1. Bioenergetics MDSC1101 – Digestion & Metabolism Dr. J. Foster
Biochemistry Unit, Dept. Preclinical Sciences
Faculty of Medical Sciences, U.W.I.

2. What do you think “bioenergetics” means?
bio = biology
energetics = branch of physics that studies energy flow

3. What then is the significance of “bioenergetics”?
Bioenergetics – study of how such energy flows,
transforms & is harnessed
Processes by which the body meets energy demands
e.g. digestion & other metabolism

4. Bioenergetics
The study of transformation and flow of energy within biological systems, and with their environment
Concerned with the initial and final energy states of reactants and not the mechanism or kinetics
Simply put - biochemical thermodynamics

5. Bioenergetics
Thermodynamics - laws & principles describing the flow and interchanges of heat, energy, & matter in systems
Concepts very applicable to biological systems
System – portion of the universe we are concerned with
Surroundings – everything else!

6. Systems
Three types
Isolated - cannot exchange matter or energy with its surroundings
Closed - may exchange energy, but not matter, with the surroundings
Open - may exchange matter, energy, or both with the surroundings

7. Garrett, Grisham. Biochemistry, 2nd ed © 2000

8. What kind of system would you classify the human body as?
Justify your choice.
Exchanges matter (food in & waste out)
Exchanges heat (homeostasis)
The human body is an open system

9. Which laws do you think impact on thermodynamic events within systems?
Can you recall any laws of physics that may apply?
clue : thermal  heat  energy

10. Laws of Thermodynamics
1st Law - the total energy of a system (including surroundings) remains constant
energy cannot be gained or lost
it can be transferred from part to part
It can be converted from one form to another
What exactly determines how energy flows and whether reactions occur?

11. Laws of Thermodynamics
Gibbs free energy (G)
energy available to reactants/products in rxn
determines the feasibility of chemical reactions i.e. direction & extent (predictive)
Two forms of G used in defining chemical reactions
ΔG, the change in G of rxn
ΔG°, the standard ΔG mol/L)
ΔG° useful only under standard conditions

12. Laws of Thermodynamics
1st Law - the total energy of a system (including surroundings) remains constant
2nd law - total entropy of a system must increase for a process to occur spontaneously

13. Free Energy Given a reaction where A ⇆ B, if ΔG
is negative – rxn is exergonic*, energy is lost from system, spontaneous from A → B
is positive – rxn is endergonic*, energy is required by system from surroundings for rxn to occur
equals zero – rxn is at equilibrium; no direction favoured
also Δ G A→B = - Δ G B→A
Spontaneous reactions equilibrium
*differ from exothermic/endothermic which relate to only heat
Exergonic/endergonic vs endothermic/exothermic – energy of many forms not only heat!!

14. Garrett, Grisham. Biochemistry, 2nd ed © 2000

15. Free Energy ΔG is determined by two factors
Enthalpy (ΔH) – change in heat of reactants and products of a rxn (e.g. chemical bonds)
Entropy (ΔS) – change in randomness/disorder of reactants & products
Neither ΔH or ΔS can predict rxn feasibility alone

16. ΔG= ΔH – TΔS as Δ S increases, ΔG becomes more -ve °K = 273 + °C J/mol
J/mol/K
J/mol
as Δ S increases, ΔG becomes more -ve

17. Free Energy ΔG can also be defined by the concentrations of A & B:
ΔG = ΔG° + RT ln [B]/[A]
At constant P (pressure) & T (absolute ) – thermal equilibrium
R = gas constant (8.315 J/mol/K)
In = natural logarithm
[B] = concentration of product
[A] = concentration of reactant
Note that ΔG and ΔG° can have different signs

18. Free Energy ΔG and ΔG° can differ greatly depending on [A], [B]
Under standard conditions [A]=[B]= 1 mol/L
ΔG = ΔG° + RT ln [B]/[A]
ΔG = ΔG° + RT ln (ln 1 = 0)
ΔG = ΔG°
ΔG° is predictive only under standard conditions
ΔG and ΔG° can differ greatly depending on [A], [B]

19. Free Energy At equilibrium (steady-state) At equilibrium ΔG = 0
[A] / [B] = constant = Keq
Thus, ΔG = ΔG° + RT ln [B]/[A]
becomes ΔG = ΔG° + RT lnKeq
At equilibrium ΔG = 0
0 = ΔG° + RT lnKeq
ΔG° = -RT lnKeq

20. Bioenergetics of pathways
Biochemical pathway - series of rxns each with characteristic ΔG
Thermodynamically for a pathway
ΔGpathway can be considered additive
feasibility depends on sum of individual ΔG’s
As long as the sum of ΔG is –ve the pathway is feasible

21. A rxn can still occur even if ΔG is +ve if it is kinetically favoured… how??
Enzymes – they reduce the activation energy needed for a rxn (kinectics)

22. Bioenergetics of pathways
Many biological systems have rxns that have +ve ΔG
How do biological systems overcome +ve ΔG’s?
Exergonic reactions are usually coupled with endergonic ones
Such coupling of rxns involves using a common intermediate – an energy coupler

23. E.g. First step of Glycolysis
The individual half-reactions in aqueous solution:
ATP + H2O  ADP + Pi DGo' = -31 kJ/mol (exg)
Pi + glucose  glucose-6-P + H2O DGo' = +14 kJ/mol (end)
Hexokinase catalyses rxn (active site excludes H2O & promotes coupled over individual rxns)
ATP + glucose  ADP + glucose-6-P DGo' = -17 kJ/mol
ATP is thus the coupler for the reaction

24. Garrett, Grisham. Biochemistry, 2nd ed © 2000

25. ATP as an energy coupler
The energy currency of cells – universal coupler
Intermediate in the rank of high-energy phosphates
Allows it to accept and donate energy in numerous rxns

26. ATP as an energy coupler
Terminal phosphate bonds are “high energy” (~) i.e. release a large amount of energy on hydrolysis
Phosphate bonds allow for
ATP to release energy for metabolic processes when hydrolysed to ADP + Pi
ADP to store energy from catabolic processes as chemical potential energy in the form of ATP

27. Harper’s Biochemistry 26th ed, Appleton and Lange, USA

28. Anabolic – complex molecules from simple ones (endergonic)
Catabolic – simple molecules from complex ones (exergonic)
ATP bridges the 2 types of metabolism
Lehninger, Biochemistry, 4nd edition © 2005

29. Thermodynamics vs Kinetics
A high activation energy barrier usually causes hydrolysis of a “high energy” bond to be very slow in the absence of an enzyme catalyst.
Such kinetic stability is essential to the role of ATP and other compounds with ~ bonds

30. Why is this kinetic stability for ATP hydrolysis a good thing??
Rapid hydrolysis (due to low barriers) would hinder ATP’s role in metabolism
enzymes lower these barriers, and most importantly couple the rxn with other useful ones
prevents free energy released from ATP hydrolysis being wasted

31. Redox Reactions
Redox (oxidation-reduction) rxns are inherently coupled & involve both
donating of e- (oxidation)
accepting of e- (reduction)
The two halves of a redox rxn are considered separately:
Fe2+ + Cu2+ ↔Fe3+ + Cu+
Can be rewritten in half-reactions as
Fe2+ → Fe3+ = e-
Cu2++ e- → Cu+

32. Redox Reactions
Free energy is also transferred via the movement of these electrons
The transfer of e- can be measured as reduction potentials (E)
This is the tendency of a chemical species to acquire electrons and thereby be reduced

33. Redox Reactions For a rxn, E can be used to calculate G: ΔG = -nFΔE
likewise, ΔGo = -nFΔEo
n = number of electrons transferred,
F = Faraday’s constant (96,480 J/V/mol),
E = reduction potential
Eo = std reduction potential
+ve E favours a –ve G (forward rxn)
–ve E favours a +ve G (backward rxn)

35. Summary
Organisms are open systems that utilise free energy (in the form of chemical energy) to live
Use chemical coupling of spontaneous exergonic rxns to overcome the energy demand of endergonic ones
ATP is the most important coupler and acts as energy currency of cells
Redox reactions also determined by free energy

36. Further Reading
Harper’s Biochemistry, 26th Ed. - Chapter 10