Effects of pressure on visible spectra of complexes of myoglobin, hemoglobin, cytochrome c, and horse radish peroxidase

Effects of Supercritical CO2 Treatment on Color, Lipid Oxidation, Heme Iron, Non-Heme Iron and Metmyoglobin Contents in Ground Pork

The color, lipid oxidation, heme iron (HI) and non-heme iron (NHI) contents, metmyoglobin content and Soret band of myoglobin of ground pork subjected to supercritical CO2 treatment under different conditions, or to heat treatment (40°C, 2 h) and subsequent storage at 4°C were evaluated during 9-day period. Supercritical CO2 treatment significantly increased CIE L* and CIE b* values of ground pork during subsequent storage, while the HI content was slightly affected. In general, CIE a* value and metmyoglobin content were decreased. Supercritical CO2 treatment for 2 h could increase the thiobarbituric acid-reactive substances (TBARS) value, while treatment for 1 h or less had no effect. The NHI content could be increased only after treatment at above 40°C or 17.2 MPa for 2 h. The Soret band of myoglobin was shifted to longer wavelength. Increasing treatment temperature from 35°C to 45°C could increase CIE L*, CIE a*, CIE b* and TBARS values, HI and NHI contents of the ground pork, while decreasing metmyoglobin content. As the treatment pressure increased from 13.8 MPa to 20.7 MPa, CIE b* and TBARS values were decreased, while the NHI and metmyoglobin contents were increased. However, the other parameters were unchanged. Extending exposure time from 0.5 h to 2 h could increase CIE L*, CIE b* and TBARS values, HI contents, while decreasing CIE a* value and metmyoglobin content. Correlation analysis showed that the TBARS value was significantly and negatively correlated with the HI content or metmyoglobin content in samples treated at 40°C or above for 2 h.

The Interplay between Molten Globules and Heme Disassociation Defines Human Hemoglobin Disassembly

Hemoglobin functions as a tetrameric oxygen transport protein, with each subunit containing a heme cofactor. Its denaturation, either in vivo or in vitro, involves autoxidation to methemoglobin, followed by cofactor loss and globin unfolding. We have proposed a global disassembly scheme for human methemoglobin, linking hemin (ferric protoporphyrin IX) disassociation and apoprotein unfolding pathways. The model is based on the evaluation of circular dichroism and visible absorbance measurements of guanidine-hydrochloride-induced disassembly of methemoglobin and previous measurements of apohemoglobin unfolding. The populations of holointermediates and equilibrium disassembly parameters were estimated quantitatively for adult and fetal hemoglobins. The key stages are characterized by hexacoordinated hemichrome intermediates, which are important for preventing hemin disassociation from partially unfolded, molten globular species during early disassembly and late-stage assembly events. Both unfolding experiments and independent small angle x-ray scattering measurements demonstrate that heme disassociation leads to the loss of tetrameric structural integrity. Our model predicts that after autoxidation, dimeric and monomeric hemichrome intermediates occur along the disassembly pathway inside red cells, where the hemoglobin concentration is very high. This prediction suggests why misassembled hemoglobins often get trapped as hemichromes that accumulate into insoluble Heinz bodies in the red cells of patients with unstable hemoglobinopathies. These Heinz bodies become deposited on the cell membranes and can lead to hemolysis. Alternatively, when acellular hemoglobin is diluted into blood plasma after red cell lysis, the disassembly pathway appears to be dominated by early hemin disassociation events, which leads to the generation of higher fractions of unfolded apo subunits and free hemin, which are known to damage the integrity of blood vessel walls. Thus, our model provides explanations of the pathophysiology of hemoglobinopathies and other disease states associated with unstable globins and red cell lysis and also insights into the factors governing hemoglobin assembly during erythropoiesis.

Softening of the packing density of horseradish peroxidase by a H-donor bound near the heme pocket

We use pressure tuning of spectral holes to estimate the compressibility of protein molecules by optical means. We found that the compressibility of mesoporphyrin-substituted horseradish peroxidase increases by a factor of three when it incorporates small aromatic H-donor molecules that bind in the vicinity of its heme pocket. Such a dramatic softening of its packing density corresponds to a jump from a compressibility range characteristic for the solid state into that characteristic for liquids.

Allosteric Effectors Influence the Tetramer Stability of Both R- and T-states of Hemoglobin A

The contribution of heterotropic effectors to hemoglobin allostery is still not completely understood. With the recently proposed global allostery model, this question acquires crucial significance, because it relates tertiary conformational changes to effector binding in both the R- and T-states. In this context, an important question is how far the induced conformational changes propagate from the binding site(s) of the allosteric effectors. We present a study in which we monitored the interdimeric interface when the effectors such as Cl-, 2,3-diphosphoglycerate, inositol hexaphosphate, and bezafibrate were bound. We studied oxy-Hb and a hybrid form (alphaFeO2)2-(betaZn)2 as the T-state analogue by monitoring heme absorption and Trp intrinsic fluorescence under hydrostatic pressure. We observed a pressure-dependent change in the intrinsic fluorescence, which we attribute to a pressure-induced tetramer to dimer transition with characteristic pressures in the 70-200-megapascal range. The transition is sensitive to the binding of allosteric effectors. We fitted the data with a simple model for the tetramer-dimer transition and determined the dissociation constants at atmospheric pressure. In the R-state, we observed a stabilizing effect by the allosteric effectors, although in the T-analogue a stronger destabilizing effect was seen. The order of efficiency was the same in both states, but with the opposite trend as inositol hexaphosphate > 2,3-diphosphoglycerate > Cl-. We detected intrinsic fluorescence from bound bezafibrate that introduced uncertainty in the comparison with other effectors. The results support the global allostery model by showing that conformational changes propagate from the effector binding site to the interdimeric interfaces in both quaternary states.

Effect of pressure on pulse radiolysis reduction of proteins

Pulse radiolysis experiments were performed on proteins under pressure. Whereas many spectroscopic techniques have shown protein modifications at different pressure ranges, the present measurements performed using the water radiolysis allowed to generate radical species and to study the mechanisms implied in their reactions with proteins. This work gives the first results obtained on the effects of pressure on the rate constants of the proteins reduction by the hydrated electron at pressures up to 100 MPa. The reaction with the hydrated electron was investigated on two classes of protein: the horse myoglobin and the mussel metallothioneins. We have successively studied the influence of the pH value of metmyoglobin solutions (pH 6, 7 and 8) and the influence of the metals nature (Zn,Cu,Cd) bound to metallothioneins. For both protein, whatever the experimental conditions, the pressure does not influence the value of the reduction rate constant in the investigated range (0.1-100 MPa).

High-pressure FTIR study of the stability of horseradish peroxidase. Effect of heme substitution, ligand binding, Ca++ removal, and reduction of the disulfide bonds

The pressure stability of horseradish peroxidase isoenzyme C and the identification of possible stabilizing factors are presented. The effect of heme substitution, removal of Ca(2+), binding of a small substrate molecule (benzohydroxamic acid), and reduction of the disulfide bonds on the pressure stability were investigated by FTIR spectroscopy. HRP was found to be extremely stable under high pressure with an unfolding midpoint of 12.0 +/- 0.1 kbar. While substitution of the heme for metal-free mesoporphyrin did not change the unfolding pressure, Ca(2+) removal and substrate binding reduced the midpoint of the unfolding by 2.0 and 1.2 kbar, respectively. The apoprotein showed a transition as high as 10.4 kbar. However, the amount of folded structure present at the atmospheric pressure was considerably lower than that in all the other forms of HRP. Reduction of the disulfide bonds led to the least pressure stable form, with an unfolding midpoint at 9.5 kbar. This, however, is still well above the average pressure stability of proteins. The high-pressure stability and the analysis of the pressure-induced spectral changes indicate that the protein has a rigid core, which is responsible for the high stability, while there are regions with less stability and more conformational mobility.

High-pressure effects on horse heart metmyoglobin studied by small-angle neutron scattering

Small-angle neutron scattering experiments were performed on horse azidometmyoglobin (MbN3) at pressures up to 300 MPa. Other spectroscopic techniques have shown that a reorganization of the secondary structure and of the active site occur in this pressure range. The present measurements, performed using various concentrations of MbN3, show that the compactness of the protein is not altered as the value of its radius of gyration remains constant up to 300 MPa. The value of the second virial coefficient of the protein solution indicates that the interactions between the molecules are always strongly repulsive even if their magnitude decreases with increasing pressure. Taking advantage of the pressure-induced contrast variation, these experiments allow the partial specific volume of MbN3 to be determined as a function of pressure. Its value decreases by 5.4% between atmospheric pressure and 300 MPa. In this pressure range the isothermal compressibility of hydrated MbN3 is found to be almost constant. Its value is (1.6 +/- 0.1) 10-4 MPa-1.

The effects of osmotic and hydrostatic pressures on macromolecular systems

Osmotic pressure and hydrostatic pressure can be used effectively to probe the behavior of biologically important macromolecules and their complexes. Using the two techniques requires a theoretical framework as well as knowledge of the more common pitfalls. Both are discussed in this review in the context of several examples.

Unusual pressure effects on ligand rebinding to the human myoglobin Leucine 29 mutants

Using high pressure flash photolysis, we revealed that the side chain of Leu(29) controls the reaction volume of the ligand migration process in myoglobin, which is the primary factor for the unusual activation volume of ligand binding in some Leu(29) mutants. As we previously reported (Adachi, S., Sunohara, N., Ishimori, K., and Morishima, I. (1992) J. Biol. Chem. 267, 12614-12621), CO bimolecular rebinding in the L29A mutant was unexpectedly decelerated by pressurization, suggesting that the rate-determining step is switched to ligand migration. However, very slow CO bimolecular rebinding of the mutants implies that bond formation is still the rate-determining step. To gain further insights into effects of the side chain on ligand binding, we prepared some new Leu(29) mutants to measure the CO and O(2) rebinding reaction rates under high hydrostatic pressure. CO bimolecular rebinding in the mutants bearing Gly or Ser at position 29 was also decelerated upon pressurization, resulting in apparent positive activation volumes (DeltaV), as observed for O(2) binding. Based on the three-state model, we concluded that the increased space available to ligands in these mutants enhances the volume difference between the geminate and deoxy states (DeltaV(32)), which shifts the apparent activation volume to the positive side, and that the apparent positive activation volume is not due to contribution of the ligand migration process to the rate-determining step.

Hereditary late-onset chorea without significant dementia: genetic evidence for substantial phenotypic variation in Huntington's disease

We examined five individuals and obtained information concerning six other members from two unrelated families, nearly all of whom developed chorea after age 50 (one patient developed chorea at age 40). The severity of chorea progressed in all patients and became disabling in some individuals approximately 15 years after onset. Cognitive impairment was absent or minimal. All five examined patients were cognitively normal, even 10 to 30 years following the onset of chorea. Formal neuropsychometric testing demonstrated mild cognitive impairment in two individuals. Nevertheless, all patients were able to maintain employment or carry on with their usual household tasks until chorea was severe. One individual first became demented 30 years after the onset of chorea. Neuroimaging (with CT or MRI) in four patients failed to demonstrate significant caudate or putaminal atrophy 8 to 15 years following the onset of chorea. Three other family members (who were not available for examination) were said to have suffered chorea (without any mental decline) beginning after age 50, with subsequent survival of 20 years (in one) and 30 years (in two). Given this constellation of history and findings, three experienced neurologists and two medical geneticists concluded that these patients had a familial chorea syndrome distinct from Huntington's disease (HD). However, genetic analysis of the trinucleotide (CAG) repeat length associated with HD (in 4p16.3) determined repeat lengths of 44 and 46 in four patients tested (within the HD range). We conclude that these patients have HD and that such families represent further convincing examples of significant phenotypic variation for HD.

Hydration-dependent conformational states of hemoglobin. Equilibrium and kinetic behavior

The equilibrium and kinetics of methemoglobin conversion to hemichrome induced by dehydration were investigated by visible absorption spectroscopy. Below about 0.20 g water per g hemoglobin only hemichrome was present in the sample; above this value, an increasing proportion of methemoglobin appeared with the increase in hydration. The transition between the two derivatives showed a time-dependent biphasic behavior and was observed to be reversible. The rates obtained for the transition of methemoglobin to hemichrome were 0.31 and 1.93 min-1 and for hemichrome to methemoglobin 0.05 and 0.47 min-1. We suggest that hemichrome is a reversible conformational state of hemoglobin and that the two rates observed for the transition between the two derivatives reflect the alpha- and beta-chains of hemoglobin.

The effects of pressure on yeast cytochrome c peroxidase

The effects of pressure on cytochrome c peroxidase [CcP(FeIII)], its cyano derivative (CcP X CN) and its enzyme-substrate complex (ES) have been studied. The effects of pressure on the binding of the substrate analog porphyrin cytochrome c (porphyrin c) to CcP X CN and ES have also been studied. High pressure causes CcP(FeIII) to undergo a high-spin to low-spin transition but has no detectable effect on either CcP X CN, which is already low spin, or on ES. The low-spin CcP(FeIII) structure at pressure is similar to the low-spin form at low temperature and the low-spin form of horseradish peroxidase at high pressure. delta V degree associated with the spin equilibrium is about 30 ml/mol and is independent of temperature. delta G degree is small, 4.7 kJ/mol at 0 degree C, while delta H degree is 14.2 kJ/mol at 1 bar (100 kPa). Pressure has no detectable effect on the binding equilibria of mixtures of CcP X CN plus porphyrin c or ES plus porphyrin c. This indicates that the interaction of CcP and porphyrin c results in little or no volume change; the same is true in the case of cytochrome c oxidase and porphyrin c.

Heme protein fluorescence versus pressure

Fluorescence spectra of several ferric heme proteins have been measured vs. pressure to 6,000 bars. Sperm whale myoglobin (SW Mb), Aplysia myoglobin, leghemoglobin (Lb), and cytochrome P450 all show excitation and emission spectra characteristic of tryptophan in proteins with peak emission at 330-340 nm. At one bar, the fluorescence is weak due to energy transfer to the heme group, which makes the yield a sensitive probe of protein unfolding at high pressure. After an initial decrease of a few percent per kbar, the protein shows a large increase in fluorescence at high pressure. The increase is pH dependent and the results indicate that several high pressure states occur. For SW Mb at 15 degrees C an increase of a factor of 20 occurs with midpoint at 2,000 bars at pH 5 and is only partially reversible, while the increase at pH 7 occurs at 4,000 bars and is only half as large and is completely reversible. Aplysia Mb and Lb show a similar effect, but unfold at a higher pressure than SW Mb. P450 also shows a transition to a state of higher fluorescence, but the transition in this case is irreversible as a stable form, P420, is formed. The fluorescence intensity measurements permit an estimation of the increase in the TRY-heme distance in the high pressure state.

High-pressure effect on the equilibrium and kinetics of cyanide binding to chloroperoxidase

The kinetics of cyanide binding to chloroperoxidase were studied using a high-pressure stopped-flow technique at 25 degrees C and pH 4.7 in a pressure range from 1 to 1000 bar. The activation volume change for the association reaction is delta V not equal to + = -2.5 +/- 0.5 ml/mol. The total reaction volume change, determined from the pressure dependence of the equilibrium constant, is delta V degrees = -17.8 +/- 1.3 ml/mol. The effect of temperature was studied at 1 bar yielding delta H not equal to + = 29 +/- 1 kJ/mol, delta S not equal to + = -58 +/- 4 J/mol per K. Equilibrium studies give delta H degrees = -41 +/- 3 kJ/mol and delta S degrees = -59 +/- 10 J/mol per K. Possible contributions to the binding process are discussed: changes in spin state, bond formation and conformation changes in the protein. An activation volume analog of the Hammond postulate is considered.

The pressure dependence of the spin equilibrium in camphor-bound ferric cytochrome P-450

The spin equilibrium of camphor-bound ferric cytochrome P-450 has been measured between 1-1000 bar (10(5)-10(8) Pa). Increasing pressure shifts the absorption spectrum from the high-spin form at 392 nm to the low-spin form at 417 nm. The molar volume change for the spin states delta V = -RT delta ln Ke/ delta P and the equilibrium coefficient Ke = [high spin]/[low spin] depend on the solvent conditions. At pH 5.6 the equilibrium coefficient at 1 bar, K1 = 0.5 and delta V = 312 cm3/mol. A sample with 10 mM KCl at pH 7 has K1 = 7.0 amd delta V = 52 cm3/mol. Solvent changes producing a larger K1 also result in a larger delta V which ranged over 16-74 cm3/mol. The correlation can be approximated as delta V = 36 + 18 log K1, which implies that there is a pressure, 3000 bar, for camphor-bound ferric cytochrome P-450 at 4 degrees C, at which the changes in delta V are compensated by the other thermodynamic parameters leaving Ke independent of the solvent conditions. Although the protein is not stable about 1000 bar for most sample conditions, the extrapolated log Ke versus pressure curves for all sample conditions intersect near 3000 bar. Camphor-bound cytochrome P-450 appears to be a rather flexible protein, having a low denaturing pressure, a large volume change, and a high sensitivity to the protein environment.

Increase in apparent compressibility of cytochrome c upon oxidation

The apparent molal adiabatic compressibilities of ferri- and ferrocytochrome c have been determined from measurements of density and sound velocity. The values found were +2.99 X 10(-8) and -2.40 X 10(-8) cm5 mol-1 dyne-1 for the ferri and ferro forms, respectively. Experiments were performed on identical solutions containing either the oxidized or reduced form of protein. Solutions of ferricytochrome c were found to have significantly greater adiabatic compressibility than equivalent solutions of ferrocytochrome c at 25 degrees C and pH 7.15. The remarkable similarity of the three-dimensional structures of the ferri and ferro proteins [Takano, T. & Dickerson, R.E. (1980) Proc. Natl. Acad. Sci. USA 77, 6371-6375] strongly suggests that this difference in compressibility is due to an increase in volume fluctuations within ferricytochrome c relative to the ferro form rather than a change in equilibrium structure or hydration. Such a difference in the dynamic properties of the structures is consistent with both the crystallographic thermal B factors and the observed increase in amide hydrogen exchange kinetics when ferrocytochrome c is oxidized. The relative magnitude of the root mean square volume fluctuations is approximated from an ideal solution treatment of the compressibility data and yields a ratio of delta Vrms (ferri cyt c)/ delta Vrms (ferro cyt c) = 1.3.

Effects of pressure and temperature on the reactions of horseradish peroxidase with hydrogen cyanide and hydrogen peroxide

Reactions of ferric horseradish peroxidase with hydrogen cyanide and hydrogen peroxide were studied as a function of pressure. Activation volumes are small and differ in sign (delta V = 1.7 +/- 0.5 ml/mol for peroxidase + HCN and -1.5 +/- 0.5 ml/mol for peroxidase + H2O2). The temperature dependence of cyanide binding to horseradish peroxidase was also determined. A comparison is made of relevant parameters for cyanide binding and compound I formation.

Diffusion, patching, and capping of stearoylated dextrans on 3T3 cell plasma membranes

Fluorescence-labeled trinitrophenylated stearoylated dextrans have been used as controllable analogues of cell membrane proteins on model membranes and on a variety of natural cell membranes. This paper reports their behavior on 3T3 mouse fibroblast plasma membranes. Spatial distribution on the membrane was studied by fluorescence microscopy, and molecular mobility was measured by fluorescence photobleaching recovery. At concentrations from 10(2) to 3 X 10(3) molecules/micron2 essentially homogeneous fluorescence was observed after treatment with these stearoyldextrans in culture. Diffusion coefficients and fractional recovery of fluorescence after photobleaching were cvoncentration independent. For 3 X 10(3) molecules/micron2 we found at 23 degrees C D = (3.0 +/- 1.8) X 10(-10) cm2/s with 65 +/- 17% recovery and at 37 degrees C D = (7.0 +/- 5.0) X 10(-10) cm2/s without a change of the fractional recovery. Cross-linking with antibodies stopped diffusion on a macroscopic scale and sometimes induced patching, mottling (defined as the development of gaps in the fluorescence layer), and capping (defined as the confinement of the fluorescence to less than 50% of the cell). Capping required approximately 3 h at 37 degrees C and was inhibited by metabolic poisons and cytochalasin B. These drugs did not affect stearoyldextran diffusion or fractional recovery. Colchicine, which did not dramatically affect capping, slowed diffusion two- to threefold but did not affect fractional recovery. The antibody inhibition of the diffusion of stearoyldextrans precedent to capping did not affect the diffusion of a lipid probe or fluorescein isothiocyanate labeled membrane proteins. When the trinitrophenylated stearoyldextran was cleared from most of the surface by capping and the surface subsequently relabeled with stearoyldextran, the diffusion coefficient and fractional recovery of the second label were identical with those of the first label prior to capping. Thus, capping does not clear an immobilizing factor from the membrane.

High-pressure proton nuclear magnetic resonance studies of hemoproteins. Pressure-induced structural change in heme environments of myoglobin, hemoglobin, and horseradish peroxidase

Hyperfine shifted proton NMR spectra of metmyoglobin, methemoglobin, and their complexes with azide, imidazole, and cyanide as well as the spectrum of native horseradish peroxidase were obtained at high pressures up to 2000 atm with a specially designed high-pressure cell for 220-MHz superconducting NMR spectrometer. For the azide complexes of metmyoglobin, in all of which the iron atoms are in thermal spin equilibrium between high- and low-spin states, the increased pressure shifted their heme methyl proton signals to the upfield side. For the cyanide complexes of metmyoglobin and methemoglobin and for the fluoride complex of metmyoglobin, which are in purely low- and high-spin states, respectively, the spectra were almost insensitive to changes in pressure up to 2000 atm. The heme methyl proton signals of aquometmyoglobin, its formate complex, and horseradish peroxidase showed appreciable upfield shifts upon pressurization. These results were interpreted to indicate that the primary effect of pressure on the hemoprotein structure is to shift the spin equilibrium in favor of the low-spin form. Hemichrome formation of methemoglobin at high pressures was also observed, and the effect of pressure on the heme environmental structure of deoxyhemoglobin and deoxymyoglobin was also discussed.