Chapter 12: Biologic Oxidation & Energy Production

The controlled reaction of hydrogen with oxygen to form water is the basis of respiration, the critical process by which higher animals generate energy in the form of ATP. The propensity of reactants to donate or accept electrons is quantified by the oxidation–reduction or redox potential (E'0), typically measured at pH 7.0, which allows scientists to predict the movement of electrons between biochemical pairs. The enzymes that govern these reactions, known as oxidoreductases, are divided into four main categories: oxidases, dehydrogenases, hydroperoxidases, and oxygenases. Oxidases utilize oxygen as a hydrogen acceptor, generating either water or hydrogen peroxide; a key example is Cytochrome Oxidase, a hemoprotein and the final component of the mitochondrial respiratory chain that transfers electrons directly to oxygen. Dehydrogenases perform two essential tasks: transferring reducing equivalents between substrates using coenzymes like NAD+ (crucial for oxidative pathways like glycolysis) or NADP+ (used in reductive biosynthesis like fatty acid and steroid synthesis), and transporting electrons within the respiratory chain. Hydroperoxidases, including peroxidases (like glutathione peroxidase containing selenium) and the hemoprotein catalase, are vital defense systems that protect the body from the harmful accumulation of Reactive Oxygen Species (ROS) and hydroperoxides. Oxygenases directly incorporate molecular oxygen into their substrates and are categorized as dioxygenases (incorporating both oxygen atoms) or monooxygenases (incorporating one atom while reducing the other to water). The Cytochrome P450 superfamily is a critical group of monooxygenases situated in the endoplasmic reticulum and mitochondria that plays a dominant role in the detoxification and modification of medicinal drugs and xenobiotics, as well as the biosynthesis of crucial steroid hormones. Finally, tissues are protected from the dangerous superoxide anion-free radical (O2-), which results from the transfer of a single electron to oxygen, by the enzyme Superoxide Dismutase (SOD), which converts superoxide into oxygen and hydrogen peroxide.