Oxygen Responses, Reactivities, and Measurements in Biosystems

S. N. Mukhopadhyay, Dipak K. Das

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000141528
• Topic: Medicine
• Subtopic: Endocrinology & Metabolism

Oxygen Responses, Reactivities, and Measurements in Biosystems meets the pressing needs of the twentieth-century biotechnological and bioengineering sciences in covering oxic reactions and oxygen transport phenomena in a single book. This book is intended for teaching senior or graduate level courses and as a self-study text for practicing biochemical and chemical engineers, biotechnologists, applied and industrial microbiologists, cell biologists, scientists involved in oxygen-free radical research, and others in related fields. The text includes thought-provoking numerical problems and short questions, conventional biochemical engineering approaches and related concepts with mathematical formulations and analysis, concepts of cell biology, basic microbiology and applied biochemistry in oxy radical research, practical approaches for the development of laboratory experiments and industrial design, and an introduction of oxygen-free radical chemistry to biotechnology and bioengineering.

Chapter 1

Introduction

CONTENTS

I.	Evolutionary Aspects of Oxygen
II.	Atmospheric Oxygen Cycle and Biosphere
References

I. EVOLUTIONARY ASPECTS OF OXYGEN

Oxygen response in biosystems has manifested itself since the existence of oxygen and living organisms on earth.^{1, 2, 3, 4, 5} The environmental conditions of global anoxic atmosphere changed gradually by geological transformations, terrestrial rays, temperature, and later biospheric factors. Those organisms which acquired the ability to resist environmental and stress conditions survived as anaerobes. These anaerobic organisms, including photosynthetic bacteria, had oxygen-sensitive superoxide dismutase (SOD) enzyme in their cells.^6,7 Probably, adaptive and mutational mechanisms caused some to be transformed to cyanobacteria and were responsible for dioxygen evolution in addition to geological transformations in the atmosphere. The dioxygen species of the biosphere worked as an electron acceptor in biosystems for the generation of biochemical energy and also as the substrate for developing oxidase and oxygenase enzyme systems^{4, 5, 6, 7, 8} in the biosynthesis of a variety of cell components and organelles. Thus, based on oxygen responses, anaerobic, facultative, and aerobic organisms came into existence in the biosphere through biological oxidation system reactions.⁹ However, even aerobic organisms cannot survive without defense mechanisms against oxidative stresses and damages. In this respect, the response of dioxygen concentration in the atmosphere plays a significant role in the evolution of microbes, plants, and animals of the biosphere and in their signaling, regulatory, and control mechanisms. It may appear that organisms acquired defense and other regulatory systems before the accumulation of dioxygen in the atmosphere by cyanobacteria, and that oxygen, even in trace concentrations, may be toxic to cells and organelles. The observation of the existence of three different types of SOD in organisms (e.g., FeSOD, MnSOD, CUZnSOD) has an intimate relation with the time of appearance of the organisms on Earth, as shown in Figure 1.1. The catalase enzyme system distributes ubiquitously in organisms. More effective scavenging enzyme peroxidase uses different electron donors between plants (ascorbate) and animals (glutathione, GSH). GSH has been found in almost all organisms, both prokaryotic and eukaryotic. In contrast, tocopherols occur in prokaryotic and eukaryotic photosynthetic organisms, but not in other prokaryotes. Therefore, during evolution organisms could develop the capacity to activate dioxygen by reduction via enzymic reaction-mediated electron and H atom transfer, developing high scavenging capacity of active oxygen species in organelles of cells where production of reactive oxygen is high. However, the scavenging enzyme of hydrogen peroxide is different in each cellular organelle (peroxidases in mitochondria and chloroplasts, catalase in peroxisomes).¹⁰

Therefore, oxygen in free and combined states can exist in the form of different species. Various oxygen species may react very significantly in expressing different properties and functions or characteristics of biological cells. In the biosphere, there are three major biosystems, namely, microbial, animal, and plant cells (Figure 1.2). Each of these cells in nature may react or respond to oxygen species to a great extent either during their physiological or metabolic and product formation functions through oxidative and reductive processes. Depending on the nature of the biosystem, the participation of dioxygen species such as free and dissolved molecular oxygen (O₂, ${\mathrm{O}}_2^{-}$, HOO⁻, HOOH) and mono-oxygen species (O, O⁻, OH, OH⁻) in biomedia have been proposed in order to elucidate redox kinetics and thermodynamics.^{11, 12, 13, 14, 15} Substantial evidence is now available in the literature to indicate that besides molecular oxygen, oxygen free radicals or singlet oxygen play a significant role in the complex course of nutritional participation, industrial bioreactions, toxicity, multi-step carcinogenesis, etc. in biosystems.^{4,16, 17, 18, 19} Several nineteenth century investigators^{20, 21, 22} observed that a number of enzymes present in living microorganisms catalyzed oxidations by molecular oxygen. These enzymes in cells used to be referred to as oxidases. It was believed that during catalysis, these enzymes interacted with oxygen in such a way that oxygen was activated to carry out a direct reaction with the substrate. Later it was proposed that one such enzyme belonging to the so-called “oxygenases” catalyzed the reaction of oxygen with an acceptor to produce a peroxide and another enzyme called “peroxidase” next catalyzed the reaction of the peroxide with the substrate. This concept and subsequent studies on the human system, including heme-containing respiratory enzymic reactions, drew attention to the probability that O₂ reacts with substrate in enzymic oxidations. However, the focus did shift when it eventually became clear that many enzymes that catalyze oxidation reactions do not need the presence of oxygen. Thus it was known that oxidations could effectively occur by some enzymes under completely anoxic (anaerobic) conditions as long as an electron acceptor was present. One of the classic examples of the same is the reversible enzymic oxidation of alcohol to aldehyde in biocells. Such findings as well as the results of some nonenzymic reaction studies have led to the proposal of a general mechanism for enzymic oxidation-reduction reactions in which the enzymes participated merely as catalysts of trans-hydrogenation reactions, with O₂ sometimes being the ultimate electron acceptor. For rationalizing the oxidation of other substrates where a new oxygen ends up in the product, it was proposed that the substrate is initially hydrated reversibly and is then oxidized enzymatically again by transhydrogenation.^{23, 24, 25, 26} This concept prevailed for a long period of time until investigators like Michaelis, and his co-worker²⁷ proposed that all oxidation reduction reactions (including those catalyzed by enzymes) had to proceed in successive univalent steps. As all stable substrates and products of redox enzymic reactions virtually contain an even number of electrons, this meant that free radicals were being proposed as intermediates in these reactions. It is based on the quantum mechanical argument that the probability of two electrons being transferred simultaneously is vanishingly small. However, it is now known that a large number of nonenzymic redox reactions, especially those involving organic compounds, are known to proceed by nonradical mechanisms. This pointed out fallacies in the arguments put forward by Michaelis²⁷ and others,²⁸ in that they did not consider that many redox reactions do occur not only by electron transfer, but by atom or group transfer as well. In such reactions, when an atom or group is transferred, it can be accompanied by any number of electrons. As a consequence, the overall two electron oxidations or ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Preface
• Foreword
• Table of Contents
• Chapter 1: Introduction
• Chapter 2: Biocells: Organelles and Fundamental Characters
• Chapter 3: Oxygen Source and Properties
• Chapter 4: Responses and Functions of Oxygen in Biosystems
• Chapter 5: Oxygen Solubilities and Affinity in Aqueous Biomedia
• Chapter 6: Oxygenation in Biosystems
• Chapter 7: Regulation and Control of Oxygen In Biosystems
• Chapter 8: Model Numerical Problems on Oxygen Interactions in Biosystems
• Index