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A Riggs

Publications and source records attributed to A Riggs.

At least 37 records · Page 2Linked to original sources
PubMed1976

Tetramer-dimer dissociation in homoglobin and the Bohr effect.

The pH dependence of the apparent tetramer to dimer dissociation constant has been determined at 20 degrees for both oxy- and deoxyhemoglobins A and Kansas. These measurements were made by three different procedures: gel chromatography, sedimentation velocity, and kinetic methods in either of three buffer systems: 0.05 M cacodylate, Tris, or glycine with 1 mM EDTA and 0.1 M NaCl between pH 6.5 and 11. The tetramer-dimer dissociation constant of human oxyhemoglobin A decreases from about 3.2 X 10(-6) M at pH 6.0 to about 3.2 X 10(-8) M at pH 8.5. The slope of this line indicates that the dissociation of tetramer to dimer is accompanied by the uptake of about 0.6 protons per mol of tetramer in this region. The corresponding dissociation constant for deoxyhemoglobin in the same pH region increases apparently almost linearly from 1.0 x 10(-12) M at pH 6.5 to about 1.0 x 10(-5) M at pH 11. To dimer is associated with the release of about 1.6 protons per mol of tetramer. Comparison of these data with the known proton release accompanying the oxygenation of tetramers confirms that the pH dependence of oxygen binding by dimers must be very small. The present data predict that the overall proton release or uptake per oxygen bound by dimer should be less than 0.1. The tetramer-dimer dissociation equilibria of oxy- and deoxyhemoglobins above pH 8.5 have identical pH dependences. In this range the dissociation constant of deoxy-Hb is about one-tenth that of oxyhemoglobin. Human oxyhemoglobin Kansas is known to have an enhanced tetramer-dimer dissociation compared with that of hemoglobin A. Below pH 8.5 the tetramer-dimer dissociation constant of Hb Kansas is about 400 times greater than that of HbA in the absence of phosphate buffers. In contrast, the tetramer-dimer dissociation constants of deoxyhemoglobins A and Kansas appear to be identical. These findings are consistent with previous structural observations on these hemoglobins. The data on the tetramer-dimer dissociation of human hemoglobin were used to calculate the total free energy of binding of oxygen to the tetramer and the median oxygen pressure on the basis of fundamental linkage relations and a pH-independent estimate of the total free energy of binding oxygen to dimer. Simulated oxygen binding curves were generated with the equations of Ackers and Halvorson (Ackers, G. K., and Halvorson, H. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 4312-4316) by making two assumptions: (a) that the dimers are noncooperative and pH-independent in O2 binding and (b) that the distribution of cooperative energy in the oxygenation of tetramers is independent of pH. We have compared these simulations with experimental data obtained at low protein concentrations (30 to 124 muM heme) to show that the variation in oxygen affinity with pH can be described in terms of the subunit equilibria. We conclude that an accurate analysis of the contributions of individual oxygen binding steps to the Bohr effect cannot be made without considering the contributions of the dimers to oxygen binding...

Hemoglobins↗
PubMed1976

Hemoglobins of the tadpole of the bullfrog, Rana catesbeiana. Temperature dependence of oxygen binding and pH dependence of subunit dissociation.

The temperature dependence of the oxygen equilibrium of tadpole hemoglobin has been determined between 0 degrees and 32 degrees for the unfractionated but phosphate-free lysate and between 12 degrees and 32 degrees for each of the four isolated components between pH 6 and 10 in 0.05 M cacodylate, Tris, or glycine buffers containing 0.1 M NaCl and 1 mM EDTA. Under these conditions the Bohr effect (defined as deltalog p50/deltapH) of the unfractionated lysate is positive at low temperatures between pH 6 and 8.5 and is negative above pH 8.5 to 8.8 at any temperature. As the temperature rises the Bohr effect below pH 8.5 changes greatly. In the interval pH 7.0 to 7.5, the magnitude of the Bohr effect decreases from + 0.28 at 0 degrees to zero at about 24 degrees and becomes negative, as in mammalian hemoglobins, above this temperature. Measurements with the isolated components show that the temperature dependence of oxygen binding for Components I and II and for Components III and IV is very similar. For both sets of components the apparent overall enthalpy of oxygenation at pH 7.5 is about -16.4 kcal/mol and -12.6 kcal/mol at pH 9.5. The measured enthalpies include contributions from the active Bohr groups, the buffer ions themselves, the hemoglobin groups contributing buffering, and any pH-dependent, oxygenation-dependent binding of ions such as chloride by the hemoglobin. The apportioning of the total enthalpy among these various processes remains to be determined. Between pH 8 and 10.5 tadpole oxyhemoglobin undergoes a pH-dependent dissociation from tetramer to dimer. The pH dependence of the apparent tetramer-dimer dissociation constant indicates that at pH 9.5 the dissociation of each tetramer is accompanied by the release of approximately 2 protons. In this pH range the oxygen equilibrium measurements indicate that about 0.5 proton is released for each oxygen molecule bound. The results are consistent with the conclusion that one acid group per alphabeta dimer changes its pK from about 10 to 8 or below upon dissociation of the tetramer.

Animals↗
PubMed1976

Factors in the evolution of hemoglobin function.

The packaging of vertebrate blood hemoglobins within cells places subtle constraints on hemoglobin evolution. Since the concentration of hemoglobin is near the solubility limit a selective advantage should exist for a noncomplementary external topology of amino acid residues. Further, any change in charge on the protein should alter ion distribution across the cell membrane and so modify ion-sensitive oxygen transport. An efficient hemoglobin must not only combine readily with oxygen at prevailing environmental oxygen pressures, but must also release it at metabolically appropriate pressures. These adaptations frequently employ different strategies to achieve the same objective in different animals. Some hemoglobins have evolved special properties unrelated to the transport of oxygen to metabolizing tissues. Thus many teleost fish have hemoglobins that discharge much of their oxygen at low pH even at high oxygen pressures. This property appears to aid in filling the swim bladder with oxygen. The hemoglobins of elasmobranchs have evoked a unique resistance to urea as a consequence of the high urea content of their blood. Sometimes the functional adaptations of hemoglobins are achieved by multiple hemoglobins in the same cells. Often, however, different red cell populations with functionally unique hemoglobins arise sequentially during ontogeny.

Animals↗
PubMed1975

Hemoglobins of the tadpole of the bullfrog, Rana catesbeiana. Structure and function of isolated components.

Four major components of the hemoglobin of the bullfrog tadpole, Rana catesbeiana, have been isolated and characterized structurally and functionally. These components fall into two clear functional classes. Components I and II have substantially higher affinities for oxygen than do components III and IV. Components I and II predominate in very young tadpoles and are largely replaced by components III and IV in older tadpoles. The data (Broyles, R.H., and Frieden, E. (1973) Nature New Biol. 241, 207-209) indicate that component I arises in the kidney and components III and IV in the liver. The synchrony of appearance and functional similarity o components I and II suggest that component II probably also arises in the kidney. Thus the development of the tadpole is associated with the successive proliferation of three distinct populations of red cells, first from the kidney, then from the liver, and finally, after metamorphosis, from bone marrow...

Amino Acid Sequence↗
PubMed1973

Oxygen equilibrium and kinetics of isolated subunits from hemoglobin Kansas.

The isolated beta subunit of hemoglobin Kansas has an oxygen affinity that is as low relative to the oxygen affinity of the beta(A) subunit as the affinity of hemoglobin Kansas is low relative to hemoglobin A. Thus the low affinity properties of hemoglobin Kansas are almost completely reflected in the properties of the isolated subunits. The kinetic results show that the equilibrium affinity difference results both from a much larger oxygen dissocation rate constant in beta(Kansas) (k = 37 sec(-1) and 18 sec(-1) for beta(Kansas) and beta(A), respectively) and from a lower association reaction rate, The properties of the alpha chains from hemoglobins A and Kansas appear to be identical, as expected.

Carboxyhemoglobin↗