Bioenergetics
Quantitative study of energy transformations in living systems
Thermodynamics
More movement / transformation of energy- heat, potential, kinetic, electrical, radiant energy
The conversion of energy into work
- chemical work
- osmotic work
- physical work
Reactions that yield energy (exergonic reactions) go from organised to disorganised, they involve and reduction in organisation. Conversely, reactions that require energy lead to a greater degree of organisation. Entropy is the driving force of all reactions.
The laws of thermodynamics
- for any physical or genial change, the total
Amount of energy in the universe remains constant. Energy may change form and may be moved, but it cannot be created or destroyed - in all natural processes, the entropy of the universe increases
Metabolism and the energy cycle
The sum of all chemical changes that occur in living cells.
- anabolism- build up
- catabolism - break down
Fuel molecules, a source of potential chemical energy, are degraded via enzymatic reactions
The energy released is utilised in the formation of a few different energy rich compounds
These are then used to drive the synthesis of other macromolecules required by the host
Gibbs free energy and delta G of a reaction
Amount of energy available to perform work during a reaction
The portion of the total energy that is useful, potentially available to drive chemical biosynthesis and other processes in the cell that require energy. But free energy cannot be measured experimentally.
But change in free energy can be measured and is the difference between the free energy of the products and reactants
Relationship between delta G and other thermodynamic properties of a system
- reflects number and types of chemical bonds in reactants/ products
- can be determined in a calorimeter
- reactions that release heat- exothermic, neg delta G (energetically favourable)
- reactions that take up heat: endothermic, pos delta G
- t is temp, measured in K where 0K is no kinetic energy, equivalent to -273
- delta S is the change in entropy in system
Delta G and equilibrium position
If Gp is lower than Gs, the reaction involves a decrease in free energy, so delta g is neg, it’s exergonic, if starting with equal concentrations, reaction will proceed spontaneously from S to P
If Gp is there same as Gs: involves no change in energy, delta G is zero, neither exergonic nor endergonic, for starting with equal concentrations, will not proceed in either direction as equilibrium is already reached
If Gp is higher than Gs, the reaction: involves an increase in free energy, so delta G is positive, endergonic, is starting with equal concentrations will proceed spontaneously in reverse direction until equilibrium established.
Standard free energy change delta G
Free energy change under standard conditions, when reactants and products are present in their standard stages
- all solutes present at “unit concentrations”
- if hydrogen ion is produced or used in the reaction, it’s concentration is also taken to be 1M (so pH=0)
- any gases produced are at 1 atmosphere pressure (101.3kPa)
However under physiological conditions:
- no solute exists at concentration of 1M
And no reaction occurs at pH 0 - biological
Systems are buffered, so most reactions occur at pH 7, if a proton is not directly consumed or produced in the reaction, then delta g will
Be dependent of pH.
Energy rich compounds
Show a large decrease in free energy when they hydrolyse because their products are significantly more stable than the reactants. Due to:
- bond strain
- stabilisation of products by ionisation
- stabilisation of products by isomerisation
- stabilisation of the products by electron resonance
Features of ATP-ADP system: Charges on ATP and ADP at physiological pH7:
- dissociable protons on interior phosphate groups have a pKas of 2.15 and 4.2; fully deprotonated
- terminal phosphate groups have both primary hydrogen (fully deprotonated) and secondary hydrogen (75% deprotonated)
- so at pH7 ATP and ADP have net charge of -4 and -3 respectively
Due to negative charge, ATP/ADP/AMP/Pi cannot readily traverse membranes; trans locates needed for movement within cells (loss of 25% of respiration energy yield) each cell must make its own ATP
Special features of the ATP-ADP system: Hydrolysis of terminal phosphate group of ATP Has a large and negative free energy change
- Electrostatic repulsion between formally ionised O- atoms (essentially fully deprotonated ionised at pH7)
- Electro negativity of O>P, confers slightly neg charge on carbonyl (=O) atoms which causes additional repulsion
- Induced slightly pos charge on P atoms causes additional repulsion
Special features of the ATP-ADP system: Resonance stabilisation of he inorganic phosphate product
Pi (HPO42-) stabilised by existence of several forms caused by electron resonance (entropic stabilisation)
Additional entropic stabilisation caused by ionisation
Special features of the ATP-ADO system: Ionisation stabilisation of the ADP product
Relatively low [H+] of only 10-7M in cells at pH 7 causes other product of hydrolysis (ADP2-) to immediately ionise to real ease a protein to surroundings
Another example of entropic stabilisation
Special features of the ATP-ADO system: Mass action effects on the equilibrium
This contributing factor to high energy yield of ATP hydrolysis involved the behaviour of equilibrium
- cellular concentrations of the products of ATP hydrolysis are very much lower than the equilibrium concentrations, so mass action effects also favour the hydrolysis reaction under cellular conditions
- reaction is pulled from left to right in favour of hydrolysis
Pyrophosphate cleavage
Hydrolysis of interior pyrophosphate bond of ATP to yield AMP and inorganic pyrophosphate (PPi)
- releases even more energy than orthophosphate cleavage
- used to drive some important reactions that have particularly high energy requirements
- essential that AMP formed by pyrophosphate cleavage is converted back to ADP or ATP, and that pyrophosphate (PPi) is converted back to Pi
- achieved by two enzymes: Adenylate kinase and pyrophosphatase
Coupling of chemical reactions
In cells, energy made available in exergonic reactions is commonly used to do work by driving related endergonic reactions. Achieved by coupling reactions that have common intermediates.
- standard free energy changes are additive
- reverse reactions have delta G of equal magnitude but opposite sign
- common intermediate in a reaction scheme involving multiple reactions can be cancelled out of the equation for the overall coupled reaction
- the delta g of the reaction is independent of the pathway by which the reaction occurs- it depends only on the nature and concentration of the initial reactants and the final products
Therefore an energetically unfavourable reaction can be driven by coupling it to an exergonic reaction through a common intermediate
Free energy transfer from ATP
I. Part of ATP molecule is transferred to substrate molecule to form covalently linked, short lived high energy intermediate structure
Then
The phosphate containing structure is displaced, releasing Pi or PPi as the product is formed
