A two-state thermodynamic and kinetic analysis of the allosteric functioning of the haemoglobin of an extreme poikilotherm

Ligand binding kinetics and dissociation of the human embryonic haemoglobins

The three human embryonic haemoglobins have been studied using a range of stopped-flow and flash photolysis experiments. The association and dissociation kinetics and equilibrium constants for the tetramer-dimer reactions of the deoxy and oxygenated forms have been investigated and found to be characterized by constants similar to those of the human adult protein. The rates of oxygen dissociation from the embryonic haemoglobins have been measured and appear to be responsible for the high oxygen-binding affinity associated with the embryonic proteins compared with the adult protein. The pH dependence of the oxygen dissociation rate constants also accounts for the rather unusual, previously described, Bohr effects characteristic of the embryonic haemoglobins. A general scheme has been developed coupling both the dimer-tetramer equilibria and ligand-binding steps observed following photolysis of the liganded forms of the human embryonic haemoglobins.

Role of dimerization in the control of the functioning of the human haemoglobin mutant haemoglobin Howick (beta 37 Trp-->Gly)

Haemoglobin Howick shows a high oxygen affinity (p50 = 1 mmHg) and a low co-operativity (n = 1.3). Equilibrium studies show the protein to be essentially totally dimeric in the oxygenated form. A wide range of rapid kinetic experiments indicate that the deoxygenated form of the protein exists in a tetramer<-->dimer equilibrium with an associated equilibrium constant of 3 microM. These kinetic data also indicate that the oxygenated form of the protein exists in a tetramer<-->dimer equilibrium with an associated equilibrium constant of 35 mM, and furthermore clearly identifies a large increase in the rate of the tetramer-to-dimer dissociation process as the origin of the vastly increased dissociation equilibrium constants. Simulations of the protein-concentration-dependence of the oxygen-binding properties of haemoglobin Howick, based on the measured equilibrium parameters, closely fits the experimental data. The change in dimerization constant for the deoxygenated form of the protein corresponds remarkably well to the free-energy change predicted for the simple transfer of the amino acid side chain at position beta 37 from a hydrophobic to a hydrophilic environment during the dimerization process.

Some reactions of carbon monoxide and oxygen with carbodi-imide-modified cytochrome c

The reactivity of carbodi-imide-modified tuna and horse heart cytochromes c with the ferrous ion ligands CO and O2 has been studied. Both modified cytochromes bind one molecule of CO. Stopped-flow and flash-photolysis experiments indicate the presence of three kinetic processes in the reaction of the cytochromes with CO. The second-order rate constants associated with all three kinetic process are pH-independent being 2.8 x 10(5) M-1.s-1, 3.8 x 10(4) M-1.s-1 and 4 x 10(3) M-1.s-1 under all conditions studied. The concentration-dependence of the contributions made by each of the processes to the overall absorbance change indicates that the fast and slow kinetic phases are associated with two forms of the cytochromes which are in equilibrium, whereas the intermediate phase arises from a separate cytochrome species. The quantum yield for the photodissociation of CO from the ferrous cytochromes is unusually low. Both modified cytochromes are capable of binding and reducing O2. In the presence of excess reductant, the modified cytochromes can catalytically reduce large molar excesses of O2. In the absence of excess reducing agent, the oxy complex initially formed undergoes a pH-dependent intramolecular electron-transfer process with half-life approx. 10 min. EDC [1-ethyl-3-(3-dimethylaminopropyl)carbodi-imide]-promoted internal cross-linking is proposed to account for differences between the EDC-modified proteins and carboxymethylated cytochrome c.

Cooperativity and allosteric regulation in non-mammalian vertebrate haemoglobins

1. This review illustrates the vast range of molecular functions expressed in non-mammalian vertebrate haemoglobins; with particular reference to the degree of aggregation of haemoglobin subunits and their interactions with allosteric effectors. 2. In at least the broadest sense, these properties suggest that haemoglobin function in non-mammalian vertebrates can be viewed against the evolutionary hierarchy of organisms rather than from a purely adaptive perspective.

Hemoglobin from the Antarctic fish Notothenia coriiceps neglecta. 1. Purification and characterisation

Antarctic fishes live at a constant temperature of -1.8 degrees C, in an oxygen-rich environment. In comparison with fishes that live in temperate or tropical waters, their blood contains less erythrocytes and hemoglobin. A study was initiated on the structure and function of Antarctic fish hemoglobin. The erythrocytes of the Antarctic benthic teleost Notothenia coriiceps neglecta, of the family Nototheniidae, have been shown to contain two hemoglobins, accounting for about 90% and 5% of the total content. These hemoglobins have been isolated, and obtained in crystalline form. They are tetramers and contain two pairs of globin chains. The globin chains of each hemoglobin have been purified and characterised. The two hemoglobins appear to have one of the two globin chains in common. The Root and Bohr effects have been investigated in erythrocytes, 'stripped' hemolysates and pure hemoglobins, indicating that the functional properties are finely regulated by pH and allosteric effectors.

A kinetic and equilibrium study of ligand binding to the monomeric and dimeric haem-containing globins of two chitons

The radular muscles of the amphineuran molluscs Amaurochiton glaucus and Sipharochiton pelliserpentis contain both a dimeric and a monomeric form of myoglobin. The dimeric form of the protein is composed of two polypeptide chains covalently linked to each other via one or more disulphide bonds. The dimeric protein shows co-operative O2-binding curves. Kinetic investigations indicate that CO binding is co-operative in the dimeric protein, subsequent to full photolysis, but mono-exponential following 10% photolysis. O2 recombination following part photolysis is mono-exponential in the dimeric form, whereas O2 dissociation kinetics indicates the presence of chain heterogeneity. The monomeric form of the protein exhibits mono-exponential time courses in all the experimental situations explored. Although the rate constants associated with the reactions of individual dimer and monomer molecular species are very different, the two species of chiton investigated show remarkably similar properties when compared with each other. All the reactions studied are pH-independent in the range pH 6-8. Amino acid analysis indicates that the monomeric units that combine to form the dimeric species are not identical with the naturally occurring monomeric form. A comparison is made between the chiton myoglobins and other similar O2-binding proteins.

An investigation of the functioning of the two major haemoglobins of the Sphenodon using fast reaction kinetic methods

The blood of the Sphenodon (Sphenodon punctatus) has been fractionated into two major and one minor haemoglobin components by ion-exchange chromatography. The two major haemoglobins have been studied in terms of their kinetic reactions with both O2 and CO. The combination of flash photolysis and stopped-flow indicates kinetic differences between two states of the proteins identified with the allosteric T and R forms. The major kinetic findings show that (i) in these haemoglobins the T state is retained to a higher level of ligation than that commonly found in mammals, (ii) the rate of conversion from the R to the T state (k0RT) is some three orders of magnitude lower in the Sphenodon haemoglobins than in mammalian systems. A comparison between the kinetic and equilibrium data for these haemoglobins indicates that the very weak cooperativity exhibited by these proteins arises from the close similarity of the affinities of the two allosteric states together with a very low value for the allosteric equilibrium constant, although very significant kinetic differences exist between the two states.

A study of the kinetics of the reaction of ligands with the liganded states of mouse embryonic haemoglobins

The reactivities of the liganded states of the embryonic haemoglobins of the mouse with both O2 and CO were measured and compared with the reactivities of the adult protein. Laser-photolysis experiments on the recombination of O2 with the partially oxygenated proteins indicates chain heterogeneity in the adult and embryonic EII and EIII species, with the difference in subunit reactivity being greatest in the embryonic species. Haemoglobin EI shows chain equivalence in these experiments. The homogeneous time courses observed for the O2-dissociation reactions are consistent with chain equivalence within all the proteins with regard to this reaction. The specific values obtained for the respective rate constants from each of these studies indicates that the high O2 affinity previously reported for haemoglobin EI is, in greatest part, due to its low O2 dissociation rate. Flash-photolysis studies on the binding of CO with the partially liganded forms of the proteins show the same patterns of chain heterogeneity as seen in O2-binding studies.

Kinetic studies on the nitrite reductase of Wolinella succinogenes

The six haem groups of the nitrite reductase enzyme isolated from Wolinella succinogenes are rapidly reduced by the addition of dithionite (S2O4(2-)). The reduction, however, is not homogeneous. Two of the haem groups, namely those that show spectral characteristics typical of five-co-ordinated haem groups, are reduced in a dithionite-concentration-dependent fashion with a rate limit of 1.5 S-1. The other four haem groups, which show spectral characteristics very similar to those of normal six-co-ordinate c-haem groups, reduce in a linear dithionite-concentration-dependent manner with a second-order rate constant of 150 M-1/2 X S-1. The ratio of the amplitudes of the two reduction phases observed in stopped-flow studies is found to be dependent on the concentration of dithionite used. A model is proposed to account for these observations, and computer simulations show that the model represents a good fit to the experimental data. The two haem groups with five-co-ordinate spectral characteristics bind CO. Flash photolysis of the CO complex exhibits one major recombination process with a linear dependence in rate on CO concentration with a second-order rate constant of 2 X 10(6) M-1 X S-1. By contrast, stopped-flow mixing of the reduced protein with CO shows a very complex pattern of combination, with most of the observed absorbance change associated with a concentration-independent step. These findings are rationalized in terms of structural changes in the protein consequent to ligand binding.

Characterization of the changes in the state of aggregation induced by ligand binding in the hemoglobin system of a primitive vertebrate, the hagfish Eptatretus cirrhatus

Hemoglobin (Hb) from the hagfish, Eptatretus cirrhatus, is composed of subunits of approx. 20,000 mol. wt. Aggregation of the deoxy subunits occurred, particularly at low pH and at high protein concentration. Oxygen equilibrium studies indicated slight cooperativity and the presence of a small, phosphate-independent Bohr effect. Equilibrium properties were protein concentration dependent indicating an oxygen-linked dissociation in the millimolar concentration range. Kinetic studies indicated a dimer to monomer transition in the micromolar concentration range. The ligand-binding character of hagfish Hb was similar to that of lampreys, but was governed by different kinetic and equilibrium parameters.

The effect of temperature on the equilibrium and kinetic properties of a root effect haemoglobin from the marlin Tetrapturus audax

The ligand binding properties of the Root effect haemoglobin of the marlin have been investigated in the temperature range 12-35 degrees C. An essentially symmetric displacement of the binding isotherms to higher concentration is observed on raising the temperature. Thermodynamic parameters have been obtained for the equilibrium binding constants, in terms of the two state model for co-operativity. Kinetic measurements indicate chain heterogeneity in both the T and R states. Activation parameters have been obtained for both chains in both quaternary states.

A kinetic and equilibrium study of ligand binding to a Root-effect haemoglobin

The blood of the striped marlin (Tetrapturus audax) contains one major Root-effect haemoglobin. Titrations of this haemoglobin with CO show that at high pH the molecule is highly co-operative (Hill coefficient 2.8) whereas at low pH the titration data can best be described as the sum of contributions from non-co-operating subunits of different affinity. In terms of the two-state model the R-state affinity constant is much more sensitive to pH than is that of the T state. Flash-photolysis studies were used to characterize the kinetics of ligand binding to this haemoglobin. Both T and R states show kinetic heterogeneity in their recombination time courses, associated with the alpha- and beta-chains of the molecule. The rate constants for ligand binding to each chain, in each quaternary state, were determined, and in conjunction with the allosteric equilibrium parameters determined at pH8.0 were used in the two-state analysis of reaction curves, over a range of ligand concentration. The two-state model, extended to take account of chain difference, adequately fits the homotrophic effects observed for this haemoglobin. The two-state model is, however, inadequate in its description of the heterotropic effects produced by protons.