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Biomedical subjects

W A Eaton

Publications and source records attributed to W A Eaton.

At least 73 records · Page 4Linked to original sources
PubMed1980

The relationship between coding sequences and function in haemoglobin.

Gilbert has suggested that the presence of intervening sequences in DNA, called introns, can speed evolution by allowing novel proteins to be constructed from the pieces of existing ones. This hypothesis further suggests that the coding sequences, called exons, correspond to functional parts of the protein. The most striking example so far is the case of the immunoglobulin gamma heavy chain, where the four polypeptide sequences corresponding to the four coding sequences form structurally and functionally distinct parts of the molecule. The relation between the three coding sequences of the beta globin gene and structure or function is not as obvious, but the central coding sequence does code for the part of the globin chain which forms the haem crevice. To further test the idea that coding sequences correspond to functional units of proteins we consider the relationship between the coding sequences of alpha and beta globin genes and the corresponding parts of the complete, tetrameric haemoglobin molecule.

Amino Acid Sequence↗
PubMed1980

Kinetic studies on photolysis-induced gelation of sickle cell hemoglobin suggest a new mechanism.

The kinetics of deoxyhemoglobin S gelation have been investigated using photolytic dissociation of the carbon monoxide complex to initiate the process. Measurements over a wide range of times, 10(-3)-10(4) show that both the concentration dependence of the tenth-time (i.e., the time required to complete one-tenth the reaction) and the time dependence of the process decrease as gelation speeds up. In slowly gelling samples, where single domains of polymers are formed in the small sample volumes employed with this technique (1-2 x 10(-9) cm3), there is a marked increase in the variability of the tenth-times. These results are explained by a mechanism in which gelation is initiated by homogeneous nucleation of polymers in the bulk solution phase, followed by heterogeneous nucleation on the surface of existing polymers. At the lowest concentrations, homogeneous nucleation is so improbable that stochastic behavior is observed in the small sample volumes, and heterogeneous nucleation is the dominant pathway for polymer formation, thereby accounting for the high time dependence. At the highest concentrations homogeneous nucleation becomes much more probable, and the time dependence decreases. The decrease in concentration dependence of the tenth-time with increasing concentration results from a decrease in size of both the homogeneous and heterogeneous critical nuclei. The model rationalizes the major observations on the kinetics of gelation of deoxyhemoglobin S, and is readily testable by further experiments.

Carboxyhemoglobin↗
PubMed1978

Spectroscopic studies of oxy- and carbonmonoxyhemoglobin after pulsed optical excitation.

The photolysis of HbO2 and HbCO has been investigated with picosecond laser techniques. Transient absorption spectra were measured in the Soret and visible regions after excitation with 353- or 530-nm pulses. The photoproducts appeared within 8 psec and exhibited considerably broadened deoxyhemoglobin-like spectra, which persisted to 680 psec. The altered spectra are attributed to the production of deoxyheme conformational and spin states that might result from the intense excitation.

Carboxyhemoglobin↗
PubMed1976

Supersaturation in sickle cell hemoglobin solutions.

The kinetic inhibition of the gelation of hemoglobin S is compared to the change in hemoglobin S soulbility, when the solubility is altered by carbon monoxide, pH, or urea. By means of a new technique, the delay time and the extent of gelation are measured on the same sample. They delay time, td, is found to be proportional to a high power (30-40) of the hemoglobin S solubility. Togehter with the previously reported concentration dependence, this result demonstrates that the rate is proportional to a high power of the supersaturation, S, defined as the ratio of the total hemoglobin S concentration to the equilibrium solubility. The results obey the supersaturation equation td-1 = gammaSn, where gamma is an empirical constant (about 10(-7) sec-1) and n is about 35. The supersaturation equation can successfully account for observations on the kinetics of cell sickling and is therefore used to estimate the increase in the delay time for sickling necessary to produce significant clinical benefit to patients with sickle cell disease.

Carbon Monoxide↗
PubMed1975

A crystallographic study of deoxy cobalt (II) mesoporphyrin IX myoglobin.

The crystal structures of acid metmyoglobin and deoxy cobalt(II)mesoporphyrin IX myoglobin were compared by a difference Fourier analysis at 2.5 A resolution. No large differences in protein conformation were observed. The greatest density of structural differences was found in the heme region. There was a loss of the histidine-bound sulfate ion and of the metal-bound water molecule, as well as a shift in the position of the prosthetic group with associated changes in the adjacent globin. The structural changes resulting from the substitution of ethyl for the vinyl side chains of the porphyrin were clearly observed. There was also a suggestion of a conformational change of the porphyrin ring. It was not clear whether there was any change of the metal position relative to the porphyrin plane or proximal histidine.

Animals↗
PubMed1975

Optical spectra and electronic structure of flavine mononucleotide in flavodoxin crystals.

The polarized single-crystal absorption spectra of the oxidized and semiquinone forms of flavodoxin from Clostridium MP have been measured with a double beam recording microspectrophotometer. The spectra establish that the radical species in the crystal is the neutral (blue) falvine semiquinone. Combination of the spectra reported here with polarization data from previous fluorescence and stretched-film studies provides transition moment directions for the first two phi-phi transitions of the oxidized form. Predictions of molecular orbital theory are in good agreement with these experimental directions. The crystal spectra of the semiquinone indicate that the two lowest frequency transitions have the same detailed orbital origin as the corresponding transitions of the oxidized form; in the semiquinone these transitions appear at lower frequency, are closer together, and, as predicted from detailed considerations of transition probabilities, exhibit approximately half the absorption intensity. Our hypothesis of a common orbital origin suggests that semiquinone formation takes place by the addition of an electron to the lowest empty phi orbital of the oxidized form without any gross electronic rearrangement.

Bacterial Proteins↗
PubMed1974

Kinetics and mechanism of deoxyhemoglobin S gelation: a new approach to understanding sickle cell disease.

We report the results of a kinetic investigation on the gelation of purified deoxyhemoglobin S. Gelation was induced by raising the temperature and was monitored by measuring both the heat absorbed, with a microcalorimeter, and the appearance of linear birefringence, with a microspectrophotometer. The kinetics are unusual. Prior to the onset of gelation there is a delay period, followed by a sigmoidal progress curve. The delay time is formally dependent on approximately the 30th power of the deoxyhemoglobin S concentration; a decrease in concentration from 23 to 22 g/dl increases the delay time by a factor of four. It is also extremely temperature dependent; a 1 degrees C temperature rise in the range 20-30 degrees C almost halves the delay time. From these results we conclude that the initial rate is controlled by the nucleation of individual fibers. We present a kinetic model that accounts for the concentration, temperature, and time dependence of the initial phase of the gelation reaction. Extrapolation of our data to physiological conditions predicts that changes in intracellular hemoglobin concentration and oxygen saturation, realizable in vivo, produce enormous changes in the delay time. The range of delay times spans both the mean capillary transit and total circulation times. This result points to the delay time as an extremely important variable in determining the course of sickle cell disease, and suggests a new approach to therapy.

Chemical Phenomena↗
PubMed1973

Structure of hemoglobin S fibers: optical determination of the molecular orientation in sickled erythrocytes.

Possible orientations of deoxyhemoglobin S molecules within sickle-cell fibers are delimited by polarized absorption measurements on single sickled cells and single crystals of deoxyhemoglobin A. The polarization ratio of cells provides a lower limit for that of an individual fiber and, coupled with the absorption properties of the deoxyhemoglobin molecule, restricts the orientation of the long molecular (x) axis to within 22 degrees of the fiber axis. Adopting the stacked ring model of Finch et al. for the molecular positions and the additional constraint that at least one mutated (beta6) site is part of an intermolecular contact, our optical result requires that the true molecular dyad (y) axis pass through some part of an adjacent molecule in the same ring. This range of orientations for the y axis is approximately perpendicular to those described in existing models and places at least one beta6 residue in position to be part of a contact between molecules in the same ring.

Anemia, Sickle Cell↗