02 have not been made.Based upon these observations we suggested that in R-state hemoglobin or isolated hemoglobin chains the protein served only to keep the heme five-coordinated and to prevent oxidation, imparting no special effect on the heme affinity for CO or 02-Other hemoproteins having lower CO or 02 affinities were presumed to accomplish this alteration from the standard hemoglobin chain behavior by one of the effects mentioned above (18).The more elaborate iron complexes such as cyclophane heme (21,22), capped heme (23), or picket-fence heme (7,24,25), all of which have bulky groups around one face of the heme, either do not mimic the hemoproteins or do so only in part. Picket-fence heme was reported to bind 02 in toluene or in solid state (7) with an affinity similar to that of chelated heme or R-state hemoglobin but to bind CO "irreversibly" in the solid state (24) and with greatly increased affinity in solution (25). These observations led to the conclusion that model compounds bind CO more strongly than do hemoproteins (24), in contrast to the proposal discussed above.We now report the thermodynamics of the binding of 02 and CO (Eqs. 1 and 2) and the replacement reaction (Eq. 3) which are very nearly the same as those of hemoglobin (R-state) Kinetics were determined with a microprocessor-controlled laser flash photolysis apparatus as described (9,26

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mentioning

confidence: 64%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

“…The third postulate, of a distal side steric effect, has been the subject of some controversy in model compound studies (7,9).…”

mentioning

confidence: 99%

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Binding of O2 and CO to hemes and hemoproteins.

Traylor¹,

Berzinis²

1980

Proc. Natl. Acad. Sci. U.S.A.

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Trans-substitution of the proximal hydrogen bond in myoglobin: II. Energetics, functional consequences, and implications for hemoglobin allostery

Barrick

2000

Proteins

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The trans-substituted histidine to glycine mutant of sperm whale myoglobin (H93G Mb) is used to study energetics of proximal hydrogen bonding, proximal ligand-heme interactions, and coupling to distal ligand binding. Comparison of mono- and dimethylimidazole structural isomers shows that the hydrogen bond between the proximal ligand and the neighboring Ser92 hydroxyl (position F7) is stabilizing. The range of hydrogen bond stabilities measured here for different distal ligand complexes ranges from -0.7 kcal/mol (monomethylimidazole isomers to MbCO) to -4.1 kcal/mol (dimethylimidazole isomers to MbCN). This range of hydrogen bond stabilities, which is similar to that seen in protein mutagenesis unfolding studies, demonstrates the high sensitivity of the hydrogen bond to modest structural perturbations. The degree to which the 2-methyl group destabilizes proximal ligand binding is found to depend inversely on the total electronic spin. For monomethylimidazole proximal ligands, distal ligand binding weakens the proximal hydrogen bond compared to deoxyMb. Surprisingly, this trend is largely reversed for the dimethylimidazole proximal ligands. These results demonstrate strong coupling between the proximal protein matrix and distal ligand binding. These results provide an explanation for the strong avoidance of hydrogen bonding residues at position F7 in hemoglobin sequences.

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Iron Porphyrin Chemistry

Walker¹,

Simonis²

2005

Encyclopedia of Inorganic Chemistry

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This article reviews most aspects of the chemistry of iron porphyrins, from Fe(0) to Fe(V), including occurrence and roles of natural iron porphyrins (hemes) and their synthetic analogs, structures and electron configurations of iron porphyrins of all oxidation and spin states, π electron configuration of the porphyrin ring, synthesis of metal‐free porphyrins and other related macrocycles, insertion of iron into free‐base porphyrins and related macrocycles, the properties, reactions, uses and biological relevance of iron(0), ‐(I), ‐(II) porphyrins (the latter with S = 0, 1, and 2 spin state possibilities), of iron(II) porphyrin π‐cation radicals, of iron(III) porphyrins (with S = 1/2, 3/2, and 5/2 spin state possibilities), of iron(III) porphyrin and corrole π‐cation radicals, of iron(IV) porphyrins (including five‐ and six‐coordinate ferryl (FeO) 2+ , iron(IV) phenyl, carbene and hydrazine complexes, and the bis‐methoxide complex) and a comparison of iron(IV) porphyrins to iron(III) porphyrin π‐cation radicals, of iron(IV) porphyrin π‐cation radicals, and of the possible existence of iron(V) porphyrins. Included in the Fe(II) part are sections on addition of ligands to four‐coordinate iron(II) porphyrins, including equilibrium binding constants, photodissociation of ligands from PFeL 2 complexes, binding of small molecules (O 2 , CO, NO, HNO) to 5‐coordinate iron(II) porphyrins and design of porphyrin ligands that will mimic the active sites of heme proteins such as myoglobin and hemoglobin, the cytochromes P450 and nitric oxide synthases, and the nitrophorins and guanylyl cyclases. Included in the iron(III) part are sections on both 5‐ and 6‐coordinate high‐spin complexes and their similarities and differences, bridged or through‐space magnetically coupled complexes of high‐spin iron(III) porphyrins with other metal complexes as possible models for cytochrome oxidase and the assimilatory sulfite reductases, coupled oxidation of hemes by hydrogen peroxide or its equivalent, and the relationship of this reactivity to the reactions of heme oxygenase, iron(III) porphyrins as reduction catalysts, and photochemistry of iron(III) porphyrins, possible electron configurations of low‐spin iron(III) porphyrins, the phenomenon and possible electronic consequences of ruffling of the porphinato core in iron(III) porphyrins, the preferred orientation of planar axial ligands bound to low‐spin iron(III) porphyrins, NO complexes of iron(III) porphyrins, reduction potentials, equilibrium constants and rates of axial‐ligand addition and exchange, kinetics of axial‐ligand rotation and porphyrin ring inversion, kinetics of reduction and autoreduction of iron(III) porphyrins, electron self‐exchange between low‐spin iron(III) and iron(II) porphyrins, synthetic ferriheme proteins, and synthesis of five‐coordinate low‐spin iron(III) porphyrins having σ‐alkyl or σ‐aryl groups as axial ligands. The iron(IV) and iron(IV) cation radical sections discuss the high‐valent states of cytochromes P450 and related enzymes.

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Model compounds for the T state of hemoglobin.

Cited by 98 publications

References 33 publications

Binding of O2 and CO to hemes and hemoproteins.

Binding of O2 and CO to hemes and hemoproteins.

Trans-substitution of the proximal hydrogen bond in myoglobin: II. Energetics, functional consequences, and implications for hemoglobin allostery

Iron Porphyrin Chemistry

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