6 - Bioenergetics 1

• Thermodynamics tell us which reactions are spontaneous and how much energy is required
• The 1st law of thermodynamics is that the total energy of a system and its surrounding is constant
• Energy is the capacity to carry out a change
• Equations quantifying energy

  delta E = E2 - E1 (energy after - energy before)

  delta E = Q - W (energy lost as heat - work carried out)
• Enthalpy - the heat of the system
• Change in energy is approximately the same as change in enthalpy for a biochemical reaction
• Enthalpy equation:

  delta H = delta E + p delta V

  (Total enthalpy = total energy + pressure/volume work done)
• Exothermic - energy is released by the system
• endothermic - energy is taken up by the system
• a mini universe is another term for an isolated system
• Entropy - also known as disorder, is when the energy in a system spreads out to more molecules so the energy is less concentrated
• The 2nd law of thermodynamics is that entropy is always increasing in an isolated system
• The 1st of of thermodynamics tells you which reactions are permissible
• The 2nd law of thermodynamics tells you which permissible reactions will happen spontaneously
• Gibbs free energy equation:

  delta G = delta H - T delta S

  (change in free energy = change in enthalpy - temperature in Kelvin x change in entropy)
• Change in Enthalpy is

  delta G = delta E - T delta S

  (change in free energy = change in energy - temp in Kelvin - change in entropy)
• Gibbs Free Energy (delta G) is the amount of energy available to do work
• negative delta G means the reaction is spontaneous
• negative delta G means that the entropy of the universe is increasing overall, however in a small system like water freezing entropy can decrease
• If delta G is negative - the reaction is spontaneous and free energy is lost. This is an EXERGONIC reaction.
• If delta G is positive - the reaction is unfavourable (not spontaneous). This is an ENDERGONIC reaction. The reverse reaction is spontaneous.
• If delta G = 0 - no free energy change takes place. This is a dynamic equilibrium.
• In Glucose-6-phosphate <---> Fructose-6-phosphate

  During glycolysis delta G is negative (F-6-P is formed)

  During gluconeogenesis delta G is positive (G-6-P is formed)

  delta G = 0 - G-6-P and F-6-P are interconverted with equal rate in both directions
• The same enzyme catalyses the forwards and backwards reactions in glycolysis ad gluconeogenesis
• delta G does not predict reaction rates. Reactions that need catalysis can be controlled by controlling the activity of the enzyme
• 3 method of controlling enzymes:

  1. Allosteric effects (i.e in glycolysis ATP/AMP and Fructose-2,6-bis P)
  2. Phosphorylation of enzymes
  3. Transcriptional regulation of enzymes (controlling the no. of enzymes around)
• Phosphorylation of glucose:

  Glucose + ATP --> glucose-6-phosphate + ADP

  delta G = -30kJ/mol (large delta G is irreversible)

  But, the reaction can only happen at a significant rate if hexokinase can catalyse it. So to control the reaction you regulate the activity of hexokinase.
• Gibbs free energy:

  1. is the energy available to do work

  2. tells us whether reactions are spontaneous

  3. depends on both the entropy (disorder) and the enthalpy (energy) change of the system