Chapter 28: Nucleosides, Nucleotides and Nucleic Acids
Bioenergetics
Bioenergetics is the study of thermodynamics as applied to biological
systems.
For biological systems the focus is usually on free energy changes, DG,
rather than just changes in enthalpy DH (an
approximation that works well for organic chemists).
Recall that reactions are spontaneous if DG
is favourable (i.e. -ve) : this is an exergonic process.
Reactions are unfavourable reactions if DG is
+ve : this is an endergonic process.
For biological purposes, the standard state is modified, DGo',
to be for 1 M concentrations and at neutral pH = 7.
The Role of ATP
The phosphate ester linkages can be viewed the energy source for biological
systems.... like a coiled spring.
The key chemical reaction for bioenergetics is the interconversion of ATP
and ADP:
In general terms it is the transfer of the phosphate unit from ATP
to another nucleophilic site that acts as the "source" of energy, for example
by creating a reactive intermediate, such as the mixed anhydride of phosphoric
acid and a carboxylic acid.
Such reactions are phosphorylations and are catalysed by enzymes
called kinases.
The reverse reaction, the formation of ADP to ATP is driven
by the metabolism of fats and carbohydrates.
For example, 1/3 of the energy released from glucose glycolysis is used
to convert ADP to ATP.
