Reactive Species and Antioxidants. Redox Biology Is a Fundamental Theme of Aerobic Life

Abstract

The field of antioxidants and free radicals is often perceived as focusing around the use of antioxidant supplements to prevent human disease. In fact, antioxidants/free radicals permeate the whole of life, creating the field of redox biology. Free radicals are not all bad, nor antioxidants all good. Life is a balance between the two: Antioxidants serve to keep down the levels of free radicals, permitting them to perform useful biological functions without too much damage (Halliwell and Gutteridge, 2006). This is especially true in plants, as the rest of this issue reveals. Yet some damage is inevitable, requiring repair systems to maintain cell viability. The purpose of this article is to take a broad view of the field, and highlight some of the fascinating differences between plants and other organisms. All animals need O2 for efficient production of energy in mitochondria. This need for O2 obscures the fact that it is a toxic mutagenic gas; aerobes survive only because they have evolved antioxidant defenses (Halliwell and Gutteridge, 2006). Oxygen appeared in significant amounts in the Earth's atmosphere over 2.2 billion years ago, largely due to the evolution of photosynthesis by cyanobacteria. They evolved to use energy from the sun to split water. Thereby, they gained reducing power (hydrogen equivalents) to drive their metabolism, but the byproduct, tonnes of O2, was discarded into the atmosphere (Lane, 2002) as an early case of air pollution. Initially, most of this O2 was consumed by the formation of the metallic oxide deposits that exist in rocks and ores today. Only when this was largely complete did O2 build up in the atmosphere. The rise in atmospheric O2 was advantageous in at least two ways; it led to formation of the ozone (O3) layer in the stratosphere that protects living organisms from UV-C radiation (that may have helped organisms to leave the sea and colonize land), and it removed ferrous iron (Fe2+) from aqueous environments by forming insoluble ferric complexes. Most Fe2+ was precipitated from solution, leaving sea and river waters today containing only trace amounts of soluble iron (Lane, 2002). When living organisms first appeared on Earth, they did so under an atmosphere containing much N2 and CO2 but little O2, i.e. they were anaerobes (Kasting, 1993). Anaerobes still exist today, but usually their growth is inhibited or they are killed by exposure to 21% O2, the current atmospheric level. As atmospheric O2 rose, many anaerobes must have died out. Present-day ones are presumably the descendants of organisms that adapted to rising O2 by restricting themselves to anoxic microenvironments. Other organisms instead began to evolve antioxidant defenses (producing new ones and realigning ancient molecules to new functions) to protect against O2 toxicity. One of the earliest of these may have been to develop proteins that bind and detoxify iron to protect DNA against Fenton chemistry (Wiedenheft et al., 2005). In retrospect, this development of antioxidants was a fruitful path to follow. Organisms that tolerated O2 could further evolve to use it for metabolic transformations catalyzed by oxidase, oxygenase, and hydroxylase enzymes, such as the Pro and Lys hydroxylases needed for collagen biosynthesis. Indeed, large multicellular animals need collagen for making their support tissues (bone and cartilage). Best of all, O2 could facilitate efficient energy production, employing electron-transport chains with O2 as the terminal electron acceptor. This switch to aerobic metabolism increased the yield of ATP that could be made from food molecules such as Glc by over 15-fold. Evolution of efficient energy production allowed the development of complex multicellular organisms, which then also needed systems to ensure that O2 can be distributed throughout their bodies (mechanisms varying from spiracles in insects to the human vascular system). A further advantage of evolving such systems is that delivery of O2 can be controlled: for example, most cells in the human body are never exposed to the full force of atmospheric O2 because blood pO2 is much lower than that of air. Similarly, insects control the opening of their spiracles to keep internal O2 low (Hetz and Bradley, 2005). Under an atmosphere mainly composed of N2 and CO2 with no O3 screen, UV radiation must have bombarded the face of the Earth. Even at the low O2 levels present 3.5 billion years ago (<0.1%), there could have been substantial H2O2 levels in rainwater generated by photochemical reactions with traces of O2 (Lane, 2002). Since Fe2+ was freely available, Fenton chemistry was a threat, so H2O2 must be eliminated. One suggestion is that the evolutionary precursors of PSII used H2O2 as a substrate (Olson and Blankenship, 2004) and only later evolved the increased chemical ferocity needed to split water. Atmospheric O2 levels may have been higher at periods in the Earth's history, perhaps reaching 35% by the late Carboniferous era. As