1,721,092 research outputs found
Hemoglobin and Cooperativity: Experiments and Theories
No full textCooperative interactions within biological macromolecules are of fundamental physiological relevance and have been studied in great detail. Yet, even in the best investigated case of oxygen binding by hemoglobin, our understanding of the structural and thermodynamic bases of cooperativity is far from satisfactory. Several theoretical models have been proposed to explain cooperative O(2) binding to hemoglobin, among which the two-state model by Monod, Wyman and Changeux, has been the most successful and the most thoroughly tested. This model explains the functional properties of hemoglobin as resulting from the equilibrium of two quaternary conformations, named R and T, characterized by different ligand affinity, and is capable of very accurate (but not always exact) predictions. This review focuses on the experiments carried out to test the models of cooperativity, and especially the two-state model, and identifies two major deviations, or groups of deviations, between the predictions of this model and the actual experimental results, namely (i) the changes in the behaviour of the T-and R-state due to solvent components; (ii) the appearance of R-like reactivity under experimental conditions in which the T-state should be largely prevalent. Modern models of cooperativity, devised to account for these discrepancies while maintaining the basic two-state hypothesis of Monod, Wyman and Changeux, are also reviewed
Logica e Fatti nelle Teorie Freudiane
No full textL'opera freudiana è difficile da valutare, perché affronta argomenti assai disparati, dallo studio e dalla terapia di singoli casi clinici alle ipotesi sulle cause delle malattie mentali, allo sviluppo psichico del bambino, alle teorie antropologiche, al commento di opere d'arte. Il volume presenta una rassegna storica dello sviluppo della psicoanalisi e delle critiche che le sono state mosse. Le teorie freudiane vengono analizzate attraverso più chiavi di lettura: distinguendo le interpretazioni cllniche da quelle scientifiche e, su un piano diverso, gli approcci empirico-deduttivi da quelli ermeneutici. Con un atteggiamento improntato a libertà di pensiero, curiosità intellettuale e grande rispetto verso la materia trattata, l'autore prende in considerazione la possibilità che la psicoanalisi non sia affatto una "scienza", ma piuttosto una disciplina affine all'ermeneutica
Ligand-linked association-dissociation In transport proteins and hormone receptors
No full textNelle macromolecole biologiche possono verificarsi cambiamenti dello stato di aggregazione controllati dalla combinazione con piccole molecole. Questi fenomeni hanno grande importanza in vari processi fisiologici quali la risposta dei recettori ormonali la cooperatività e altri ancora.I formalismi matematici che descrivono questi processi sono complessi perché devono descrivere simultaneamente osservazioni sperimentali condotte in condizioni sperimentali diverse che prevedono la variazione di molti parametri; devono inoltre consentire casi in cui la stechiometria ed il meccanismo di legame variano con lo stato di aggregazione. In questo articolo vengono esaminati alcuni casi paradigmatici con l'ambizione di essere comprensivi sul numero dei possibili meccanismi, se non sul numero delle proteine che presentano fenomeni di aggregazione-disaggregazione ligando-dipendenti.Ligand-linked changes in the aggregation state of biological macromolecules occur and have importance in several physiological processes, e.g., the response of hormone receptors, cooperative ligand binding, and others. The mathematical formalisms that express the thermodynamics governing these processes are complex, as they are required to describe observations made under experimental conditions in which many parameters may be simultaneously varied. The description of the functional behaviour of proteins that present ligand-linked association-dissociation events must accommodate cases where both the binding stoichiometries and reaction mechanisms are variable. In this paper, we review some paradigmatic cases that cover different structural arrangements and binding modes, with special attention to the case of dissociating homodimeric transport proteins and receptors. Even though we cannot pretend to be comprehensive on the proteins presenting this behaviour, we believe that we can attempt to be comprehensive on the structural arrangements and thermodynamic properties of these systems, which fall into a limited set of possible types
On the measurement of cooperativity and the physico-chemical meaning of the hill coefficient
No full textCooperative ligand binding is a fundamental property of many biological macromolecules, notably transport proteins, hormone receptors, and enzymes. Positive homotropic cooperativity, the form of cooperativity that has greatest physiological relevance, causes the ligand affinity to increase as ligation proceeds, thus increasing the steepness of the ligand binding isotherm. The measurement of the extent of cooperativity has proven difficult, and the most commonly employed marker of cooperativity, the Hill coefficient, originates from a structural hypothesis that has long been disproved. However, a wealth of relevant biochemical data has been interpreted using the Hill's coefficient, and is being used in studies on evolution and comparative physiology. Even a cursory analysis of the pertinent literature shows that several authors tried to derive from the Hill's coefficient more sound biochemical information, often unaware of each other. As a result, a perplexing array of equations interpreting the Hill's coefficient is available in the literature, each responding to specific simplifications or assumptions. In this work we summarize and try to order these attempts, and demonstrate that the Hill coefficient (i) provides a minimum estimate of the free energy of interaction, the other parameter used to measure cooperativity, and (ii) bears a robust statistical correlation to the population of incompletely saturated ligation intermediates. Our aim is to critically evaluate the different analyses that have been advanced to provide a physical meaning to the Hill coefficient, and possibly to select the most reliable ones to be used in comparative studies that may make use of the extensive but elusive information available in the literature
Reversible ligand binding: theory and experiment
No full textReversible Ligand Binding: Theory and Experiment discusses the physical background of protein-ligand interactions—providing a comprehensive view of the laws that govern reversible, as well as irreversible, ligand binding. Special consideration is devoted to enzymology, a field usually treated separately from ligand binding, but actually governed by identical thermodynamic relationships. Attention is given to the design of experiments, including how to uncover evidence of biochemical features that may otherwise escape notice. Classical experiments are reviewed in order to further highlight the importance of the experimental design. Overall, the book supplies students with understandingnecessary for interpreting ligand binding experiments, formulating plausible reaction schemes, and analyzing the data according to a chosen model.
Topics covered include: theory of ligand binding to monomeric proteins; practical considerations and commonly encountered problems; oligomeric proteins with multiple binding sites; ligand binding kinetics; hemoglobin and its ligands; single-substrate enzymes and their inhibitors; two-substrate enzymes and their inhibitors; and rapid kinetic methods for studying enzyme reactions
Approaches to the engineering of hemoglobin based oxygen carriers
No full textMolecular biology offers the opportunity to construct hemoglobin molecules as blood substitutes tailored to specific therapeutic applications. Oxygen affinity can be manipulated by amino acid substitutions in the heme pocket or on the protein surface. A response to the concentration of plasma-Cl- that mimics the effect of 2,3-DPG was also introduced in human Hb. Polymerization of tetrameric Hb prevents the vasoconstriction associated with infusion of Hb solutions. Polymers have been obtained through the formation of intermolecular S-S bonds between cysteine residues introduced on the Hb surface. In a mouse model, transfusion of polymeric hemoglobins reduced the volume of cerebral infarction. This effect was particularly evident with polymers having a high oxygen affinity (P50≤ ̃2.0 Torr) and no cooperativity (n = 1). These are the same functional characteristics of myoglobin. Polymers of myoglobin have been constructed and could be a viable alternative to the use of polymeric hemoglobins in some pathological conditions
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