Table of Contents
Overview of Anabolic Metabolism
Anabolic metabolism (anabolism) includes all metabolic processes in which simple molecules are built up into more complex substances. These reactions:
- require an input of energy (they are mostly endergonic),
- often use ATP and reduced coenzymes (e.g. NADPH) as energy and electron sources,
- are essential for growth, repair, and storage of substances in cells.
In contrast to catabolic processes, which break down molecules and release energy, anabolic pathways create order and structure in the organism.
Typical anabolic processes include:
- synthesis of carbohydrates (e.g. in photosynthesis, glycogen synthesis),
- synthesis of lipids (fatty acids, triglycerides, membrane lipids),
- synthesis of amino acids and proteins,
- synthesis of nucleotides and nucleic acids,
- formation of specific storage substances (e.g. starch, fats, reserve proteins).
This chapter focuses on the general principles common to anabolic metabolism; details of important special cases (photosynthesis, chemolithoautotrophy, storage of chemical energy) are treated in their own chapters.
General Features of Anabolic Pathways
Energy Requirement and Coupling
Most anabolic reactions are thermodynamically unfavorable on their own: the change in Gibbs free energy is positive:
$$
\Delta G > 0
$$
To proceed in cells, they are coupled to energy‐providing (exergonic) reactions, especially the hydrolysis of ATP:
$$
\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}
$$
By coupling, the overall reaction becomes favorable:
$$
\Delta G_{\text{overall}} = \Delta G_{\text{anabolic}} + \Delta G_{\text{ATP hydrolysis}} < 0
$$
Examples of such couplings:
- activation of monomers (e.g. aminoacyl-tRNA formation in protein synthesis),
- phosphorylation of intermediates in carbohydrate synthesis,
- activation of fatty acids (formation of acyl-CoA).
Reducing Power: NADPH and Other Coenzymes
Anabolic processes often involve reductions (addition of electrons and usually hydrogen atoms) to build more energy‐rich, reduced organic molecules from more oxidized precursors (e.g. CO₂, pyruvate, acetyl-CoA).
For this, cells use reduced coenzymes, especially:
- NADPH (nicotinamide adenine dinucleotide phosphate, reduced form),
- sometimes NADH and reduced ferredoxin, depending on organism and pathway.
NADPH is particularly characteristic of biosynthetic (anabolic) reactions:
- It donates electrons (and a proton) to anabolic enzymes (reductases),
- is itself produced mainly in specific pathways (e.g. pentose phosphate pathway, light reactions of photosynthesis).
In simple form:
$$
\text{NADPH} + \text{X}_{\text{oxidized}} \rightarrow \text{NADP}^+ + \text{X}_{\text{reduced}}
$$
Thus, anabolism not only consumes ATP, but also reducing equivalents.
Use of Precursor Metabolites
Anabolic metabolism does not usually start with completely inorganic substances. Instead, it uses so-called precursor metabolites that arise in central catabolic pathways (glycolysis, citric acid cycle, pentose phosphate pathway).
Typical precursor metabolites include:
- pyruvate,
- acetyl-CoA,
- oxaloacetate,
- α-ketoglutarate,
- 3-phosphoglycerate,
- phosphoenolpyruvate,
- ribose-5-phosphate and erythrose-4-phosphate.
From these, cells build:
- amino acids (e.g. from oxaloacetate, α-ketoglutarate, 3-phosphoglycerate),
- fatty acids (from acetyl-CoA),
- nucleotides (from ribose-5-phosphate and other intermediates),
- complex carbohydrates (from glycolysis intermediates).
Thus, central catabolism delivers building blocks that anabolism extends and combines.
Specificity and Regulation
Anabolic pathways are:
- highly specific: each step catalyzed by a particular enzyme,
- strongly regulated: synthesis only when needed, to save energy and material.
Important regulatory principles in anabolism:
- Feedback inhibition
End products of a biosynthetic pathway inhibit early enzymes of the same pathway.
Example: an amino acid that is present in sufficient quantity inhibits its own synthesis pathway. - Reciprocal regulation of anabolism and catabolism
Opposite pathways (e.g. glycolysis vs. gluconeogenesis, fatty acid synthesis vs. breakdown) are regulated so that both are not fully active simultaneously in the same cell region.
This is achieved by: - different key enzymes,
- regulation via allosteric effectors (ATP, AMP, citrate, etc.),
- hormonal control in animals (e.g. insulin vs. glucagon).
- Compartmentation in eukaryotes
Anabolic and catabolic processes can occur in different cellular compartments: - fatty acid synthesis mainly in cytosol (and chloroplasts in plants),
- fatty acid breakdown (β-oxidation) in mitochondria (and peroxisomes),
- photosynthetic carbon fixation in chloroplast stroma, etc.
- Integration with nutrient supply and growth signals
Rate of biosynthesis adapts to: - availability of precursors (e.g. nitrogen, carbon source),
- energetic situation of the cell,
- growth and division signals (hormones, growth factors).
Types of Anabolic Products
Structural and Functional Macromolecules
Anabolism builds macromolecules that constitute the structure and function of cells:
- Proteins: enzymes, structural proteins, transport proteins, receptors.
- Nucleic acids: DNA as information storage, RNA as information and functional molecules.
- Polysaccharides: cell walls, extracellular matrices, recognition molecules.
- Lipids: membranes, myelin sheaths, certain hormones and signaling molecules.
The synthesis of these macromolecules has characteristic features:
- They are chain polymers assembled from a limited set of monomers.
- Polymerization is template-controlled (DNA, RNA, protein) or pattern-controlled (polysaccharides, some lipids).
- The sequence or arrangement determines the specific function.
Storage Substances
An important part of anabolism is the formation of storage molecules. These allow organisms to store energy and material for later use, e.g. in fasting, darkness, or during periods without nutrients.
Typical storage substances:
- Carbohydrate storage
- plants: starch (amylose, amylopectin), deposited in plastids, seeds, tubers, etc.
- animals and fungi: glycogen, mainly in liver and muscles (animals).
- Lipid storage
- triacylglycerols (triglycerides) in fat droplets and adipose tissue, very energy-dense.
- Protein storage
- reserve proteins in seeds, eggs, and some tissues (as nitrogen and sulfur storage).
The balance between immediate use of energy (via catabolism) and storage (via anabolism) is crucial for survival.
Sources of Energy and Reducing Power for Anabolism
Different organisms use different primary energy sources to drive anabolic biosynthesis.
Phototrophs
In photoautotrophic organisms (e.g. plants, algae, cyanobacteria):
- light is the primary energy source,
- water often supplies electrons (and protons),
- ATP and NADPH are generated in the light reactions of photosynthesis,
- these drive the assimilation of CO₂ into carbohydrates (and subsequently into other organic molecules).
Thus, inorganic carbon is converted into organic building blocks via anabolic processes.
Chemotrophs
Chemotrophic organisms obtain energy from chemical compounds:
- Chemoorganotrophs use organic matter (e.g. glucose) as energy and carbon source,
- Chemolithotrophs gain energy from oxidation of inorganic substances (e.g. H₂, H₂S, NH₃, Fe²⁺) and can fix CO₂.
In both cases:
- catabolic oxidation reactions provide ATP and reducing equivalents,
- which are then used to drive anabolic reactions (e.g. synthesis of cell components).
Anabolism and Cellular Growth
Cell growth requires coordinated anabolism on several levels:
- Increase in biomass
Synthesis of: - proteins (enzymes, structural elements),
- lipids (new membranes),
- nucleic acids (for DNA replication, RNA synthesis),
- polysaccharides (cell walls, matrix).
- Cell division
Before division, all cellular components must be doubled. Anabolism provides the necessary material. - Repair and renewal
- Proteins and lipids damaged by oxidation, heat, or chemicals are continuously replaced.
- This “turnover” is a constant anabolic effort, even without obvious growth.
Rates of anabolic processes are therefore directly linked to:
- growth rate,
- nutrient supply,
- environmental conditions (temperature, stress),
- signaling pathways that report “nutrient and energy status” of the cell.
Anabolic vs. Catabolic Pathways: Cooperation and Separation
Although anabolism and catabolism have opposite directions, they are not simply reverse processes of each other.
Important aspects:
- Many steps are shared (e.g. reversible reactions),
- but key steps are catalyzed by different enzymes and are often irreversible in each direction.
Example of such a principle (without details of specific pathways):
- Pathway A (catabolic):
substrate → intermediate 1 → intermediate 2 → end product + ATP - Pathway B (anabolic):
end product + ATP → other activated intermediate → intermediate 2 → substrate
Thus:
- the direction of flux is controlled by enzyme regulation and energy status,
- organisms can adapt flexibly to conditions (e.g. transition between energy consumption and storage, fasting and feeding).
Ecological and Physiological Significance of Anabolism
Anabolic processes are essential at multiple levels:
- Individual level
- growth of organisms (size increase, development of organs),
- regeneration after injury (wound healing, tissue replacement),
- adaptation (e.g. building of new enzymes in response to new food sources, formation of protective substances).
- Population and ecosystem level
- primary production by autotrophs (conversion of inorganic carbon to organic matter) forms the energetic basis of most ecosystems,
- storage substances determine how long organisms can survive unfavorable phases,
- biosynthesis of specific molecules (toxins, pigments, signaling molecules) shapes interactions such as defense, pollination, symbiosis.
Summary
- Anabolic metabolism comprises all biosynthetic processes in which complex, energy-rich molecules are built from simpler components.
- These processes:
- require ATP and reducing equivalents (especially NADPH),
- use precursor metabolites from central catabolic pathways,
- are highly specific and strongly regulated.
- Anabolism builds:
- structural and functional macromolecules (proteins, nucleic acids, polysaccharides, lipids),
- storage substances (starch, glycogen, fats, reserve proteins).
- The energy and reducing power for anabolism come from:
- light (phototrophs) or
- oxidation of chemical compounds (chemotrophs).
- Anabolic and catabolic pathways are functionally linked but use partly distinct enzymes and are regulated in opposite ways, enabling organisms to grow, adapt, and survive in changing environments.