Premed · Premed · Biochemistry
Lecture 13: Bioenergetics and ATP
Biochemistry
Learning Objectives
By the end of this lecture, students will be able to:
- Explain the laws of thermodynamics as they apply to biochemical reactions
- Define free energy, enthalpy, and entropy and their relationship (Gibbs equation)
- Distinguish between standard free energy change and actual free energy change
- Explain why ATP is the universal energy currency of the cell
- Describe the concept of energy coupling and phosphoryl group transfer potential
- Describe other high-energy compounds and their roles in metabolism
Lecture Content
I. Thermodynamics in Biological Systems
The First Law of Thermodynamics states that energy is conserved -- it can be converted from one form to another but cannot be created or destroyed. The Second Law of Thermodynamics states that the total entropy of the universe always increases in any spontaneous process. Living organisms maintain low internal entropy by increasing the entropy of their surroundings, functioning as open systems that exchange matter and energy with the environment.
Gibbs Free Energy (G) is the thermodynamic function that predicts whether a process will occur spontaneously at constant temperature and pressure. It is defined by the equation delta-G = delta-H - T(delta-S), where delta-H is the enthalpy change (heat absorbed or released), delta-S is the entropy change (disorder), and T is the absolute temperature in Kelvin. When delta-G is negative, the reaction is exergonic (spontaneous and favorable). When delta-G is positive, it is endergonic (non-spontaneous). When delta-G equals zero, the system is at equilibrium. Importantly, delta-G tells us nothing about the rate of a reaction -- only about its thermodynamic favorability.
II. Standard Free Energy Change
The standard free energy change (delta-G^0) is measured with all reactants and products at 1 M, 25 degrees C, and 1 atm. The biochemical standard free energy change (delta-G^0') adjusts for pH 7.0 and treats [H2O] at 55.5 M as constant. The relationship to the equilibrium constant is delta-G^0' = -RT ln K'eq: when K'eq is greater than 1, delta-G^0' is negative and products are favored.
The actual free energy change in the cell is given by delta-G = delta-G^0' + RT ln([products]/[reactants]), and depends on the actual concentrations present. A reaction with a positive delta-G^0' can still proceed if the concentration ratio is sufficiently favorable. In metabolic pathways, product removal by the next enzyme keeps reactions favorable.
III. Energy Coupling
Thermodynamically unfavorable reactions (delta-G greater than 0) can be driven by coupling them to favorable reactions (delta-G less than 0), since free energies are additive. If reaction A has delta-G = +20 kJ/mol and reaction B has delta-G = -35 kJ/mol, the coupled reaction has delta-G = -15 kJ/mol, which is net favorable. Coupling is usually achieved through a common intermediate, and ATP hydrolysis is the most common energy source for biological coupling.
IV. ATP: The Universal Energy Currency
Adenosine triphosphate consists of adenine, ribose, and three phosphate groups (alpha, beta, gamma). ATP hydrolysis releases significant energy: ATP + H2O yielding ADP + Pi has a delta-G^0' of -30.5 kJ/mol (-7.3 kcal/mol), while hydrolysis to AMP + PPi has a delta-G^0' of -45.6 kJ/mol (pyrophosphate is further hydrolyzed by pyrophosphatase, making this essentially irreversible). Under cellular conditions, the actual delta-G is even more negative, approximately -50 to -54 kJ/mol.
Why Is ATP Hydrolysis So Exergonic?
Four factors combine to make ATP hydrolysis highly favorable. Electrostatic repulsion among the three closely spaced negative charges is relieved by hydrolysis. Resonance stabilization is greater in the products (ADP + Pi) than in ATP. Solvation (hydration) of the products is more favorable than for ATP. And the entropy increase from generating two molecules from one adds a favorable thermodynamic contribution.
V. Phosphoryl Group Transfer Potential
Not all high-energy phosphate compounds have the same delta-G^0' of hydrolysis. Compounds with a more negative delta-G^0' have a higher phosphoryl group transfer potential. ATP occupies an intermediate position on this scale, allowing it to accept phosphoryl groups from compounds with higher transfer potential and donate them to compounds with lower transfer potential. This intermediate position makes ATP an ideal energy shuttle.
Compounds with Higher Phosphoryl Group Transfer Potential
Phosphoenolpyruvate (PEP) has the highest transfer potential among common metabolites (delta-G^0' = -61.9 kJ/mol) because its hydrolysis product, enolpyruvate, tautomerizes to the much more stable pyruvate. 1,3-Bisphosphoglycerate is an acyl phosphate (delta-G^0' = -49.4 kJ/mol). Creatine phosphate (delta-G^0' = -43.1 kJ/mol) serves as an energy buffer in muscle and brain, with creatine kinase rapidly regenerating ATP from phosphocreatine and ADP.
Compounds with Lower Phosphoryl Group Transfer Potential
Glucose-6-phosphate (delta-G^0' = -13.8 kJ/mol) and glycerol-3-phosphate (delta-G^0' = -9.2 kJ/mol) lie below ATP on the scale, meaning ATP can phosphorylate these substrates spontaneously.
<image>A vertical scale diagram showing phosphoryl group transfer potentials. The y-axis represents the standard free energy of hydrolysis (delta-G^0' in kJ/mol), with more negative values at the top. Compounds are placed at their respective energy levels: PEP at the top (-61.9), then 1,3-BPG (-49.4), creatine phosphate (-43.1), ATP (-30.5, highlighted in a box as the central energy carrier), glucose-1-phosphate (-20.9), glucose-6-phosphate (-13.8), and glycerol-3-phosphate (-9.2) at the bottom. Arrows show that phosphoryl groups flow from higher potential compounds to ATP (regenerating it from ADP) and from ATP to lower potential compounds (phosphorylating them). The direction of spontaneous phosphoryl transfer is indicated.</image>
VI. Other High-Energy Compounds
Thioesters such as acetyl-CoA have a delta-G^0' of hydrolysis of approximately -31.4 kJ/mol, comparable to ATP. The thioester bond is "high-energy" because sulfur is less electronegative than oxygen, providing less resonance stabilization than oxygen esters, so the products are more stable than the reactant. Acetyl-CoA donates acetyl groups in many biosynthetic and catabolic reactions.
Other high-energy compound classes include phosphoanhydrides (ATP, pyrophosphate), enol phosphates (PEP), acyl phosphates (1,3-bisphosphoglycerate, acetyl phosphate), and guanidinium phosphates (creatine phosphate, arginine phosphate). The hydrolysis of pyrophosphate by pyrophosphatase (delta-G^0' = -19.2 kJ/mol) makes many biosynthetic reactions irreversible.
VII. ATP Turnover and Cellular Energy Charge
ATP is not stored in large quantities but is continuously recycled. A human turns over approximately 40 to 75 kg of ATP per day, with each molecule recycled 500 to 750 times daily. The energy charge, defined as ([ATP] + 0.5[ADP]) / ([ATP] + [ADP] + [AMP]), ranges from 0 (all AMP) to 1 (all ATP) and is normally maintained at approximately 0.85 to 0.90. Catabolic pathways are stimulated when energy charge is low, and anabolic pathways are stimulated when it is high.
Adenylate kinase catalyzes the reaction 2 ADP reversibly forming ATP + AMP, helping to maintain energy charge. AMP is a particularly sensitive indicator of energy status because small decreases in ATP cause large increases in AMP. AMP activates AMP-activated protein kinase (AMPK), a master metabolic regulator that coordinates cellular responses to energy depletion.
<image>A diagram showing the ATP cycle and energy charge regulation. Panel A: The ATP cycle showing ATP being generated by catabolic pathways (glycolysis, oxidative phosphorylation) from ADP + Pi, and being consumed by biosynthesis, muscle contraction, active transport, and signal transduction to regenerate ADP + Pi. Panel B: A graph showing how the rates of catabolic pathways (increasing as energy charge decreases) and anabolic pathways (increasing as energy charge increases) respond to changes in energy charge, with the normal physiological energy charge (~0.85-0.90) marked. Panel C: The adenylate kinase equilibrium (2 ADP ⇌ ATP + AMP) and its role in amplifying the AMP signal when ATP is depleted.</image>
