Energy-rich compounds (also called high-energy compounds) are biochemical molecules that release a large amount of free energy (ΔG) when their high-energy bonds are hydrolyzed.
These molecules serve as energy carriers, transferring energy from catabolic to anabolic reactions in living cells.
Key Characteristics:
- They contain “high-energy phosphate bonds”, thioester bonds, or other unstable linkages.
- Hydrolysis of these bonds releases > –25 kJ/mol of free energy.
Standard Free Energy of Hydrolysis (ΔG°’)
| Compound | ΔG°′ (kJ/mol) |
|---|---|
| Phosphoenolpyruvate (PEP) | –61.9 |
| 1,3-Bisphosphoglycerate (1,3-BPG) | –49.3 |
| Creatine phosphate | –43.1 |
| Acetyl-CoA (thioester bond) | –31.4 |
| ATP → ADP + Pi | –30.5 |
| Glucose-6-phosphate | –13.8 (low energy) |
Compounds with ΔG°’ < –25 kJ/mol are considered high-energy compounds.
Classification of Energy-Rich Compounds
Energy-rich compounds are classified based on the type of high-energy bond they contain:
1. Phosphorylated Compounds
Contain phosphoanhydride or phosphoester bonds.
| Compound | Bond Type | Energy Released (ΔG°′) |
|---|---|---|
| ATP | Phosphoanhydride | –30.5 kJ/mol |
| GTP, CTP, UTP | Phosphoanhydride | Similar to ATP (~–30.5 kJ/mol) |
| Phosphoenolpyruvate | Enol phosphate | –61.9 kJ/mol |
| 1,3-Bisphosphoglycerate | Acyl phosphate | –49.3 kJ/mol |
| Creatine phosphate | Guanidino phosphate | –43.1 kJ/mol |
2. Thioesters
Contain a sulfur-linked ester bond (R–CO–S–R′)
| Function | Standard Free Energy Change (ΔG°′) |
|---|---|
| Donates acetyl group in TCA cycle and fatty acid synthesis | –31.4 kJ/mol |
Thioester bonds are less stable than oxygen esters, making them energy-rich.
3. Enol Phosphates
Have unstable enol forms attached to phosphate groups.
| Compound | ΔG°′ (kJ/mol) |
|---|---|
| Phosphoenolpyruvate (PEP) | –61.9 |
- The most energy-rich phosphorylated intermediate in glycolysis.
4. Acyl Phosphates
Formed between a carboxylic acid and phosphate group.
| Compound | ΔG°′ (kJ/mol) |
|---|---|
| 1,3-Bisphosphoglycerate (1,3-BPG) | –49.3 |
Important intermediate in glycolysis and substrate-level phosphorylation.
5. Guanidino Phosphates
Phosphates attached to guanidine group.
Example:
| Role | Standard Free Energy Change (ΔG°’) |
|---|---|
| Energy storage in muscle cells | –43.1 kJ/mol |
Used to quickly regenerate ATP during intense muscular activity.
Functions of Energy-Rich Compounds
| Function | Description |
|---|---|
| Energy Transfer | Carries energy from catabolic pathways to anabolic pathways |
| Biosynthesis | Powers endergonic processes like protein and DNA synthesis |
| Active Transport | Fuels ion pumps (e.g., Na⁺/K⁺-ATPase) to move substances across membranes |
| Mechanical Work | Supports muscle contraction, flagella movement, and other mechanical processes |
| Signal Transduction | ATP & GTP participate in kinase activity and G-protein signaling |
| Phosphorylation Reactions | ATP donates phosphate groups in enzymatic regulation and signaling |
Summary Table
| Class | Example | Bond Type | Energy Level |
|---|---|---|---|
| Phosphorylated | ATP, GTP, PEP, 1,3-BPG | Phosphoanhydride, Enol phosphate | High |
| Thioester | Acetyl-CoA | Thioester bond | High |
| Acyl phosphate | 1,3-Bisphosphoglycerate | Acyl phosphate bond | High |
| Guanidino phosphate | Creatine phosphate | Guanidino phosphate bond | High |
| Low-energy phosphate | Glucose-6-phosphate | Phosphoester bond | Low |
Biological Significance of ATP and Cyclic AMP (cAMP)
ATP (Adenosine Triphosphate)
What is ATP?
ATP is the primary energy carrier in all living cells. It stores and transports chemical energy within cells for metabolism and other cellular activities.
Structure:
- Adenine (a nitrogenous base)
- Ribose (a sugar)
- 3 phosphate groups (α, β, γ)
The high-energy bonds between the phosphate groups (especially the terminal γ phosphate) are what make ATP such an important energy molecule.
Biological Significance of ATP
1. Universal Energy Currency
- All living cells use ATP to power nearly all forms of biological work:
- Biosynthesis (e.g., protein, DNA, lipid synthesis)
- Muscle contraction
- Active transport (e.g., Na⁺/K⁺ pump)
- Cell division
- Signal transduction
2. Coupling of Reactions
- ATP is often hydrolyzed (ATP → ADP + Pi) to release energy (ΔG ≈ –30.5 kJ/mol) that drives endergonic reactions.
- Reactions that are not spontaneous become feasible when coupled with ATP hydrolysis.
3. ATP Regeneration
- ATP is continuously regenerated from ADP + Pi using energy from:
- Cellular respiration (mitochondria)
- Photosynthesis (chloroplasts in plants)
4. Signal Transduction (Secondary Role)
- In some pathways, ATP serves as a phosphate donor in phosphorylation cascades (e.g., protein kinases).
cAMP (Cyclic Adenosine Monophosphate)
What is cAMP?
cAMP is a cyclic nucleotide derived from ATP. It acts as a second messenger in signal transduction pathways inside cells.
- Formed from ATP by the enzyme adenylyl cyclase, which is activated by many hormones.
Biological Significance of cAMP
1. Second Messenger in Hormonal Signaling
- cAMP mediates cellular responses to extracellular signals, especially hormones such as:
- Epinephrine
- Glucagon
- ACTH
- It helps amplify the signal inside the cell, triggering rapid responses.
2. Activates Protein Kinase A (PKA)
- cAMP binds to and activates PKA, a key enzyme that phosphorylates target proteins.
- These phosphorylated proteins regulate metabolism, gene expression, and ion channels.
3. Role in Metabolic Regulation
- Example: In liver cells, glucagon and epinephrine trigger cAMP production, leading to glycogen breakdown and glucose release into the blood.
4. Neural and Sensory Functions
- cAMP is involved in:
- Memory formation
- Olfaction (sense of smell)
- Vision (photoreceptor signal transduction)
5. Short-Lived but Potent
- cAMP acts quickly and is rapidly broken down by phosphodiesterase (PDE) to AMP, ensuring signal control and short duration.
Quick Recap
| Feature | ATP | cAMP |
|---|---|---|
| Full Name | Adenosine Triphosphate | Cyclic Adenosine Monophosphate |
| Main Role | Energy currency | Second messenger in signaling |
| Formed From | ADP + Pi (during respiration) | ATP (converted by adenylyl cyclase) |
| Involvement | Metabolism, transport, biosynthesis | Hormone response, signal transduction |
| Stability | Stable; used and regenerated cyclically | Short-lived; quickly degraded by phosphodiesterase (PDE) |
| Key Pathways | Respiration, photosynthesis | cAMP-PKA pathway, hormone signaling |
| Acts On | Multiple enzymes and transporters | Protein Kinase A (PKA) and downstream targets |
Conclusion
- ATP is essential for energy transfer and storage in almost all cellular processes.
- cAMP plays a central role in cell signaling, converting external signals into cellular responses.
- Other high-energy compounds (like PEP, 1,3-BPG, Acetyl-CoA) play specific roles in metabolism and signal amplification.
- These compounds are crucial for maintaining life’s energy demands, supporting growth, reproduction, and homeostasis.