Cellular Energetics
Thermodynamics
– language chemists use to talk about energy
- Gibb’s Free Energy (G)
– most useful value – G = H - TS = enthalpy - entropy * temperature
- cannot calculate absolute free energy – use differences – D G = D H - TD S
- useful to determine what can occur sponatneously – does not indicate what does occur – e.g., dry ATP is very stable
- if D G < 0, reaction favors products (e.g., hydrolysis of ATP)
- if D G = 0, reaction is at equilibrium
- if D G > 0, reaction favors reactants
- most reactions require catalysis by enzymes even if the reaction D G is highly negative
- D Gº’ – refers to D G when reactants and products are both at 1 M and pH = 7.0
- D
G = D Gº’ + RT ln (B/A) – D G is not dependent upon D Gº’ (e.g., lactate « pyruvate in different circumstances)
- equilibrium constant (equal to (B/A)) is thus related to
D Gº’
- at equilibrium,
D G = 0 so D Gº’ = - RT ln (Keq)
- every factor of 10 in Keq
Þ 1.3 kcal/mol difference in D Gº’
Metabolic Energy
Metabolism is essentially combustion (carbohydrate + O2 Þ H2O + CO2) in which the energy is trapped in ATP
ATP drives most cellular processes (synthesis of molecules, electric work (ion pumping), temperature regulation)
- couple ATP hydrolysis to some process –
D Gº’ = -7.3 kcal/mol; D G = -12 kcal/mol under cellular conditions
e.g. glucose Þ G6P D G = 3.3 kcal/mol, but ATP Þ ADP D G = -7.3 kcal/mol, so net D G is –4.0 kcal/mol
other high- energy molecules: phosphoenolpyruvate, acetyl phosphate, creatine phosphate – all have phosphate bonds
Oxidation and Reduction – Oxidation is loss of electrons, Reduction is gaining of electrons
- no free electrons – almost always transferred as electron pairs, usually balanced by movement of two protons
- metabolism (and combustion) involve transfer of electrons from fuel to oxygen (fuels oxidized, oxygen reduced)
- principal oxygen carriers in vivo:
- (1) Nicotinamide Adenine Dinucleotide (NADH/NAD+ or NADPH/NADP+)
- (2) Flavine Adenine Dinucleotide (FAD/FADH2)