MCAT General Chemistry Review

Chapter 11: Oxidation–Reduction Reactions

Introduction

You’re on a night call in the emergency department (ED) when a 5-month-old infant patient’s chart appears on your screen. You click through the triage notes and take note of what the mother reports: poor sucking ability and loss of head control and motor skills. You’re puzzled by the findings and the previous ED admissions of lactic acidosis. You suspect diabetic ketoacidosis (DKA), liver, or kidney diseases—and possibly even poisoning—but nothing seems to fit. Minutes later, the child is brought into the examination room and does not stop crying. Over the course of an hour, another episode of lactic acidosis develops. The child is eventually admitted to the neonatal intensive care unit for long-term care.

Later, you ask the neonatologist about the patient. They point you to the charts and a genetic test performed shortly after birth. The child was diagnosed with Leigh’s disease, an extremely rare mitochondrial disorder. In Leigh’s disease, a number of key mitochondrial enzymes are disrupted and the process of oxidative phosphorylation is never achieved. Specifically, some of the most important enzymes that catalyze oxidation–reduction reactions, such as the pyruvate dehydrogenase complex and succinate dehydrogenase complex, are affected. When pyruvate cannot be oxidized to acetyl-CoA, it is instead fermented to lactic acid.

In biological systems, oxidation is coincident with the loss of electrons, sometimes in the form of hydrogen (dehydrogenation). The enzymes that catalyze these oxidations are called dehydrogenases. Many other macromolecules besides enzymes, such as vitamins, also carry out their functions by oxidizing or reducing other compounds. Iron in hemoglobin likewise undergoes rounds of oxidation and reduction as it carries oxygen from the lungs to tissues.

In this chapter, we focus our attention on the movement of electrons in chemical reactions. Such reactions are called oxidation–reduction (redox) reactions because they always occur in pairs. Oxidation–reduction reactions are particularly important because they tie into a number of topics in organic chemistry and biochemistry. In fact, Chapters 5 through 10 of MCAT Organic Chemistry Review and Chapters 9 through 11 of MCAT Biochemistry Review all touch on oxidation–reduction reactions in different sets of molecules.