On the Nature of Allosteric Transitions: A Plausible Model

This article presents evidence for the existence of a specific linear relationship between the entropy change and the enthalpy change in a variety of processes of small solutes in water solution. The processes include solvation of ions and nonelectrolytes, hydrolysis, oxidation–reduction, ionization of weak electrolytes, and quenching of indole fluorescence among others. The values of the proportionality constant, called the compensation temperature, lie in a relatively narrow range, from about 250 to 315 °K, for all these processes. Such behavior can be a consequence of experimental errors but for a number of the processes the precision of the data is sufficient to show that the enthalpy–entropy compensation pattern is real. It is tentatively concluded that the pattern is real, very common and a consequence of the properties of liquid water as a solvent regardless of the solutes and the solute processes studied. As such the phenomenon requires that theoretical treatments of solute processes in water be expanded by inclusion of a specific treatment of the characteristic of water responsible for compensation behavior. The possible bases of the effect are proposed to be temperature-independent heat-capacity changes and/or shifts in concentrations of the two phenomenologically significant species of water. The relationship of these alternatives to the two-state process of water suggested by spectroscopic and relaxation studies is examined. The existence of a similar and probably identical relationship between enthalpy and entropy change in a variety of protein reactions suggests that liquid water plays a direct role in many protein processes and may be a common participant in the physiological function of proteins. It is proposed that the linear enthalpy–entropy relationship be used as a diagnostic test for the participation of water in protein processes. On this basis the catalytic processes of chymotrypsin and acetylcholinesterase are dominated by the properties of bulk water. The binding of oxygen by hemoglobin may fall in the same category. Similarities and differences in the behavior of small-solute and protein processes are examined to show how they may be related. No positive conclusions are established, but it is possible that protein processes are coupled to water via expansions and contractions of the protein and that in general the special pattern of enthalpy–entropy compensation is a consequent of the properties of water which require that expansions and contractions of solutes effect changes in the free volume of the nearby liquid water. It is shown that proteins can be expected to respond to changes in nearby water and interfacial free energy by expansions and contractions. Such responses may explain a variety of currently unexplained characteristics of protein solutions. More generally, the enthalpy–entropy compensation pattern appears to be the thermodynamic manifestation of “structure making” and “structure breaking,” operationally defined terms much used in discussions of water solutions. If so, the compensation pattern is ubiquitous and requires re-examination of a large body of molecular interpretations derived from quantitative studies of processes in water. Theories of processes in water may have to be expanded to accommodate this aspect of water behavior.

Enthalpy-entropy compensation phenomena in water solutions of proteins and small molecules: a ubiquitous property of water.

Publisher Summary This chapter describes the advances with an emphasis on the structures of the alcohol dehydrogenases and the relationship between structure and function Yeast and mammalian alcohol dehydrogenase differ in substrate specificity and rate of catalytic activity The classic yeast enzyme is more specific for acetaldehyde and ethanol, which is consistent with its recognized physiological Significance to participate in alcohol fermentation at the end of the glycolytic pathway Enzyme forms with other functions and properties also occur in yeast The mammalian enzymes have broad substrate specificity and, even with primary alcohols, the maximum activity is not observed with ethanol Alcohols including ethanol, produced in the intestinal tracts mainly by bacterial actions, are found in the portal vein One physiological function of liver alcohol dehydrogenase may be to metabolize these products Structural studies have established that mammalian alcohol dehydrogenases have a distant evolutionary link to both the yeast and bacterial enzymes Ingested alcohol is metabolized to acetaldehyde mainly by the action of liver alcohol dehydrogenase

3 Alcohol Dehydrogenases

Cooperativity in the dissociation of nitric oxide from hemoglobin.

Conformation, co-operativity and ligand binding in human hemoglobin.

Preparation and Properties of α- and β-Chains from Human Hemoglobin

http://www.jbc.org/content/244/17/4668.full.pdf

The Reaction of Methemoglobin with Some Ligands

The Enzymatic Aminohydrolysis of 4-Aminopyrimidine Deoxyribonucleotides

Publisher Summary This chapter deals with the circular dichroism (CD) spectra of hemoglobins. The measurement of CD spectra as a tool to investigate the structural organization of protein molecules is of particular advantage for hemoglobins. These molecules contain, in addition to the protein moiety, the heme, with electronic transitions that are quite intense, diverse, and very sensitive both to the surrounding environment and to ligand binding. This situation offers three distinct regions of investigation, each containing information concerning a part of the structural organization of the hemoglobin molecule. Consequently, the optical activity of hemoglobin spectra depends on the relative positions of the different chromophores in the tridimensional organization of the molecule. Alterations of the relative positions in different conformational states are likely to cause alterations in the interactions from which the CD spectra of the different chromophores derive. This is the reason that CD is so sensitive to the conformational states of the molecule. The chapter also presents the informational content of CD spectra.

Giuseppe Geraci

Papers

Preparation and Properties of α- and β-Chains from Human Hemoglobin

The Reaction of Methemoglobin with Some Ligands

The Enzymatic Aminohydrolysis of 4-Aminopyrimidine Deoxyribonucleotides

[17] Circular dichroism spectra of hemoglobins

The reaction of liver alcohol dehydrogenase with reduced diphosphopyridine nucleotide.