Contrary to the case in mammals and birds, a multiplicity of hemoglobin components is generally found in the blood of fish, reptiles, and amphibians. Allosteric effects have become apparent in studies on hemoglobins those on the structural and functional properties of hemoglobins have been largely carried out on the proteins extracted from mammals.
#Carbon bohr model series#
Under the pressure of the variable oxygen requirements, a series of adaptive mechanisms involving the different “gears” of the machine deputed to the supply of oxygen to the tissues has come into operation. The existence of specific carriers deputed to the transport of oxygen from the outer environment to the place of utilization is widespread in nature. The data indicate, semiquantitatively, that the apparent enthalpy change corresponding to the allosteric equilibrium constant is pH-dependent and positive. From this it appears that the essential feature of the functional behavior of trout IV hemoglobin, which is characteristic of hemoglobins of teleost fish in general, lies in a proton-induced shift in the allosteric equilibrium constant. Further analysis in terms of a modified two-state allosteric model shows how the three model parameters k T, k R, and L vary with pH and temperature. Analysis of the results of these three different sets of experiments at a purely phenomenological level shows that they are in satisfactory agreement. The reaction of hemoglobin trout IV with carbon monoxide has been studied in three parallel sets of experiments comprising: (1) microcalorimetric measurements in different buffers at two pH values (7.1 and 8.5) and two temperatures (20 ☌ and 5 ☌) to determine the overall value of the enthalpy change of ligand binding as well as its point values as a function of fractional saturation with CO (2) a study of the CO binding curves at three pH values (from 5.7 to 7.4) and at 23 ☌ and 5 ☌ in order to explore the pH and temperature dependence of the model-independent binding parameters which describe the system (3) differential titrations in 0.2 m-sodium chloride at 23 ☌ and 5 ☌ of the deoxy and CO saturated forms of the molecule in order to determine the overall CO Bohr effect as a function of temperature. A comparison of these models shows clear evidence for novel mechanisms of pH-dependent control of ligand affinity. We have solved the crystal structure of tuna hemoglobin in the deoxy form at low and moderate pH and in the presence of carbon monoxide at high pH. Sequence alignments have signally failed to pinpoint the residues involved, and site-directed mutagenesis has not yielded a human hemoglobin variant with this property. X-ray crystal structures of Root effect hemoglobins have not, to date, provided convincing explanations of this effect. Some fish hemoglobins are almost completely insensitive to pH (within physiological limits), whereas others show extremely low oxygen affinity under acid conditions, a phenomenon called the Root effect. Among these, fish hemoglobins are well known for their widely varying interactions with heterotropic effector molecules and pH sensitivity. Several animal hemoglobins have properties that cannot be readily explained in terms of their amino acid sequence and known atomic models of hemoglobin. The crystal structure of hemoglobin has been known for several decades, yet various features of the molecule remain unexplained or controversial.
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