Q3 LONG ANSWER 10M ★ High-yield

Define high-energy compounds. Classify them with examples and explain the central role of ATP.

Biochemistry · 1st Professional — Pre-Clinical

Bioenergetics and Free Energy

Bioenergetics = the study of energy transformations in living systems.

Term	Meaning
ΔG (free energy change)	The energy available to do useful work
Exergonic	ΔG negative — energy released; spontaneous
Endergonic	ΔG positive — energy required; not spontaneous
ΔG = 0	The reaction is at equilibrium
ΔG°′	Standard free energy change at pH 7.0, 25 °C, 1 M

ΔG depends on the actual concentrations, not only on ΔG°′ — which is why reactions with a positive ΔG°′ still proceed in the cell when the product is removed
Endergonic reactions are driven by coupling to an exergonic one, usually ATP hydrolysis, through a shared intermediate

Definition of a High-energy Compound

High-energy compounds = compounds whose hydrolysis releases a large amount of free energy, conventionally ΔG°′ more negative than −7 kcal/mol (−30 kJ/mol).

The bond is written ~ (a squiggle), though it is not a special kind of chemical bond — the energy comes from the greater stability of the products, through resonance and relief of charge repulsion

Classification with Values

Compound	ΔG°′ (kcal/mol)	Type
Phosphoenolpyruvate	−14.8	Enol phosphate — the highest
Carbamoyl phosphate	−12.3	Acyl phosphate
1,3-Bisphosphoglycerate	−11.8	Acyl phosphate
Creatine phosphate	−10.3	Phosphagen (guanidino phosphate)
Acetyl-CoA	−7.5	Thioester
ATP → ADP + Pi	−7.3	Pyrophosphate
Glucose-1-phosphate	−5.0	Low energy
Glucose-6-phosphate	−3.3	Low energy
Glycerol-3-phosphate	−2.2	Low energy — the lowest

CLINICAL PEARL

ATP sits deliberately in the middle of the range. That is the whole point. It can be formed by compounds above it and can donate phosphate to those below it — making it a universal intermediate rather than a dead end. A molecule at either extreme could not serve as currency.

ATP — the Energy Currency

Structure — adenine + ribose + three phosphates; the β and γ phosphates carry high-energy bonds
Turnover is enormous — a resting adult makes and uses about 40–70 kg of ATP a day, while the body contains only about 50 g at any moment
Total cellular concentration is only 2–5 mM; it is a transmitter of energy, not a store
ATP → amp + PPi releases about 10 kcal, since the pyrophosphate is then hydrolysed — used to drive reactions that must not reverse, such as fatty acid activation and aminoacyl-tRNA formation

Uses of ATP

Chemical work — biosynthesis
Mechanical work — muscle contraction, cilia, chromosome movement
Transport work — Na+/K+-ATPase and other pumps
Electrical work — maintaining membrane potential
Heat production
Precursor of cAMP, and a component of NAD, FAD and coenzyme A

Creatine Phosphate — the Immediate Buffer

Rest: ATP + creatine → creatine phosphate + ADP → Exercise: creatine phosphate + ADP → ATP + creatine → Enzyme: creatine kinase, freely reversible

Present at 4–5 times the concentration of ATP in muscle
Supplies energy for the first 5–8 seconds of maximal effort, before glycolysis takes over
Because it is higher on the scale than ATP, it can regenerate ATP directly — a phosphagen

Sources of High-energy Phosphate

Source	Mechanism	Contribution
Oxidative phosphorylation	Proton gradient	About 90%
Glycolysis	Substrate-level (2 steps)	Small but oxygen-independent
TCA cycle	Substrate-level (succinate thiokinase → GTP)	1 per turn
Creatine phosphate	Creatine kinase	Immediate buffer
Adenylate kinase	2 ADP ↔ ATP + amp	Salvages energy; generates the amp that signals depletion

Applied Aspects

Cyanide, carbon monoxide and any block of the respiratory chain stop ATP production, and death follows within minutes — the brain has almost no reserve
Rigor mortis occurs because ATP is required to release the actin–myosin cross-bridge; without it the muscle stays contracted
Creatine supplements raise the muscle phosphocreatine pool and improve performance in short bursts of high-intensity exercise
Creatine kinase measurement — CK-MB in myocardial infarction, CK-MM in muscular dystrophy and rhabdomyolysis
Ischaemia depletes ATP within seconds, and the amp formed is degraded to urate — which is why serum urate rises after a myocardial infarction or seizure
The Na+/K+-ATPase alone consumes 20–30% of resting ATP, and far more in the kidney and brain
Hypothermia during cardiac surgery lowers ATP demand and protects the brain — the practical use of the temperature dependence of metabolism

All questions in Bioenergetics & Biological Oxidation

Every Biochemistry chapter →