Mitochondria are the “powerhouses” of the cell, generating most of the cell’s supply of adenosine triphosphate (ATP) for chemical energy. These small, oval-shaped structures are found in the cells of complex organisms like plants and animals. The chemical reactions inside mitochondria are facilitated by specialized proteins called enzymes.
Enzymes are biological catalysts that speed up chemical reactions without being consumed. Each enzyme is highly specific, designed to work on a particular molecule known as a substrate. Mitochondria are packed with these enzymes, which are responsible for converting energy from food into a usable form.
Enzymes of the Citric Acid Cycle
The citric acid cycle, also known as the Krebs cycle, is a metabolic pathway that occurs in the mitochondrial matrix. Its purpose is to process acetyl-CoA, a molecule derived from carbohydrates, fats, and proteins, by systematically oxidizing it to release stored energy. The cycle consists of a sequence of eight enzyme-driven reactions.
The cycle begins when the enzyme Citrate Synthase combines acetyl-CoA with a four-carbon molecule, oxaloacetate, to form a six-carbon molecule called citrate. This initial step is what gives the cycle its name. As the citrate molecule is progressively modified, its atoms are rearranged and oxidized by a dedicated team of enzymes.
A primary regulatory point is controlled by the enzyme Isocitrate Dehydrogenase. This enzyme catalyzes the conversion of isocitrate into α-ketoglutarate, releasing one molecule of carbon dioxide and transferring high-energy electrons to NAD+ to form NADH. The activity of Isocitrate Dehydrogenase is controlled by the cell’s energy levels, slowing when ATP is abundant and speeding up when more energy is needed.
Through a full turn of the cycle, the original acetyl-CoA is completely oxidized, releasing two molecules of carbon dioxide. The energy from this breakdown is captured in the form of high-energy electron carriers, NADH and FADH2, which carry the captured energy to the next stage of cellular respiration.
Enzymes of the Electron Transport Chain and ATP Synthesis
The high-energy electron carriers NADH and FADH2 deliver their electrons to the electron transport chain (ETC). The ETC is a series of four large enzyme complexes (Complex I, II, III, and IV) embedded within the highly folded inner mitochondrial membrane. These folds, known as cristae, increase the surface area of the membrane, providing more space for the enzyme complexes. The function of these complexes is to manage a sequential transfer of electrons, with each transfer releasing a small amount of energy.
As electrons are passed down the chain, the released energy is used by Complexes I, III, and IV to pump protons (H+ ions) from the mitochondrial matrix into the intermembrane space. This action creates an electrochemical gradient, with a high concentration of positively charged protons on one side of the membrane. This gradient represents a form of stored potential energy, similar to water held behind a dam.
This stored energy is harnessed by the enzyme ATP Synthase, also known as Complex V, which functions like a molecular turbine. Protons that were pumped into the intermembrane space flow back down their concentration gradient into the matrix by passing through a channel in the ATP Synthase enzyme.
The flow of protons through ATP Synthase causes a part of the enzyme to spin. This rotational mechanical energy drives the catalytic portion of the enzyme to convert adenosine diphosphate (ADP) and inorganic phosphate (Pi) into ATP.
This process, where energy from the proton gradient powers ATP production, is called chemiosmosis. It is the primary source of ATP in aerobic organisms, generating the majority of the energy that cells need.
Enzymes for Fatty Acid Oxidation
Mitochondria also break down fats for energy through a pathway in the mitochondrial matrix known as beta-oxidation. Before this process begins, fatty acids must be “activated” by attaching to coenzyme A (CoA) and then transported across the inner mitochondrial membrane by a carnitine shuttle system.
Once inside the matrix, the fatty acid undergoes a repeating four-step sequence of enzymatic reactions. Each cycle of beta-oxidation shortens the fatty acid chain by two carbon atoms, releasing one molecule of acetyl-CoA, one molecule of NADH, and one of FADH2.
A primary family of enzymes in this process is the Acyl-CoA Dehydrogenases. These enzymes are responsible for the first step in each cycle, and different versions specialize in processing fatty acids of varying lengths.
The acetyl-CoA produced from beta-oxidation enters the citric acid cycle, while the generated NADH and FADH2 deliver their electrons to the electron transport chain.
Role in Cellular Health and Disease
Errors in mitochondrial enzyme function can lead to serious medical conditions known as mitochondrial diseases, which arise from mutations in the genes that code for these enzymes. The instructions for building mitochondrial proteins come from two separate sources: nuclear DNA (nDNA) from the cell’s nucleus and the mitochondria’s own DNA (mtDNA).
This dual genetic origin means diseases can be inherited through various patterns, including from nuclear genes or maternally from mtDNA. A defect in a single enzyme can disrupt an entire metabolic pathway, leading to an energy deficit in the cells. Tissues and organs with high energy demands, such as the brain, heart, and muscles, are often the most affected.
One example is Leigh syndrome, a neurological disorder that appears in infancy. It can be caused by defects in different mitochondrial enzymes, most commonly those in the electron transport chain or ATP synthase, which leads to the degradation of motor and cognitive skills.
Another example is Medium-Chain Acyl-CoA Dehydrogenase (MCAD) deficiency, a disorder of fatty acid oxidation. This condition is caused by a defect in the enzyme that breaks down medium-chain fatty acids.
Without this enzyme, the body cannot properly use these fats for energy, which can lead to low blood sugar, lethargy, and severe outcomes during periods of fasting or illness.