Functional consequences of ligand-linked dissociation in hemoglobin from the sea cucumber Molpadia arenicola

It has been established that Molpadia hemoglobin tends to dissociate into subunits as oxygen is bound. The kinetics and equilibria of carbon monoxide and ethylisocyanide binding reported here show a dependence on protein concentration that supports the conclusion that the aggregated hemoglobin has a lower ligand affinity than the dissociated subunits. This is true for the isolated D-chain as well as for the unfractionated hemolysate that contains four distinct polypeptide chains (A-D). This indicates that even homopolymers of Molpadia hemoglobin have lower ligand affinity than the dissociated subunits. At high protein concentration hemolysates of Molpadia hemoglobin show slight cooperativity. The time course of ligand binding to the deoxy D-chain also suggests cooperative interactions. The low affinity of the aggregated state may have a different molecular explanation than in human hemoglobin where tetramers of identical subunits (as in Hb H) show high oxygen affinity. The absence of tyrosine and histidine at the C-terminal of the Molpadia D-chains also suggests a different stabilization of the low affinity deoxy state. An additional functional difference between Molpadia hemoglobin and human hemoglobin is that organic phosphates do not alter the ligand affinity of the sea cucumber hemoglobin.

Electron transfer between soluble and immobilized mammalian cytochrome c. Equilibrium and kinetic studies on immobilized cytochrome c

Horse heart cytochrome c was covalently bound to Sepharose 4B and its redox properties were measured under various experimental conditions. The equilibrium constant for the electron exchange between the oxidized and the reduced form of cytochrome c when one of the two forms was in the semi-solid state and the other one in solution was close to 1. Matrix-bound ferrocytochrome c is very stable to autoxidation and is not oxidized by O2 even in the presence of mammalian cytochrome oxidase. Oxidation occurs if catalytic amounts of soluble cytochrome c are added to the reaction mixture. The rate of oxidation of matrix-bound ferrocytochrome c in the presence of cytochrome oxidase and catalytic amounts of soluble cytochrome c may be correlated with the rate of electron transfer between soluble and matrix-bound cytochrome c. This rate is more than two orders of magnitude lower than that reported for the homonuclear (between identical species) electron transfer in solution.

Hemoglobin Abruzzo (beta143 (H21) His replaced by Arg). Consequences of altering the 2,3-diphosphoglycerate binding site

Hemoglobin Abruzzo is an abnormal human hemoglobin with a substitution at a residue known to be involved in the binding of 2,3-diphosphoglyceric acid. It has increased oxygen affinity and reduced heme-heme interaction in the absence of organic or inorganic phosphate cofactors. In inorganic phosphate buffers the Bohr effect and heme-heme interaction are normal, but the oxygen affinity remains higher than that of hemoglobin A. CO combination in inorganic phosphate is more strongly autocatalytic than in normal hemoglobin and a slower rate of oxygen dissociation is observed. Although many of the functional differences of this variant may be attributed to the high oxygen affinity of the mutant beta chains, the interactions between subunits are also affected by the histidine to arginine substitution at beta143. Stripped hemoglobin Abruzzo appears to be significantly more dissociated than hemoglobin A. Kinetic studies indicate that interaction with organic or inorganic phosphates decreases its subunit dissociation. In all of the functional properties examined, hemoglobin Abruzzo is more sensitive to the allosteric influence of organic and inorganic anions than is hemoglobin A.

Allosteric interactions in non-alpha chains isolated from normal human hemoglobin, fetal hemoglobin, and hemoglobin Abruzzo (beta143 (H21) His replaced by Arg)

Oxygen-linked effects of inositol hexaphosphate occur in heme-containing non-alpha chains isolated from normal human hemoglobin, fetal hemoglobin, and the abnormal human hemoglobin Abruzzo, beta143(H21) His leads to Arg. The occurrence of these effects implies that the chains undergo ligand-linked conformational changes. Inositol hexaphosphate lowers the oxygen affinity of isolated beta and gamma chains by differential binding to their deoxy conformations. Neither 2,3-diphosphoglycerate nor inorganic phosphate produces such an effect. In the case of Abruzzo beta chains, the binding of inorganic phosphate and 2,3-diphosphoglycerate is also oxygen-linked. Stripped beta chains isolated from hemoglobin Abruzzo have much higher oxygen affinity than beta chains isolated from HbA. Their higher oxygen affinity and enhanced allosteric interactions with phosphates account, in large part, for the abnormal functional behavior of the hemoglobin Abruzzo tetramer. In this hemoglobin variant the substitution of arginine for histidine at beta143 involves a residue known to interact with anionic allosteric effectors of hemoglobin. It is of interest that the effect of inositol hexaphosphate observed with isolated gamma chains is comparable to the effect observed with isolated beta chains, even though the gamma143 position is occupied by an uncharged serine residue.

Enzymatic preparation of 20 beta-hydroxysteroids in a two-phase system

The behavior of 20 beta-hydroxysteroid dehydrogenase in a two-phase system consisting of an emulsion of water with an immiscible organic solvent was investigated. The effect of several organic solvents on the stability, activity, and kinetic profile of the enzyme was considered. The most suitable system for carrying out the enzymatic reaction proved to be water-butyl acetate. The production of high quantitied of 20 beta-hydroxysteroids in 100% yield using catalytic amount of cofactor was achieved by coupling the 20 beta-hydroxysteroid dehydrogenase-and the alcohol dehydrogenase-catalyzed reactions.

Identification of chloride-binding sites in hemoglobin by nuclear-magnetic-resonance quadrupole-relaxation studies of hemoglobin digests

35Cl minus-nuclear magnetic resonance (NMR) studies indicate that various digests of human hemoglobin with carboxypeptidase A and B, or a combination of the two, may be used for the identification of chloride binding sites. All the digestion products contain, like hemoglobin itself, at least two classes of binding sites, one of high, the others of low affinity. The pH dependence of the excess linewidth of the 35Cl minus NMR signal indicates that in the simple digests with either carboxypeptidase A or B, chloride is bound with high affinity at or near His-beta146-Asp-beta94 and at or near Val-alpha1-Arg-alpha141. The high-affinity sites show, in the case of the simple digests, a strong oxygen linkage which is lost in the forms digested with both carboxypeptidase A and B; this linkage may thus be correlated to the presence of conformational changes. Organic phosphates, like inositol hexaphosphate, show competition for some of the high-affinity chloride binding sites in hemoglobin and in the simple digests. This competition is likewise lost in the doubly digested hemoglobins.

A study of the kinetics of iron and copper binding to hen ovotransferrin

The kinetics of iron and copper binding to hen's-egg apo-ovotransferrin were studied by using citrate chelates of these metals at pH9.3 in borate buffer in the presence of bicarbonate. The kinetics of the absorbance change associated with the formation of the final product show a fast process, which is pseudo-first-order, where the reagents are in excess with respect to the protein, and the citrate concentration is higher than 25mM. At lower citrate concentration, the progress curves are clearly biphasic. There is marked dependence of the rate of the reaction on bicarbonate concentration, which may be interpreted as a displacement reaction of the ligand-metal-protein ternary complex. The kinetics have been interpreted in the framework of a reaction scheme which involves bimolecular reaction of a metal chelate to the protein and subsequent colour development by displacement of the chelator by bicarbonate. The pH-dependence of this reaction supports the belief that tyrosine residues are involved in the process of iron-binding. The overall similarity of kinetics for iron and copper binding, notwithstanding their different co-ordination preferences, suggests that the process of metal-binding or chromophore development for the two metal complexes must be similar.

Equilibrium and kinetics of the reaction of Aplysia myoglobin with azide

The present paper reports a study on the equilibria and kinetics of the acid-alkaline transition and the azide binding reaction by ferric Aplysia myoglobin. A single completely reversible spectrophotometric titration curve is found over the pH range from similar to 5 to similar to 9, with an apparent pK equals to 7.5 for the acid-alkaline transition. The kinetics of the process, followed by the temperature-jump method, gives, at pH values close to the pK of the transition, one single, well-resolved, relaxation independent of protein concentration and of type of buffer used. The pattern accords to a simple pH dependent reaction, in buffered medium, between the two forms of the protein. The results of the azide binding reaction show that the process conforms to simple equilibrium as expected for a single site protein. The méasured association constant is reported as a function of pH. The kinetics of the reaction of Aplysia metMb with N3- minus shows, on the other hand, a complex behavior. The relaxation pattern is found to strongly depend on pH and ligand concentration in such a way to suggest a linkage between ligand binding and acid-alkaline transition. The system is discussed on the basis of two simplifying conditions, i.e., at low and higher pH with respect to the pK of the acid-alkaline transition. At acid pH the reaction corresponds to a single bimolecular process as expected for a simple binding reaction; at alkaline pH, the dependence of relaxation time on ligand concentration implies the existence of a rate-limiting monomolecular step. On the basis of a reaction scheme implying that binding of the ligand can only occur through the acid (aquomet) form of the protein via the displacement of the water molecule, the experimental data are quantitatively accounted for.

Kinetic studies on the reaction between cytochrome c oxidase and ferrocytochrome c

In stopped-flow experiments in which oxidized cytochrome c oxidase was mixed with ferrocytochrome c in the presence of a range of oxygen concentrations and in the absence and presence of cyanide, a fast phase, reflecting a rapid approach to an equilibrium, was observed. Within this phase, one or two molecules of ferrocytochrome were oxidized per haem group of cytochrome a, depending on the concentration of ferrocytochrome c used. The reasons for this are discussed in terms of a mechanism in which all electrons enter through cytochrome a, which, in turn, is in rapid equilibrium with a second site, identified with 'visible' copper (830 nm-absorbing) Cud (Beinert et al., 1971). The value of the bimolecular rate constant for the reaction between cytochromes c2+ and a3+ was between 10(6) and 10(7) M(-1)-S(-1); some variability from preparation to preparation was observed. At high ferrocytochrome c concentrations, the initial reaction of cytochrome c2+ with cytochrome a3+ could be isolated from the reaction involving the 'visible' copper and the stoicheiometry was found to approach one molecule of cytochrome c2+ oxidized for each molecule of cytochrome a3+ reduced. At low ferrocytochrome c concentrations, however, both sites (i.e. cytochrome a and Cud) were reduced simultaneously and the stoicheiometry of the initial reaction was closer to two molecules of cytochrome c2+ oxidized per molecule of cytochrome a reduced. The bleaching of the 830 nm band lagged behind or was simultaneous with the formation of the 605 nm band and does not depend on the cytochrome c concentration, whereas the extinction at the steady-state does. The time-course of the return of the 830 nm-absorbing species is much faster than the bleaching of the 605 nm-absorbing component, and parallels that of the turnover phase of cytochrome c2+ oxidation. Additions of cyanide to the oxidase preparations had no effect on the observed stoicheiometry or kinetics of the reduction of cytochrome a and 'visible' copper, but inhibited electron transfer to the other two sites, cytochrome a3 and the undetectable copper, Cuu.

Complexes of metal phthalocyanines with globin as the models of heme proteins

The reaction between iron and cobalt tetrasulfonated phthalocyanines and globin results in the formation of the green complexes, as has been proved by difference spectroscopy. Spectrophotometric titration data indicate the formation of those complexes at the molar ratio 1:1. The complexes of ferrous, ferric and cobaltous tetrasulfonated phthalocyanines with globin have been isolated from the reaction mixtures by separation on Sephadex G-50 and precipitation of the protein fractions with ammonium sulfate. The visible spectra of these complexes are characterised by the main intensive peak at 641 nm, 678 nm, and 675 nm for ferric, ferrous and cobaltous derivatives, respectively. The new globin complexes have the property of reversible combination with oxygen and coordination with cyanide ions. It is evidence from the results of the spectrophotometric titrations of hemoglobin and methemoglobin with cobaltous tetrasulfonated phthalocyanine that iron protoporphyrins are displaced by this cobalt derivative; this suggests that phthalocyanine and porphyrin are bonded in a similar manner.

Electron transfer between azurin and cytochrone c-551 from Pseudomonas aeruginosa

The electron-transfer reaction between azurin and cytochrome c1 isolated from Pseudomonas aeruginosa was investigated by rapid-reaction techniques. Temperture-jump studies clearly reveal two chemical relaxations, the amplitudes of which have ikentical spectral distributions, but relaxation times show different dependencies on reactant concentrations. Stopped experiments also showed complex kinetics. A model is proposed which is consistent with the kinetic and equilibrium data obtained. The central feature of this model is the proposal that two intercenvertible forms of reduced azurin exist in solution, only one of which si able to participate directly in the electron-transfer reaction with cytochrome c-551. Support for the hypothesis that two forms of reduced azurin exist is derived from studies on the electron-transfer reaction between azurin and Pseudomonas cytochrome oxidase. The possible physiological significance of such a situation is discussed.

Nuclear Magnetic resonance quadrupole relaxation studies of chloride binding to the isolated hemoglobins from trout (Salmo irideus)

NMR studies of chloride binding to the main components of trout blood, Hb Trout I and Hb Trout IV, indicate that although the affinity of chloride is high for both hemoglobins, the characteristics of the binding process are markedly differnet. In Hb Trout IV chemical exchange at the chloride binding site(s) is fast and quadrupole effects determine the linewidth; chloride binding has a definite pH dependence, but there is no significant oxygen linkage. In contrast Hb Trout I represents a unique case of slow chemical exchange, which may depend on unusual stereoche mical characteristics of the chloride binding site; chloride binding is pH independent, but shows a significant oxygen linkage, which may be attributed to changes of the lifetime of chloride at the binding site. The chloride binding properties displayed by Hb Trout I and IV have been compared with those of normal and modified human hemoglobins and discussed in terms of the structural differences in the C- and N-terminal regions of the alpha- and beta-chains.

The kinetics of oxidation of ferroperoxidase by molecular oxygen. A model of a terminal oxidase

1. The decay of oxyferroperoxidase to ferriperoxidase was studied by rapidly mixing solutions of ferroperoxidase with various amounts of oxygen and following the time-course of appearance of oxyferroperoxidase and its subsequent decay to ferriperoxidase by reaction with ferroperoxidase. 2. The scheme can be accommodated by and [Formula: see text] and occurs without detectable intermediates being observed in the millisecond time-range. 3. Analogue-computer simulation of the reaction is in agreement with the initial rate-limiting reaction being an intermolecular, not intramolecular, electron-donating process, and analysis of the data leads to quantitative values for the rate constants of the overall process. 4. The reaction of oxyferroperoxidase with ferroperoxidase is a model of a terminal oxidase, and the results are discussed in terms of the possible importance of this reaction in peroxidase function, and also in the light it throws on autoxidation of haem compounds.