Table of Contents
Overview
Catabolic metabolism (catabolism) includes all metabolic pathways in which complex, energy-rich molecules are broken down into simpler ones and part of their chemical energy is converted into usable free energy (mainly in the form of ATP). This chapter focuses on the role of catabolic pathways in living organisms and prepares the ground for the specific chapters on cellular respiration and fermentation.
Role of Catabolic Metabolism in the Cell
Catabolism is one side of metabolism, complementing anabolic metabolism (anabolism):
- Catabolism
- Breakdown of large, complex molecules (e.g. carbohydrates, lipids, proteins)
- Usually releases energy (exergonic)
- Produces small molecules (e.g. CO₂, H₂O, NH₃) and energy carriers (ATP, NADH, FADH₂)
- Anabolism
- Uses small building blocks and energy to synthesize complex molecules
- Is generally energy-consuming (endergonic)
The two sides are tightly linked: catabolic pathways supply both the energy and many precursors that anabolic pathways require.
Important overall functions of catabolic metabolism:
- Energy provision: generation of ATP and reduced coenzymes (NADH, FADH₂) for cellular work
- Production of basic building blocks: such as acetyl-CoA, pyruvate, and various intermediates that feed into biosynthetic pathways
- Waste removal: conversion of organic carbon to CO₂ and nitrogen-rich compounds to excretable products (e.g. urea, ammonia in animals; nitrate reduction in microbes)
Main Classes of Catabolized Substrates
In everyday metabolism, three main groups of biomolecules are degraded catabolically:
1. Carbohydrate Catabolism
Carbohydrates are often the primary energy source:
- Polysaccharides (e.g. starch, glycogen) are first hydrolyzed to monosaccharides (mainly glucose).
- Glucose is a key starting compound of catabolic metabolism and is central in:
- Cellular respiration (with O₂ as final electron acceptor)
- Fermentation (without O₂, using organic molecules as electron acceptors)
Typical catabolic stages of glucose (described in later chapters):
- Partial oxidation and cleavage into smaller units (e.g. glycolysis to pyruvate)
- Complete oxidation of carbon atoms to CO₂ (citric acid cycle)
- Use of high-energy electrons for ATP production (electron transport chain and oxidative phosphorylation)
2. Lipid (Fat) Catabolism
Lipids, especially triacylglycerols, are very energy-rich molecules:
- First split into glycerol and fatty acids.
- Glycerol can be converted to an intermediate that enters carbohydrate pathways.
- Fatty acids are broken down stepwise by β-oxidation, producing:
- Acetyl-CoA (feeds into the citric acid cycle)
- Reduced coenzymes NADH and FADH₂
Lipids provide more ATP per gram than carbohydrates, so many organisms use them as long-term energy storage and mobilize them catabolically during fasting, migration, or hibernation.
3. Protein Catabolism
Proteins are primarily functional and structural molecules. Catabolic use is often a “last resort” or occurs during controlled remodeling:
- First, proteins are hydrolyzed into amino acids.
- The amino group is removed (deamination), producing:
- Ammonia (NH₃) or its derivatives, which must be detoxified and excreted (e.g. as urea, uric acid, or ammonium compounds, depending on the organism).
- A carbon skeleton that can be converted into:
- Pyruvate
- Citric acid cycle intermediates
- Acetyl-CoA or acetoacetate (ketogenic amino acids)
Protein catabolism thus contributes both to energy production and to the intermediate pool of central metabolism.
Common Features of Catabolic Pathways
Despite the diversity of substrates, several general principles are shared by catabolic pathways.
Stepwise Degradation
Instead of oxidizing nutrients in one big step (which would release too much energy as heat), cells use many small, enzyme-catalyzed steps:
- Each step releases or transfers only a manageable amount of energy.
- Several steps are coupled to the reduction of coenzymes or to ATP synthesis.
- Intermediates can branch off into biosynthetic or other catabolic pathways.
This modular structure of pathways helps cells regulate catabolism and adapt to changing nutrient and energy demands.
Redox Reactions and Electron Carriers
Catabolic processes mainly involve oxidation of organic molecules:
- “Oxidation” here typically means:
- Loss of hydrogen atoms (and thus electrons)
- Gain of oxygen atoms or increase in the number of bonds to oxygen
Cells do not usually transfer electrons directly to oxygen in small steps; instead they use coenzymes as electron carriers:
- NAD⁺ / NADH
- FAD / FADH₂
In catabolic reactions:
- Substrate is oxidized (loses electrons).
- Coenzyme is reduced (gains electrons and often protons).
These reduced coenzymes store high-energy electrons and are later reoxidized, often via an electron transport chain. The free energy released during this reoxidation is used to drive ATP formation.
ATP Production and Coupling
Much of the energy released during catabolic metabolism is first captured in chemical form before ultimately contributing to the phosphorylation of ADP to ATP:
- Some steps are directly coupled to substrate-level phosphorylation (transfer of a phosphate group from an organic intermediate to ADP).
- Other steps supply the reduced coenzymes that fuel oxidative phosphorylation (or analogous processes in anaerobic pathways).
Typical overall pattern:
- Fuel molecules → reduced coenzymes + small carbon units
- Reduced coenzymes → ATP (in a separate process)
This separation allows cells to use many different fuels while relying on a largely common mechanism for ATP synthesis.
Aerobic vs. Anaerobic Catabolism
The presence or absence of oxygen as a terminal electron acceptor leads to two major modes of catabolic metabolism.
Aerobic Catabolism
- Uses O₂ as the final electron acceptor.
- Allows complete oxidation of many organic substrates to CO₂ and H₂O.
- Typically yields much more ATP per fuel molecule than anaerobic pathways.
- Characteristic of many eukaryotic cells (including most animal and plant tissues) and numerous aerobic bacteria.
Core processes (detailed in the cellular respiration chapter) include:
- Oxidation of substrates to CO₂ and reduced coenzymes.
- Reoxidation of NADH and FADH₂ by an electron transport chain.
- ATP synthesis by oxidative phosphorylation.
Anaerobic Catabolism
- Occurs without O₂ or when O₂ is scarce.
- Uses other substances as electron acceptors, for example:
- Organic molecules (e.g. pyruvate) in fermentation
- Inorganic ions (e.g. nitrate, sulfate, CO₂) in anaerobic respiration of some microbes
- Yields less ATP per molecule of fuel than aerobic catabolism.
In many organisms, fermentation serves two key purposes:
- Regenerate NAD⁺ from NADH to allow glycolysis (and thus ATP production) to continue.
- Provide a way to dispose of electrons in the absence of an external electron acceptor such as O₂.
Integration of Catabolic Pathways
Catabolic metabolism is not a set of isolated routes for single nutrients; instead, it is a network of overlapping pathways.
Central Metabolic Intermediates
Different catabolic processes often converge on a few central intermediates, such as:
- Pyruvate
- Acetyl-CoA
- Citric acid cycle intermediates (e.g. α-ketoglutarate, succinate, oxaloacetate)
These junction points:
- Allow interconversion of carbohydrate, lipid, and protein catabolism.
- Provide precursors for biosynthesis (e.g. amino acids, fatty acids).
- Serve as regulatory nodes where the cell can modulate flux according to energy needs and nutrient availability.
Amphibolic Pathways
Some pathways, especially the citric acid cycle, are amphibolic:
- They function both in catabolism (oxidizing intermediates to CO₂ and generating reduced coenzymes)
- And in anabolism (providing precursors for biosynthetic routes)
Cells must balance:
- Use of these intermediates for energy production
- Versus their removal for biosynthesis
This balance is maintained by coordinated regulation between catabolic and anabolic processes.
Regulation of Catabolic Metabolism
Because catabolism supplies both energy and building blocks, it is tightly regulated in response to the cell’s state and the environment.
Important regulatory principles (without going into molecular detail):
- Energy charge: High ATP levels generally signal “energy sufficient” conditions:
- Catabolic pathways are slowed.
- Anabolic pathways are often activated.
- Availability of substrates: Abundant glucose, fatty acids, or amino acids can stimulate their respective degradation pathways.
- Hormonal control in multicellular organisms:
- Hormones (e.g. insulin, glucagon, adrenaline) can shift metabolism between storage and mobilization of nutrients.
- Feedback regulation by intermediates: Accumulation of certain products or intermediates can inhibit earlier steps, preventing overproduction and waste.
In this way, catabolic metabolism is not simply “on” or “off” but finely tuned to cellular and whole-organism needs.
Catabolism at the Level of the Whole Organism
On the organismal level, catabolic metabolism is linked to:
- Nutrient intake and digestion:
- Larger food molecules are first broken down extracellularly (e.g. in the digestive tract) before entering cellular catabolism.
- Physiological state:
- Rest vs. exercise
- Feeding vs. fasting
- Growth, reproduction, hibernation, migration
- Tissue specialization:
- Different tissues may rely preferentially on different fuels (e.g. fatty acids, glucose, ketone bodies) and have distinct catabolic capacities.
Catabolic rate at the organism level is reflected in measures such as metabolic rate, which are treated in detail elsewhere.
Summary
Catabolic metabolism encompasses all enzyme-driven degradation pathways that:
- Break down energy-rich molecules (carbohydrates, lipids, proteins)
- Transfer electrons to coenzymes (NAD⁺, FAD) and ultimately to terminal electron acceptors
- Generate ATP and key intermediates for anabolic processes
- Are regulated so that energy supply matches cellular and organismal demand
The following chapters on cellular respiration and fermentation examine in detail how specific catabolic pathways operate under aerobic and anaerobic conditions.