Oxygen: High Enzymatic Reactivity of Reactive Oxygen Species

Earth's life depends mainly on the availability of oxygen in the terrestrial biosphere. Based on geochemical records of existing terrestrial oxides, oxygenic photosynthesis occurred in the cyanobacterial precursors approximately 2800 Ma ago. The oxygen level in the atmosphere is now 21%. The human cells use this oxygen to extract the necessary energy through mitochondrial respiration using the reactions of the redox system that involves the transfer of electrons, enzymatic agents, and reactive oxygen species, mainly superoxide radical (O.- 2), hydroxyl radical (.OH) and hydrogen peroxide (H2O2). The different reaction mechanisms from and to produce reactive oxygen species with their reaction constants in aquatic environments are presented here, as well as their production through the Fenton reaction. Oxidative stress is an imbalance between both normal oxygen-free radicals’ production and the cell's ability to detoxify it.

Biological oxidation is the mechanism by which oxygen is supplied to living organisms and energy is produced. These processes involve the participation of oxygen incorporated into the substrates by oxidoreductase enzymes. The mechanisms of reaction of enzymes involved in both oxidation-reduction processes and electron transport chain mitochondrial-ETC, and their relationship with reactive oxygen species (ROS) production, are presented here.

Reactive oxygen species -ROS are produced by the oxidation-reduction processes of some enzymes as a product of the energy metabolism of cells and organelles and by exogenous sources. The mechanisms of oxidation-reduction and the enzymatic kinetics of some ROS source enzymes are presented here, as well as the mechanisms related to ROS production in organelles and cells and exogenous sources such as Fe, Cd, Hg, Ni, Zn, among others and xenobiotics such as paraquat-PQ.

The enzymatic and non-enzymatic reaction mechanisms of primary and secondary defense systems developed by cells to diminish the effects caused by overproduction of reactive oxygen species-ROS as a metabolic response to the damaging effects from endogenous and environmental factors are presented here. Enzymatic reaction mechanisms developed by plants as an antioxidant defense system are also presented.

The high enzymatic reactivity of reactive oxygen species can be seen in the metabolism of flavonoids, since throughout the metabolic process that presents this type of chemical compounds as trapping agents of reactive oxygen species or antioxidants, finally and due to the speed of reaction they themselves become promoting agents of the same reactive oxygen species. The flavonoids are organic molecules that, due to their chemical nature and their low redox potential (0.23 < E7 < 0.75 V), can easily react with oxygen-free radicals, inhibiting both the action of the radicals and the molecules producing them; through antioxidant action mechanisms; some of which are explained by physicochemical and molecular parameters such as heat formation (ΔHf), Ionization potential and (IP Bond dissociation energy (BDE). However, after bioavailability and absorption, the flavonoids promote oxygen free radical’s production acting as prooxidants mainly through hydrogen atom transfer HAT or simple electron transfer-SET mechanisms, properties and mechanisms of antioxidant and prooxidant reaction.

The ability to extract oxygen from the environment and deliver it to each cell of the multicellular organism through metabolism in time was the main development of organisms during evolution. The life of living organisms is absolutely dependent on oxygen supplementation for respiration, a process by which cells produce ATP to obtain energy energy from controlled reactions of hydrogen with oxygen to produce water. Depending on cell type, function and biological state, cells have a broad range in oxygen utilization. Here there is evidence on the considerable increase of reactive oxygen species-ROS caused by metabolic alterations, as response the low levels of oxygen available-hypoxia that occur in human cells.