The reactive superoxide radical, O2-, formerly of concern only to radiation chemists and radiobiologists, is now understood to be a normal product of the biological reduction of molecular oxygen. An unusual family of enzymes, the superoxide dismutases, protect against the deleterious actions of this radical by catalyzing its dismutation to hydrogen peroxide plus oxygen.

https://science.sciencemag.org/content/sci/201/4359/875.full.pdf

The biology of oxygen radicals

For present purposes, a protein-bound metal site consists of one or more metal ions and all protein side chain and exogenous bridging and terminal ligands that define the first coordination sphere of each metal ion. Such sites can be classified into five basic types with the indicated functions: (1) structural -- configuration (in part) of protein tertiary and/or quaternary structure; (2) storage -- uptake, binding, and release of metals in soluble form: (3) electron transfer -- uptake, release, and storage of electrons; (4) dioxygen binding -- metal-O{sub 2} coordination and decoordination; and (5) catalytic -- substrate binding, activation, and turnover. The authors present here a classification and structure/function analysis of native metal sites based on these functions, where 5 is an extensive class subdivided by the type of reaction catalyzed. Within this purview, coverage of the various site types is extensive, but not exhaustive. The purpose of this exposition is to present examples of all types of sites and to relate, insofar as is currently feasible, the structure and function of selected types. The authors largely confine their considerations to the sites themselves, with due recognition that these site features are coupled to protein structure at all levels. In themore » next section, the coordination chemistry of metalloprotein sites and the unique properties of a protein as a ligand are briefly summarized. Structure/function relationships are systematically explored and tabulations of structurally defined sites presented. Finally, future directions in bioinorganic research in the context of metal site chemistry are considered. 620 refs.« less

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Structural and Functional Aspects of Metal Sites in Biology

Superoxide ion (O2•–) is of great significance as a radical species implicated in diverse chemical and biological systems. However, the chemistry knowledge of O2•– is rather scarce. In addition, numerous studies on O2•– were conducted within the latter half of the 20th century. Therefore, the current advancement in technology and instrumentation will certainly provide better insights into mechanisms and products of O2•– reactions and thus will result in new findings. This review emphasizes the state-of-the-art research on O2•– so as to enable researchers to venture into future research. It comprises the main characteristics of O2•– followed by generation methods. The reaction types of O2•– are reviewed, and its potential applications including the destruction of hazardous chemicals, synthesis of organic compounds, and many other applications are highlighted. The O2•– environmental chemistry is also discussed. The detection methods of O2•– are categorized and elaborated. Special attention is given to the f...

Superoxide Ion: Generation and Chemical Implications

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 The chapter reviews the results obtained from measurements of hydrogen exchange (H–H, H–D, and H–T) in proteins and related compounds. The slowness of the rate of hydrogen exchange in a protein, relative to the exchange rate observed with simple peptides under the same experimental conditions is closely related to the conformation of the protein molecule in aqueous solution. Quantitative measurements of the rate of hydrogen exchange in a given protein, under specified experimental conditions, provide a multiparameter characterization of the protein conformation (or distribution of conformations) present under these conditions. Moreover, the chapter discusses the experimental techniques that have been used to measure quantitatively the rate of hydrogen exchange. The chapter also discusses arguments that support case, as the mechanism of exchange of the slowly exchanging hydrogen atoms.

Hydrogen exchange in proteins.

Determinations at 22 deg C of the thermodynamic dissociation constant of acetic acid in ordinary water and in deuteriumenriched water (98.0 volume% D2O) demonstrate that an ordinary Radiometer glass electrode type G 202A under convenient experimental conditions exhibits the theoretical response to variations in the hydrogen-ion concentration in both solvents in the range between 2 x 10-2 and 2 x 10-5 M. The acidity determinations involve standardization and storage of the glass electrode in solutions in H2O and subsequent drying of the glass electrode with mercury before immersing it in the 0.5-ml. deuterium enriched samples. From the electromotive forces observed with a saturated KCl-H2O calomel electrode at 22 deg C the relation between true p(DH) and apparent pH in 98% D2O is derived, p(DH) = apparent pH + 0.44. A possibility of determining the activities, aH and a/sub D/, separately, is considered from the point of view of extrapolating rate data obtained in H2 O-D2O mixtures to pure D2O.

Sigurd O. Nielsen

Papers

Acidity measurements with the glass electrode in h2o-d2o mixtures

Absorption spectrum and decay kinetics of O2- and HO2 in aqueous solutions by pulse radiolysis

Pulse radiolysis of oxalic acid and oxalates

Hydrogen-deuterium exchange of poly-dl-alanine in aqueous solution

Hydrogen-deuterium exchange of small peptides in aqueous solution.