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Dissociation of modular humeral head components: a biomechanical and implant retrieval study.

In vivo dissociation of the Morse-taper of shoulder arthroplasty modular humeral components has been reported. The incidence of this complication appears to be approximately 1:1000. The objective of this study was to identify conditions that might affect the Morse-taper interface strength in humeral components. Mechanical tests were performed to load and dissociate humeral heads from the humeral stems (titanium). The effect of loading rate, load amplitude, and number of impactions was investigated. Dissociation force was measured after the taper was contaminated with water, oil, blood, and bone cement particles. The mean dissociation force after two impactions with a mallet was 2926 +/- 955 N. Dissociation force was linearly proportional to impaction force. Repetitive loading beyond two impactions did not significantly increase taper strength. Contamination of the taper with as little as 0.4 ml of fluid could prevent fixation of the taper.

Biomechanical Phenomena↗

PubMed2006

Is dissociative amnesia a culture-bound syndrome? Findings from a survey of historical literature.

BACKGROUND: Natural human psychological phenomena, such as depression, anxiety, delusions, hallucinations and dementia, are documented across the ages in both fictional and non-fictional works. We asked whether 'dissociative amnesia' was similarly documented throughout history. METHOD: We advertised in three languages on more than 30 Internet web sites and discussion groups, and also in print, offering US$1000 to the first individual who could find a case of dissociative amnesia for a traumatic event in any fictional or non-fictional work before 1800. RESULTS: Our search generated more than 100 replies; it produced numerous examples of ordinary forgetfulness, infantile amnesia and biological amnesia throughout works in English, other European languages, Latin, Greek, Arabic, Sanskrit and Chinese before 1800, but no descriptions of individuals showing dissociative amnesia for a traumatic event. CONCLUSIONS: If dissociative amnesia for traumatic events were a natural psychological phenomenon, an innate capacity of the brain, then throughout the millennia before 1800, individuals would presumably have witnessed such cases and portrayed them in non-fictional works or in fictional characters. The absence of cases before 1800 cannot reasonably be explained by arguing that our ancestors understood or described psychological phenomena so differently as to make them unrecognizable to modern readers because spontaneous complete amnesia for a major traumatic event, in an otherwise lucid individual, is so graphic that it would be recognizable even through a dense veil of cultural interpretation. Therefore, it appears that dissociative amnesia is not a natural neuropsychological phenomenon, but instead a culture-bound syndrome, dating from the nineteenth century.

Advertising↗

PubMed2000

A capillary electrophoretic reactor with an electroosmosis control method for measurement of dissociation kinetics of metal complexes

The solvolytic dissociation rate constants of 1:2 complexes of Al3+ and Ga3+ with an azo dye ligand, 2,2'-dihydroxyazobenzene-5,5'-disulfonate (DHABS, H2L2-), have been evaluated with a capillary electrophoretic reactor (CER) system. This CER system is based on the fact that metal complexes encounter an overwhelming force to dissociate when apart from the ligand by CE resolution. Treatment of a capillary with a slightly acidic buffer solution, e.g., pH 5, reduces the double-layer potential (zeta) of the inner silica wall. Owing to slow relaxation of the deprotonation equilibria of superficial silanol groups known as the pH hysteresis, this zeta potential can be actually retained during the electrophoresis of the metal complexes in question with a neutral buffer at pH 7.0. This method enables one to manipulate migration times, namely, residence times in a capillary tube, from 5 to 90 min, depending on the prescribed conditioning pH, without changing any other operation conditions such as buffer composition and electric field strength. The excellent performance of the CER is exemplified by the accurate estimation of the dissociation degree of the complexes. The dissociation degree-time profiles for the complexes are quantitatively described using both internal and external standards; the very inert complex of [Co(III)L2]5- for the peak signal standardization and methyl orange for the injection volume correction. The solvolytic dissociation rate constants of the 1:2 complexes of Al3+ and Ga3+ ions with DHABS [AlL2]5- and [GaL2]5- into the 1:1 ones have been determined as (4.9+/-1.0) x 10(-4) and (3.7+/-0.3) x 10(-3) s(-1) at 303 K, respectively.

Journal Article↗

PubMed2005

Determination of bond dissociation energies using electrospray tandem mass spectrometry and a derived effective reaction path length approach.

A new approach for calculating bond dissociation energies (BDEs) from ES-MS/MS measurements has been developed. The new method features a "derived effective reaction path length" that has been applied to measure BDEs of alkali metal (Li+) adducts and halide (Cl-) adducts of monoacylglycerol, 1,2-diacylglycerol, and 1,3-diacylglycerol lipids. Also studied were lithium-bound dimers of monoacylglycerols, 1,2-diacylglycerols, and 1,3-diacylglycerols. BDEs for the adducts and dimers of the lipids were derived from collision-induced dissociation experiments using a triple quadrupole mass spectrometer with electrospray as the ionization source. Mass spectral data were used to empirically derive a single-exponential growth equation that relates product cross section to collision energy. From these single-exponential equations, a general second-order polynomial was derived using a multivariate growth curve model that enables prediction of BDEs of unknown complexes. Mass spectral results were compared to computer-generated bond dissociation energies using Becke-style three-parameter density functional theory (B3LYP, employing the Lee-Yang-Parr correlation functional), with excellent agreement between experimental and theoretical energy values. The newly developed method is general in nature and can be used for the measurement of metal or halide ionic adduct bond dissociation energies and for the measurement of bond energies of noncovalent interactions such as dimer dissociation energies. The validity of the method has been rigorously established using a triple quadrupole, but it may also be applied to other mass spectrometers that allow user control of the collision cell potential.

Journal Article↗

PubMed2006

Thermally assisted collision-induced dissociation in a quadrupole ion trap mass spectrometer.

Thermally assisted collision-induced dissociation (TA-CID) provides increased dissociation in comparison with CID performed at ambient temperature in a quadrupole ion trap mass spectrometer. Heating the bath/collision gas during CID increases the initial internal energy of the ions and reduces the collisional cooling rate. Thus, using the same CID parameters, the parent ion can be activated to higher levels of internal energy, increasing the efficiency of dissociation and the number of dissociation pathways. The increase in the number of dissociation pathways can provide additional structural information. A consequence of the increase in initial internal energy is the ability to use less power to effect collisional activation. This allows lower q(z) values to be used and, thus, a greater mass range of product ions to be observed. TA-CID alleviates the problems associated with traditional CID and results in more available information than traditional CID.

Journal Article↗

PubMed1997

Determination of acid dissociation constants of peptide side-chain functional groups by two-dimensional NMR.

An NMR method is described for determining residue-specific acid dissociation constants for peptides which contain more than one residue of the same acidic or basic amino acid. The method is based on using the differences in C alpha H proton chemical shifts which result from peptide sequence nearest-neighbor and possibly secondary structure effects to resolve resonances for carbon-bonded reporter protons adjacent to each side-chain acidic group in two-dimensional total correlation spectroscopy (TOC-SY) spectra. Acid dissociation constants were determined for each of the four lysine side-chain ammonium groups of the peptide Lys-Asn-Asn-Gln-Lys-Ser-Glu-Pro-Leu-Ile-Gly-Arg-Lys-Lys-Thr-NH2. Resonances for the C epsilon H2 protons adjacent to the four side-chain ammonium groups, which overlap in the one-dimensional spectrum, were resolved using the C alpha H-C epsilon H2 cross peaks in the TOCSY spectrum. Chemical shift-pH titration data were obtained for each lysine side-chain ammonium group from one-dimensional subspectra taken from two-dimensional TOCSY spectra measured as a function of pH. The pKAs of the Lys1, Lys5, Lys13, and Lys14 side-chain ammonium groups were determined to be 11.14 +/- 0.01, 10.95 +/- 0.01, 10.96 +/- 0.02, and 11.09 +/- 0.02, respectively. The chemical shift-pH titration data were also analyzed by a pH-independent procedure to obtain relative acid dissociation constants: KA(Lys1)/KA(Lys5) = 0.663 +/- 0.009, KA(Lys1)/KA(Lys13) = 0.703 +/- 0.014, and KA(Lys1)/KA(Lys14) = 0.910 +/- 0.009, which correspond to relative acidities for the side-chain ammonium groups of Lys1, Lys5, Lys13, and Lys14 of 1:1.508:1.422:1.099. To further demonstrate the utility of this method, acid dissociation constants were determined for the six acidic groups of the peptide Glu-Ala-Cys-Asn-Pro-Ala-Ala-Gly-Arg-His-Tyr-Ser-Cys-NH2. Chemical shift-pH titration curves were obtained for the C beta H2 protons adjacent to the two cysteine thiol groups using one-dimensional subspectra taken from TOCSY spectra measured as a function of pH. The pKAs of the CyS3 and Cys13 thiol groups were determined to be 9.21 +/- 0.07 and 8.60 +/- 0.06, respectively. The relative acid dissociation constants (KA(Cys3)/ (KA(Cys13)) were found to be 0.21 +/- 0.06 by the pH-independent calculation procedure.

Amino Acid Sequence↗

PubMed1995

Kinetics of annexin VI, calcium, and phospholipid association and dissociation.

Annexins VI and V are members of the annexin family of proteins that bind to phospholipid membranes in a calcium-dependent manner. The dynamics of protein, calcium, and phospholipid assembly and dissociation were investigated by stopped-flow. At relatively low calcium levels, the kinetics of the binding reaction were sensitive to calcium concentration. However, in the presence of saturating levels of calcium and at relatively low protein/vesicle (w/w) ratios (0.4 or lower), the binding reactions were rapid and the rate constants were comparable to the collisional limit, about 1.4 x 10(10) M-1 s-1 for large unilamellar vesicles (about 120 nm diameter) and about 2.7 x 10(9) M-1 s-1 for small unilamellar vesicles (about 31 nm in diameter). These constants are expressed on the basis of vesicle concentration. These limiting association rate constants were not sensitive to the phospholipid composition of the vesicles. In contrast, at these calcium levels, protein dissociation was so slow that the complexes could be regarded as stable. However, individual calcium ions that were bound to the complexes appeared to exchange rapidly with ions in bulk solution. EGTA-induced protein dissociation was rapid with first-order rate constants ranging from 10 to 50 s-1. These were dependent on the membrane composition and on the protein type (annexin VI or V). Variations in this dissociation process were found to complement the calcium concentration needed to support annexin-membrane association; increasing the acidic phospholipid component or partially replacing phosphatidylcholine by phosphatidylethanolamine in the membrane decreased both the EGTA-induced dissociation rate and the calcium concentration needed to support binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Determination of kinetic rate constants for the binding of inhibitors to HIV-1 protease and for the association and dissociation of active homodimer.

Association and dissociation rate constants for a competitive inhibitor of HIV-1 protease were determined by a novel method employing a pair of integrated rate equations. This method, termed the paired progress curve method, is both rapid and reproducible. Progress curves, taken at a single concentration of inhibitor, are analyzed simultaneously to determine association and dissociation rate constants, the concentration of active sites, and the catalytic rate constant. The method is applied to BILA 398, a compound for which the cocrystal structure with HIV-2 protease has been reported recently [Tong, L., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 8387-8391]. This compound exhibited an association constant of 1.6 x 10(7) M-1 s-1 and a dissociation constant of 1.0 x 10(-4) s-1 corresponding to a binding affinity constant of 6.4 x 10(-12) M. During the course of the analysis, nonlinearity was observed in control reactions containing enzyme and substrate only. This was subsequently shown to be due to a reversible inactivation process resulting from enzyme dilution. Integrated rate equations were developed on the basis of the dissociation of active dimeric enzyme during dilution and a reassociation of dilute monomers following the addition of substrate. The equations were modeled to the data, yielding a dissociation constant of 1.9 x 10(-3) s-1 and an association constant of 9.2 x 10(5) M-1 s-1 for the monomer-dimer interconversion process. This corresponds to an equilibrium constant of 4 x 10(-9) M for the dimerization of HIV-1 protease.

Binding, Competitive↗

PubMed1991

Structural studies of gangliosides by fast atom bombardment ionization, low-energy collision-activated dissociation, and tandem mass spectrometry.

Negative ion fast atom bombardment, low-energy collision-activated dissociation, and tandem mass spectrometry techniques were applied for the structural elucidation of gangliosides. The mass spectra were simplified by selecting a single molecular ion or fragment ion in the analysis of mixtures, and interference by background signals from the liquid matrix could be avoided. Introduction of collision-activated dissociation produced abundant fragment ions convenient for structural analysis. In the daughter scan mode, ions were produced by cleavage of the glycosidic bonds, and not by cleavage at the sugar ring. These ions all contain ceramide moieties, except the sialic acid fragment ion. In the parent scan mode, product ions resulting from cleavage at the sugar ring were detected beside the ions resulting from cleavage at the glycosidic bonds, and ions of oligosaccharide fragments were also detected. In parent scan mode spectra of gangliosides based on the sialic acid ion, all ions contained a sialic acid residue, and the observed ions were similar to those obtained in the high-energy collision-activated dissociation daughter scan mode. These results indicate the usefulness of low-energy collision-activated dissociation tandem mass spectrometry in the daughter and parent scan modes for the analysis of ganglioside structure, in combination with fast atom bombardment mass spectrometry and high-energy collision-activated dissociation mass spectrometry.

Carbohydrate Conformation↗

PubMed1984

Stability of quaternary structure and mode of dissociation of fructosediphosphate aldolase isoenzymes.

Using a highly sensitive "subunit exchange" assay, we have studied the relative strengths of interactions between different subunit types (A and C) of fructosediphosphate aldolase and have determined the mode of dissociation of aldolase tetramers in vitro. Interactions between C subunits within C4 tetramers were found to be considerably more resistant to disruption than were interactions between A subunits in A4 tetramers with regard to increasing concentrations of H+, OH-, or urea. Slight dissociation of A4 was also observed in 1.2 M magnesium chloride. These observations suggest that the quaternary structure of aldolase C4 is inherently more stable than that of aldolase A4. Also, the symmetrical heterotetramer A2C2 was found to be more resistant to urea-mediated dissociation than was the aldolase A4 homotetramer; this observation suggests that, even when in heteromeric combination, C subunits have a stabilizing influence on the quaternary structure of aldolase tetramers. In no case did we find evidence for a stable dimeric intermediate in the dissociation of aldolase tetramers to monomers. These observations are considered in terms of the tetrahedral arrangement of subunits in the aldolase tetramer. The general applicability of the subunit exchange assay described here for studying the subunit structure and mode of dissociation of oligomeric enzymes is discussed.

Animals↗

PubMed1985

Dissociation of the receptor for immunoglobulin E in mild detergents.

We previously showed that, in the absence of phospholipids, exposure of the tetrameric receptor for immunoglobulin E to mild detergents dissociates the intact beta chain and two gamma chains from the alpha chains. Having developed a practical method for assaying the dissociation, we have now explored a variety of different detergents, detergent concentrations, temperatures, times, salts, pHs, and other factors that influence the detergent-induced dissociation. Our findings should be useful for optimizing the stability of the receptor and for future studies on recombination of the subunits. The data suggest the following: (1) The critical perturbant is micellar detergent. (2) Unlike solubilization of membranes, where a molar ratio of micellar detergent:lipid of 2 is adequate, dissociation of the receptor is incomplete even at molar ratios of micellar detergent:receptor of greater than 10(5) and may be limited by a reversible component. (3) Detergents that are best for solubilizing membranes are also best for dissociating the receptors. (4) The latter observation and other data implicate bound lipid as stabilizing the receptor. Our findings may be applicable to the study of interactions between membrane proteins in general.

Animals↗

PubMed1986

ATP-induced dissociation of rabbit skeletal actomyosin subfragment 1. Characterization of an isomerization of the ternary acto-S1-ATP complex.

The adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) induced dissociation of actomyosin subfragment 1 (S1) has been investigated by monitoring the light scattering changes that occur on dissociation. We have shown that ATP gamma S dissociates acto-S1 by a mechanism similar to that of ATP but at a rate 10 times slower. The maximum rate of dissociation is limited by an isomerization of the ternary actin-S1-nucleotide complex, which has a rate of 500 s-1 for ATP gamma S and an estimated rate of 5000 s-1 for ATP (20 degrees C, 0.1 M KCl, pH 7.0). The activation energy for the isomerization is the same for ATP and ATP gamma S, and both show a break in the Arrhenius plot at 5 degrees C. The reaction between acto-S1 and ATP was also followed by the fluorescence of a pyrene group covalently attached to Cys-374. We show that the fluorescence of the pyrene group reports the isomerization step and not actin dissociation. The characterization of this isomerization is discussed in relation to force-generating models of the actomyosin cross-bridge cycle.

Adenosine Triphosphate↗

PubMed1988

Sequential dissociation of Ca2+ from the calcium adenosinetriphosphatase of sarcoplasmic reticulum and the calcium requirement for its phosphorylation by ATP.

The kinetics for dissociation of the stable enzyme-calcium complex of the sarcoplasmic reticulum calcium ATPase, cE.Ca2, were followed by assay with simultaneous addition of [32P]ATP and EGTA, which gives 70% phosphorylation of cE.Ca2 with k = 300 s-1 (25 degrees C, pH 7.0, 5 mM MgSO4, 0.1 M KCl). The binding of ATP to cE.Ca2 is described by kATP = 1.0 X 10(7) M-1 s-1, k-ATP = 120 s-1, and Kdiss = 12 microM; ATP binding is partially rate limiting for phosphorylation at less than 100 microM ATP. The sequential dissociation of Ca2+ from cE.Ca2 is described by k-2 = 55-60 s-1 for the first, "outer" Ca2+, k-1 = 25-30 s-1 for the second, "inner" Ca2+, and K0.5 = 3.4 microM, n = 1.9 (from Kdiss = 7.4 X 10(-7) M for Ca.EGTA). Dissociation of the inner Ca2+ is inhibited by external Ca2+, with K0.5 = k-1/k2 = 0.7 microM. This confirms the conclusion that dissociation of the two Ca2+ ions is sequential. The ability of cE.Ca2 to catalyze phosphorylation by ATP disappears in the presence of EGTA with k = 50-55 s-1, the same as k-2 for dissociation of the outer Ca2+ ion. This result, and the absence of the induction period that would occur if both cE.Ca2 and cE.Ca1 were catalytically competent, shows that both Ca2+ ions are required for phosphorylation. This conclusion is confirmed by the stoichiometry of 1.4/0.7 = 2.0 for the ratio of Ca2+ internalized to phosphoenzyme formed after simultaneous addition of ATP and EGTA. Phosphorylation of cE.Ca2 in the presence of 45Ca gives 0.15, not 0.3, 45Ca internalized, which corresponds to exchange of only 1 Ca2+ and is in agreement with this conclusion. The requirements for binding of two Ca2+ for catalytic specificity toward ATP and loss of two Ca2+ from E approximately P.Ca2 for specificity toward water account for the stoichiometry of Ca2+ transport and provide a possible reason for the two steps in the phosphorylation of cE.Ca2.ATP.

Animals↗

PubMed1981

Pressure-induced reversible dissociation of enolase.

A study of the polarization of the intrinsic fluorescence and the fluorescence of dansyl conjugates of enolase shows that an increase in hydrostatic pressure, in the range of 1 bar-3 kbar, promotes the dissociation of this protein into dimers. The dissociation of oligomeric proteins under pressure is predicted to be a general phenomenon by a model that assumes the existence of small "free volumes" at the intersubunit boundaries. The same model predicts a dependence of the standard volume change in the dissociation reaction upon the pressure, owing to the additional surface compressibility of the monomers, and numerical analysis of the results clearly shows that dependence for enolase. For a midpoint dissociation pressure of 1.5 kbar the standard volume change in the dissociation reaction is delta V p0 = -65 +/- 8 mL mol-1 and the dependence of the volume change upon pressure (dVp0/dp) is approximately -30 mL mol-1 kbar-1. The reversibility of the pressure effects is shown to be better than 95% by either polarization or fluorescence spectrum recovery. The pressure perturbation of the fluorescence polarization is a method of general applicability to studies of protein aggregation, and it can be also of value in characterizing the effect of ligands on the aggregation of oligomeric proteins.

Dansyl Compounds↗

PubMed1982

Effect of colchicine binding on the reversible dissociation of the tubulin dimer.

The reversible subunit-dissociation equilibria of the tubulin alpha beta dimer and of the colchicine-tubulin dimer complex have been examined by equilibrium ultracentrifugation at 4.6 degrees C. The dissociation constants (KD) of tubulin from bovine brain and from flagellar outer-doublet microtubules of the sea urchin Strongylocentrotus purpuratus were 7.4 x 10(-7) M and 15.2 x 10(-7) M, respectively. In contrast, both brain and outer-doublet colchicine-tubulin complexes dissociated less readily into their alpha- and beta-tubulin monomers; KD = 2.7 x 10(-7) M for the brain complex and KD = 5.0 x 10(-7) M for the outer-doublet colchicine-tubulin species. Podophyllotoxin (2 x 10(-4) M), tropolone (10(-4) M), or both podophyllotoxin and tropolone (2 x 10(-4) and 5 x 10(-4) M, respectively) had no effect on the dissociation constant of brain tubulin. Under these experimental conditions, the initial colchicine-binding capacities of brain and flagellar tubulins were 0.87 +/- 0.05 and 0.70 +/- 0.07 mol/mol, respectively. The colchicine-binding activities of free tubulins decayed at 4 degrees C according to first-order kinetics with half-times of 37 h for brain tubulin and 26 h for flagellar tubulin. However, colchicine-tubulin complexes (brain or flagellar) showed no decay of binding activity when stored at 4 degrees C for periods up to 4 days. These results strongly support the following conclusions: (1) colchicine binding induces or stabilizes a conformational isomer of tubulin that dissociates into its alpha and beta monomers less readily than free tubulin; (2) the conformational change in tubulin is specific for binding of the intact colchicine molecule and does not occur when ligands specific for the trimethoxyphenyl subdomain (i.e., podophyllotoxin) or tropolone subdomain of the colchicine-binding site bind singly or simultaneously to tubulin; and (3) bound colchicine stabilizes tubulin against loss of colchicine-binding activity at 4 degrees C. This altered conformation of tubulin may be involved in the substoichiometric poisoning of microtubule assembly produced by the addition of colchicine-tubulin complexes to the ends of microtubules.

Animals↗

PubMed1978

Subunit dissociation in the allosteric regulation of Glycerol kinase from Escherichia coli. 3. Role in desensitization.

The mechanism of desensitization of glycerol kinase to allosteric inhibition by fructose 1,6-bisphosphate caused by salt, urea, and high pH has been examined in the light of the model proposed in an earlier paper [de Riel, J. K., and Paulus H. (1978), Biochemistry 17] relating subunit dissociation and ligand binding. KCl (0.4 M) causes a tenfold decrease in the affinity of tetrameric glycerol kinase for fructose, 1,6-bisphosphate but has no significant effect on the dissociation process itself. Urea (2 M) causes a large increase in the equilibrium constant for the dissociation of the glycerol kinase tetramer to dimer but has no effect on the affinity of the tetramer for the allosteric inhibitor. High pH (9--10) has only a small effect on the subunit dissociation constant but greatly reduces the rates of subunit association and dissociation. Desensitization of glycerol kinase to allosteric inhibition can thus occur by three different mechanisms, two of which are directly related to the polysteric nature of the enzyme.

Allosteric Regulation↗

PubMed1977

Association-dissociation of transcarboxylase.

Transcarboxylase consists of a central subunit to which two sets of three subunits each are attached at opposite faces. Evidence obtained by various ultracentrifugal techniques has shown that there is an equilibrium between active forms of the enzyme with six, five, four, three, two, and one subunits attached to the central subunit. Since each attached subunit contains two biotins, the biotin content of these forms varies from 12 to 2. By reactive enzyme sedimentation at pH 5.5, it has been shown that the largest form of the enzyme (with 12 biotins and a molecular weight of 1.2 X 10(6) is active and that at pH 6.8 this form of the enzyme dissociates to an enzymatically active form containing three attached subunits. This result is in accord with previous observations (Wood, H.G., Chiao, J.P., and Poto, E.M.(1977), J Biol. Chem. 252, 1490; Wrigley, N.G., Chiao, J.P., and Wood, H.G. (1977), J Biol. Chem. 252, 1500) which showed that dissociation of three of the six attache subunits occurs preferentially from one face of the central subunit and that the remaining three attached subunits are quite firmly bound to the second face of the central subunit. Multiple peaks are obtained in sedimentation velocity experiments at 60 000 rpm because the rate of sedimentation outstrips the rate of equilibration of the different forms of transcarboxylase, whereas at 30 000 rpm a single peak of the multiple form of the enzyme is observed, since equilibration of the different forms keeps abreast of the sedimentation. The rate and extent of dissociation are increased by increase in temperature. It has been shown by centrifugation under oil that the association-dissociation of the large form of the enzymes with six, five, and four attached subunits is not affected by hydrostatic pressure. In contrast, the association-dissociation of the enzyme with three and two attached subunits appears to be affected by pressure. The accumulated results indicate that the native form of the enzyme in the cytosol of the cell consists of a series of enzymatically active forms with one to six attached subunits. The predominant forms in the cytosol will depend on the concentration of the constituent subunits, the pH, the concentration of ions (particularly divalent anions), and the temperature.

Biotin↗

PubMed1976

Proton-dependent dissociation equilibrium of hemoglobin. 1. A 700-nanometer light-scattering study on horse methemoglobin in the pH range 4.8 to 7.2.

The effect of proton concentration upon the subunit dissociation of horse methemoglobin has been investigated at two ionic strengths by light scattering photometry at 700 nm. Differential refractometry revealed a slight but systematic decrease of the specific refractive index increment with decreasing protein concentration for solutions in dialytic equilibrium with the solvent. In the pH range 4.8-7.2 the dissociation can be described by a simple equilibrium between tetramers and dimers. The dissociation constant Kd of the met derivative is found to be very similar to those of the O2- and CO-ligated states. From the slope of a plot of log Kd vs. pH, the number of protons bound is n = 1.3 +/- 0.1 resulting from an increase in the pK values of two groups upon dissociation. These two groups must be identical because the dissociation is symmetrical.

Animals↗