Bioenergetics of the cell

Bioenergetics of the cell

1. Definition of bioenergetics

   • Part of biochemistry concerned with the energy involved in making and breaking of chemical bonds in the molecules found in biological organisms
   • It can also be defined as the study of energy relationships and energy transformations in living organisms

2. Gibbs' free energy

   • For any process to be possible, the change in Gibbs' free energy must be negative
   • A negative Gibbs' free energy indicates a 'spontaneous' process
   • Spontaneity does not imply that the reaction goes ahead, it simply considers the possibility, or feasibility, of a particular reaction or process
   • 'Free' indicates the energy available in the form of useful work
   • Sometimes a catalyst is required
   • OILRIG
     • Oxidation is losing [electrons]
     • Reduction is gaining [electrons]
   • Change in Gibbs' free energy - energy is liberated and available for use
   • Change in Gibbs' free energy depends on difference in energy between products and substrates
   • Change in Gibbs' free energy for ATP hydrolysis
     • ATP + H20 >> ADP + Pi
       • Gibbs' change in free energy is -7.3kcal/mole
     • Equimolar solution of product and substrate favours the forward reaction (hydrolysis of ATP)
       • Change in Gibbs' free energy is negative
   • Change in Gibbs' free energy outside standard conditions depends on product and substrate concentrations
     • Reactions positive change in Gibbs' free energy can occur in the forward direction if there is enough substrate or if product is reduced significantly
     • Product can be very low if it is rapidly used in another reaction
   • Change in Gibbs' free energy values of sequential reactions are additive

3. Structure of ATP

   • Adenine
   • Ribose
   • Phosphate
   • Has 2 phosphoanhydride bonds
     • When the 1st is hydrolysed it produces ADP
     • When the 2nd is hydrolysed it produces AMP

4. Phosphoanhydride bonds are relatively weak

   • Overall, more energy is released in forming the products than used to break bonds in the reactants

5. Uses of ATP

   • Mechanical work
     • Muscle contraction
       • Direct hydrolysis of ATP is the source of energy in the conformational changes in myosin that produce muscle contraction
       • The binding of ATP dissociates myosin from actin
       • ATP is hydrolysed, inducing a conformational change that displaces the myosin head group
       • The myosin head binds to the new position on the actin filament with the release of ADP and Pi
       • The return of myosin head to its original conformation drives actin filament sliding
     • Quick synthesis of ATP
       • Phosphocreatine serves as a ready source of phosphoryl groups for the quick synthesis of ATP from ADP
       • The phosphocreatine concentration in skeletal muscle
       • Creatine kinase catalysis this reversible reaction
   • Keeping us alive
     • Transport e.g. sodium / potassium transporter
       • Membrane active transport
       • Accounts for 10-30% of BMR in humans
       • The pump binds ATP and binds 3 intracellular NA+ ions
         • ATP is hydrolysed
           • Conformational change in the pump which exposes the Na+ ions to the outside
             • The phosphorylated form of the pump has a low affinity for Na+ ions, so they are released to the cell exterior
               • The pump binds 2 extracellular K+ ions
                 • Dephosphorylation of the pump occurs, and K+ is transported into the cell
                   • The unphosphorylated form of the pump has a higher affinity for Na+ ions than K+ ions, so the 2 bound K+ ions are released
                     • ATP binds, and the process starts again
       • Functions
         • Establish an intracellular ion environment high in K+ and low in Na+
         • Control of cell volume
         • Providing electrochemical sodium gradient for driving secondary active transport systems e.g. glucose
         • Establish a resting potential
     • Anabolic or synthetic reactions
       • Glycogen synthesis
       • Fatty acid synthesis
       • DNA synthesis
       • Protein synthesis
       • Cell division

6. ATP regeneration

   • There is a set amount of ATP in the body at any one time
     • ~100mg worth
   • If we were unable to regenerate ATP, in the heart it would last for 10 heart beats
   • ATP generation
     • Glycolysis
     • Fatty acid oxidation
     • Ketone acid oxidation
     • Citric acid cycle (TCA cycle)
     • Pentose phosphate pathway
     • Individual pathways for individual pathways
   • Substrate level phosphorylation
   • Anaerobic glycolysis
   • Requires a source of energy
   • A controlled series of chemical reactions
   • Intermediates NADH and FAD2H produced
     • Glycolysis
     • Krebs cycle
   • Creates some ATP directly or via electrons

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