Table of Contents
Overview of Metabolism 🌐
Metabolism is the sum of all chemical reactions that occur in the body to maintain life. These reactions allow the body to extract energy from nutrients, build new molecules, and remove waste products. For USMLE Step 1, you do not need to memorize every pathway in full detail at this stage, but you must understand how the major types of metabolism relate to each other and to clinical scenarios.
Metabolism can be divided into two broad types. Catabolism refers to the breakdown of larger molecules such as carbohydrates, lipids, and proteins into smaller units with the release of energy. Anabolism refers to the synthesis of complex molecules such as glycogen, fatty acids, and proteins from smaller building blocks, which requires energy input. Almost all questions that involve energy use, nutrient processing, or hormone effects are rooted in this balance between catabolism and anabolism.
The key idea is that metabolism serves two main purposes. It provides energy in the form of ATP and it supplies building blocks for growth, repair, and specialized functions like hormone production. The body constantly shifts between catabolic and anabolic states, depending on nutrient availability and hormonal signals.
Metabolism = Catabolism (breakdown, usually produces ATP) + Anabolism (synthesis, usually uses ATP).
Energy Currency and Reducing Equivalents 💡
The universal energy currency in the cell is ATP, adenosine triphosphate. Hydrolysis of ATP to ADP and inorganic phosphate releases energy that can be used for muscle contraction, active transport, biosynthesis, and many other processes. ATP is not stored in large amounts. Instead, it is continuously regenerated from ADP by metabolic pathways such as glycolysis, the citric acid cycle, and oxidative phosphorylation, which are covered elsewhere.
Besides ATP, cells use reducing equivalents to transfer high energy electrons. The most important carriers are NADH and FADH$_2$, which feed electrons into the electron transport chain, and NADPH, which serves mainly in anabolic and protective reactions. For example, NADPH is essential for fatty acid synthesis and for maintaining reduced glutathione in red blood cells.
You must distinguish NADH from NADPH conceptually. NADH primarily participates in ATP production through oxidative phosphorylation, while NADPH is involved in biosynthetic pathways and antioxidant defense. The same vitamin precursor, niacin, is used to form both NAD$^+$ and NADP$^+$, but their metabolic roles differ.
NADH and FADH$_2$ = mainly energy production; NADPH = mainly biosynthesis and antioxidant defense.
Metabolic Pathways and Their Directions 🔄
Metabolic reactions are organized into pathways. Some pathways are strictly catabolic, some strictly anabolic, and some can work in both directions depending on the cellular state. It is very important to understand whether a pathway is primarily involved in energy production or in synthesis of complex molecules.
Catabolic examples include glycolysis, beta oxidation of fatty acids, and many steps of amino acid degradation. Anabolic examples include gluconeogenesis, fatty acid synthesis, cholesterol synthesis, and glycogen synthesis. Some pathways are amphibolic, which means they serve both catabolic and anabolic roles. A classic example is the citric acid cycle, which oxidizes acetyl CoA to produce energy but also provides intermediates for amino acid, heme, and glucose synthesis.
Reversibility is not uniform across a pathway. Some steps are irreversible under physiological conditions, which allows pathways in opposite directions to be regulated independently. For instance, glycolysis and gluconeogenesis share several enzymes but have distinct irreversible steps controlled by different enzymes. Drugs, toxins, and genetic defects often target key irreversible steps, leading to accumulation of substrates and characteristic clinical pictures.
Compartmentalization of Metabolism 🧱
Cells separate metabolic activities into distinct compartments. The main compartments relevant for metabolism are the cytosol and the mitochondria. This separation allows the cell to run pathways in parallel without interference and to regulate them independently.
In the cytosol, you find most of glycolysis, the pentose phosphate pathway, fatty acid synthesis, and many steps of protein and nucleotide synthesis. In the mitochondria, you find the citric acid cycle, beta oxidation of fatty acids, the urea cycle steps that are mitochondrial, and oxidative phosphorylation. Some pathways are split between the cytosol and mitochondria, which requires specific transport systems and shuttle mechanisms.
The location of a pathway is often tested indirectly through clinical scenarios. For example, mitochondrial disorders commonly affect tissues with high energy demand and can impact processes like beta oxidation or the electron transport chain, which are mitochondrial. Cytosolic pathways may be more affected by enzyme deficiencies that interfere with biosynthetic processes or redox balance.
A simplified view of compartmentalization is shown in the table.
| Compartment | Mainly Catabolic | Mainly Anabolic |
|---|---|---|
| Cytosol | Glycolysis | Fatty acid synthesis, protein synthesis, nucleotide synthesis |
| Mitochondria | Beta oxidation, TCA cycle, oxidative phosphorylation | Part of urea cycle, some steroid synthesis |
Knowing where a pathway occurs helps you predict which organelles or tissues will be particularly vulnerable in genetic or acquired metabolic diseases.
Metabolic States: Fed and Fasting 🕒
The body alternates between different metabolic states across the day. The classic model describes the fed state, the fasting or postabsorptive state, and prolonged starvation. Each state has a characteristic pattern of fuel use, storage, and release. Hormones, especially insulin, glucagon, and catecholamines, coordinate these changes.
In the fed state, which occurs during and shortly after a meal, blood glucose rises. Insulin secretion increases and glucagon levels fall. Tissues use glucose as the primary fuel. The liver takes up excess glucose to synthesize glycogen and fatty acids. Adipose tissue stores triglycerides, and protein synthesis is active in many tissues. Catabolic pathways that produce glucose, such as gluconeogenesis, are suppressed, while storage and synthesis pathways are stimulated.
During the early fasting or postabsorptive state, usually starting several hours after a meal, blood glucose begins to fall toward baseline. Glucagon rises and insulin falls. The liver breaks down glycogen to release glucose and begins gluconeogenesis. Adipose tissue increases lipolysis, releasing free fatty acids and glycerol. Muscle shifts progressively from glucose to fatty acids as a fuel, which helps conserve glucose for organs that depend on it strongly, such as the brain and red blood cells.
With prolonged fasting or starvation, glycogen stores become depleted. Gluconeogenesis becomes the dominant source of glucose. The liver increases ketone body production from fatty acids, and the brain gradually adapts to use ketone bodies as a major fuel. This adaptation reduces the rate of muscle protein breakdown required to provide substrates for gluconeogenesis. Many metabolic diseases manifest when the body fails to transition normally between these states, often due to enzyme deficiencies, hormonal defects, or organ failure.
Fed state = insulin high, storage and synthesis dominate. Fasting state = glucagon high, mobilization of stores and gluconeogenesis dominate.
Tissue Specific Metabolism 🧠🫀🧍♂️
Different tissues have distinct metabolic roles and fuel preferences. For USMLE, you must know the characteristic fuels and functions of key organs, especially brain, red blood cells, liver, adipose tissue, skeletal muscle, and heart.
The brain normally relies almost entirely on glucose for energy, because most fatty acids do not cross the blood brain barrier. During prolonged fasting, the brain can use ketone bodies significantly, which spares muscle protein from breakdown. However, the brain still requires some glucose even during starvation, so gluconeogenesis remains essential.
Red blood cells lack mitochondria, so they use anaerobic glycolysis exclusively. They convert glucose to lactate, which returns to the liver for further processing. Because of this, red blood cells are very sensitive to defects in glycolytic enzymes and in pathways that maintain redox balance, such as the pentose phosphate pathway.
The liver is the central metabolic hub. It maintains blood glucose by glycogen synthesis and breakdown, and by gluconeogenesis. It synthesizes and oxidizes fatty acids, produces ketone bodies, and converts ammonia to urea. It also makes many plasma proteins and cholesterol. Adipose tissue stores triglycerides in the fed state and releases free fatty acids in fasting. Skeletal muscle uses glucose and fatty acids and serves as a major reservoir of protein that can be catabolized during prolonged fasting. The heart primarily uses fatty acids under normal conditions but can use ketone bodies and lactate as well.
These tissue differences explain why certain metabolic diseases preferentially affect specific organs. For example, defects in beta oxidation enzymes often present with hypoketotic hypoglycemia and can be associated with cardiomyopathy and liver dysfunction, because heart and liver depend heavily on fatty acid oxidation.
Regulation of Metabolism by Hormones 🧬
Hormones provide the primary control over metabolic pathways at the whole body level. Insulin and glucagon act as opposing signals that reflect nutrient availability. Catecholamines, cortisol, thyroid hormone, and others provide additional layers of regulation. Here you should focus on the logic of hormonal control rather than the detailed mechanisms that are discussed elsewhere.
Insulin is secreted in response to elevated blood glucose, such as after a meal. It promotes glucose uptake into certain tissues, especially muscle and adipose tissue, by increasing the number of glucose transporters in the cell membrane. It stimulates glycogen synthesis, fatty acid synthesis, and protein synthesis, which are anabolic processes. At the same time, it inhibits gluconeogenesis, glycogen breakdown, and lipolysis. Overall, insulin favors storage of energy and building of new tissues.
Glucagon is secreted when blood glucose falls, as occurs during fasting, exercise, or between meals. It acts mainly on the liver to stimulate glycogen breakdown and gluconeogenesis, increasing hepatic glucose output. It also promotes lipolysis in adipose tissue indirectly by enhancing the actions of other hormones. Catecholamines, such as epinephrine, reinforce these fasting signals, especially during stress or exercise, by stimulating glycogen breakdown in muscle and lipolysis in adipose tissue.
Cortisol and other glucocorticoids support longer term fasting and stress responses. They promote protein catabolism and gluconeogenesis and reduce peripheral glucose uptake. Thyroid hormone tends to increase overall metabolic rate, which influences energy expenditure and fuel use. Disordered hormone levels, such as in diabetes mellitus or hyperthyroidism, lead to characteristic metabolic derangements that are heavily tested.
Insulin = promotes storage and synthesis. Glucagon + catecholamines = promote fuel mobilization and glucose production.
Integration of Carbohydrate, Lipid, and Protein Metabolism 🔗
Carbohydrates, lipids, and proteins enter interconnected metabolic networks instead of being processed in isolation. A central molecule, acetyl CoA, links the oxidation of glucose, fatty acids, and certain amino acids to the citric acid cycle. Although details of each pathway belong to other sections, you must appreciate how they converge and diverge at common intermediates.
Glucose from the diet or glycogen breakdown is metabolized to pyruvate, which can be converted to acetyl CoA. Fatty acids undergo beta oxidation to yield acetyl CoA units directly. Many amino acids are deaminated and transformed into intermediates that feed into the citric acid cycle or into gluconeogenic pathways. The citric acid cycle itself produces reducing equivalents that drive ATP formation and also provides intermediates for biosynthesis of glucose, amino acids, heme, and other compounds.
The direction of flow through these networks depends heavily on the metabolic state. In the fed state, incoming glucose and amino acids supply energy and serve as substrates for glycogen, triglyceride, and protein synthesis. In fasting, stored triglycerides and glycogen are broken down to supply energy, and amino acids from muscle breakdown support gluconeogenesis. Ketone bodies form when acetyl CoA from fatty acid oxidation exceeds the capacity of the citric acid cycle, especially during prolonged fasting or uncontrolled diabetes.
A frequent theme in questions is the rerouting of intermediates when one pathway is blocked. For example, if the citric acid cycle is inhibited, acetyl CoA accumulates and may be diverted toward ketone body formation. If a specific step in glycolysis or gluconeogenesis is defective, upstream metabolites accumulate and downstream products become deficient, producing biochemical patterns that can be detected in laboratory tests.
Metabolic Flux and Allosteric Control ⚙️
Beyond hormones, metabolic pathways are carefully tuned by allosteric regulation and substrate availability. Allosteric regulators are molecules that bind enzymes at sites other than the active site, changing enzyme activity. These regulators often reflect the energy status of the cell. High levels of ATP, citrate, or NADH suggest that energy is abundant and often inhibit further catabolic activity. High levels of ADP, AMP, or inorganic phosphate signal low energy and stimulate pathways that generate ATP.
Enzymes at rate limiting or irreversible steps are usually the targets of allosteric control. For example, a key glycolytic enzyme is activated by AMP and inhibited by ATP and citrate. This coordination ensures that glycolysis is active when energy is needed and slows when it is not. Similarly, enzymes in gluconeogenesis and fatty acid synthesis respond to metabolites that reflect the current fuel mix and energy supply.
Metabolic flux also depends on substrate availability. The presence of a particular substrate will drive the pathway that consumes it, within the limits set by enzyme capacity and regulation. In clinical contexts, accumulation of unusual substrates or lack of essential cofactors, such as vitamins, will alter flux and produce characteristic metabolite patterns. Many inborn errors of metabolism are essentially defects in metabolic flux through a specific step, which causes toxic accumulation or failure to produce a necessary compound.
Metabolic Adaptation and Disease 🧬🧪
Metabolism is dynamic and adapts to physiological demands such as growth, pregnancy, exercise, and aging. For example, during sustained exercise, skeletal muscle gradually shifts from primarily using glucose to increased reliance on fatty acids. During pregnancy, maternal metabolism adapts to ensure continuous nutrient supply to the fetus, often with increased insulin resistance in late pregnancy to make more glucose available to the fetus.
When regulatory systems fail, metabolic diseases arise. Some diseases are due to inherited enzyme deficiencies, summarised as inborn errors of metabolism. These often present in newborns or infants with poor feeding, vomiting, hypoglycemia, acidosis, or developmental delay. Others are acquired, such as type 2 diabetes mellitus, where insulin resistance and relative insulin deficiency lead to chronic hyperglycemia and altered lipid and protein metabolism.
Liver failure and kidney failure also produce major disruptions in metabolism, because these organs are central to waste removal, gluconeogenesis, and handling of nitrogen. Understanding basic metabolic principles helps you predict the biochemical and clinical consequences of organ failure and hormonal disorders, which is a common theme in USMLE style questions.