Energy Metabolism and Thermodynamics

Introduction to Thermodynamics

• Living things need energy to perform metabolic activities, survive, grow, and reproduce.
• Biologic systems are essentially isothermic and use chemical energy to power living processes.
• Biologic systems conform to the two general laws of thermodynamics:
  • The total energy of a system, including its surroundings, remains constant.
  • The total entropy of a system must increase if a process is to occur spontaneously.

Enthalpy (H) and Energy

• Enthalpy (H) is the heat content of the reacting system.
• Exothermic reactions release heat, have a negative ∆H, and occur spontaneously.
• Endothermic reactions take up heat from their surroundings, have a positive ∆H, and require energy input.

Free Energy (ΔG)

• Gibbs change in free energy (ΔG) is the available energy for doing work.
• If ΔG is negative, the reaction proceeds spontaneously with loss of free energy (exergonic).
• If ΔG is positive, the reaction proceeds only if free energy can be gained (endergonic).
• If ΔG is zero, the system is at equilibrium and no net change takes place.

Endergonic Processes

• Endergonic processes cannot exist independently and must be coupled with exergonic processes.
• Exergonic reactions (catabolism) break down fuel molecules, while endergonic reactions (anabolism) build up substances.
• The combined catabolic and anabolic processes constitute metabolism.

ATP (Adenosine Triphosphate)

• ATP is a nucleotide consisting of adenosine, ribose, and three phosphate groups.
• Hydrolysis of ATP to ADP and Pi or ATP to AMP and PPi is an exergonic reaction.
• ATP is a high-energy compound with high-energy phosphate bonds.

Other High-Energy Compounds

• Thiol esters involving coenzyme A (e.g., acetyl-CoA)
• Acyl carrier protein
• Amino acid esters involved in protein synthesis
• S-adenosylmethionine (active methionine)
• Uridine diphosphate glucose (UDPGlc)
• 5-phosphoribosyl-1-pyrophosphate (PRPP)

ATP/ADP Cycle and Phosphagens

• Phosphagens act as storage forms of high-energy phosphate and include creatine phosphate and arginine phosphate.
• Creatine phosphate is the major energy source in short-duration, high-intensity activities (e.g., 100m runners).

Adenylyl Kinase (Myokinase)

• Adenylyl kinase is an enzyme present in most cells.
• It catalyzes the transfer of group transfer potential between ADP, ATP, and AMP.
• Adenylyl kinase is important for maintaining energy homeostasis in cells.