ATP and Coenzymes

Metabolic reactions in cells require a constant supply of energy. This energy is provided primarily by adenosine triphosphate (ATP), often described as the universal energy currency of the cell. In addition, several coenzymes play essential roles in transferring hydrogen atoms, electrons, and acetyl groups during metabolic pathways such as respiration and photosynthesis.

---

ATP — Structure and Properties

Key Definition: ATP (adenosine triphosphate) is a nucleotide derivative consisting of the nitrogenous base adenine, the pentose sugar ribose, and a chain of three phosphate groups linked by high-energy phosphoanhydride bonds.

Structural Components

• Adenine — a purine nitrogenous base.
• Ribose — a five-carbon pentose sugar.
• Adenine + ribose = adenosine (a nucleoside).
• Adenosine + one phosphate group = AMP (adenosine monophosphate).
• Adenosine + two phosphate groups = ADP (adenosine diphosphate).
• Adenosine + three phosphate groups = ATP (adenosine triphosphate).

The bonds between the phosphate groups (phosphoanhydride bonds) store energy. When the terminal phosphate is removed by hydrolysis, energy is released.

---

Hydrolysis and Synthesis of ATP

Hydrolysis (ATP → ADP + Pi)

The enzyme ATPase (also called ATP hydrolase) catalyses the hydrolysis of ATP:

ATP + H₂O → ADP + Pi + energy (approximately 30.5 kJ mol⁻¹)

The released energy drives endergonic (energy-requiring) cellular processes such as:

• Active transport — e.g., the sodium–potassium pump (Na⁺/K⁺ ATPase) uses ATP to transport 3 Na⁺ out and 2 K⁺ into the cell against their concentration gradients.
• Muscle contraction — myosin ATPase hydrolyses ATP to power the cross-bridge cycle.
• Biosynthesis — ATP provides energy for anabolic reactions (e.g., joining amino acids to form polypeptides on ribosomes).
• Nerve impulse transmission — resetting ion gradients after an action potential.
• Secretion — exocytosis requires ATP for vesicle transport and membrane fusion.

Synthesis (ADP + Pi → ATP)

ATP is regenerated from ADP and inorganic phosphate (Pi) by the enzyme ATP synthase through phosphorylation:

ADP + Pi + energy → ATP + H₂O

Three types of phosphorylation produce ATP:

Type	Location	Process
Substrate-level phosphorylation	Cytoplasm (glycolysis) and mitochondrial matrix (Krebs cycle)	A phosphate group is transferred directly from a phosphorylated substrate to ADP
Oxidative phosphorylation	Inner mitochondrial membrane	Energy from the electron transport chain creates a proton gradient; protons flow through ATP synthase, which catalyses ATP synthesis (chemiosmosis)
Photophosphorylation	Thylakoid membrane of chloroplasts	Light energy drives the electron transport chain, creating a proton gradient across the thylakoid membrane; protons flow through ATP synthase

---

Why ATP Is the Universal Energy Currency

ATP is ideally suited as an energy currency for several reasons:

• Releases energy in small, manageable amounts — the hydrolysis of one ATP molecule releases about 30.5 kJ mol⁻¹, which is a suitable quantity for most cellular reactions. Releasing all the energy from a glucose molecule at once would generate too much heat and waste energy.
• Rapidly regenerated — a resting human uses and regenerates approximately 40 kg of ATP per day, but the total mass of ATP in the body at any time is only about 50 g. The ATP/ADP cycle is extremely fast.
• Water-soluble — can be easily transported within the cell.
• Universal — used by all living organisms, from bacteria to mammals.
• Releases energy in a single, enzyme-catalysed reaction — the hydrolysis of one phosphoanhydride bond is sufficient; no complex multi-step process is needed to access the energy.
• Cannot pass through cell membranes — ATP is made and used within the same cell, ensuring energy is available precisely where it is needed.