Lecture 3 – Enzymes
Ecosystems
• Sunlight drives photosynthesis; sees the formation of plant
material from CO2
• Herbivores act as an energy source for carnivores
• Food eaten by animals is “burnt off” during respiration,
providing energy for physical activity and growth
• Photosynthesis (in chloroplasts) à Respiration (in mitochondria)
à Photosynthesis
Metabolism
• Anabolism: The synthesis of complex molecules in cells (storage
of energy)
• Catabolism: The breakdown of food by heterotrophic
metabolism in a number of stages (release of energy)
• Autotrophic: Complex materials are built up from simple,
inorganic materials using energy from sunlight (plants)
• Heterotrophic: Metabolism in which the energy inherent in
organic matter is released as organic compounds (respiration)
(animals)
• ATP (Adenosine Triphosphate): Major intracellular currency for
the exchange of energy
• ATP + H2O = Energy for cellular work
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Redox Reactions
• Reduction/Oxidation processes (transfer of energy)
• Oxidation: Addition of oxygen, removes hydrogen, loss of
electrons, liberates energy
• Reduction: Removes oxygen, addition of hydrogen, gaining of
electrons, stores energy
Free Energy
• Each type of molecules has a certain level of internal energy
• Chemical reactions are associated with a change in free energy
• When the products of a reaction have a higher energy content
(free energy), then energy has to be put into the system
• When the reactants of a reaction have a lower energy content
(free energy), they energy can be obtained from the system
• Exothermic/Exergonic: Energy output (catabolic)
• Endothermic/Endogonic: Energy input (anabolic)
• Gibbs Free Energy: The energy change during a reaction (ΔG)
• ΔG < 0: Exothermic reaction
• ΔG > 0: Endothermic reaction
• EA: Activation energy (energy required to activate a reaction)
Activation Energy
• Catalysts speed up the rate of a reaction
• The catalysts used by living cells are the enzymes
• Reduces the activation energy required
Enzymes
• Binding the substrate (reactants) to the surface of the enzyme at
the enzyme’s active site
• Lock and key model/induced fit model
• Interactions between enzymes and substrates induces stress on
particular bonds
• Metal ions or other small organic molecules are called co-factors
• More substrate; faster reaction
• When substrate is in excess; the enzyme is saturated
• Effected by temperature and pH
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Enzyme Inhibition
• Competitive Inhibition: An inhibitor that mimics the substrate
and competes for the active site
• Non-Competitive Inhibition: An inhibitor binds to the enzyme
and changes the shape of the active site
Feedback
• Allosteric Site: Another site of the enzyme, not the active site
• Inhibitors feedback from end product of the reaction or
metabolic pathways
ATP/NAD
• ATP + H2O ßà ADP + Pi ; ΔG = -30.5 kJmol-1
• NAD+ à NADH and NADP+ à NADPH
• The reduced from of a number of biological molecules can be
used as sources of energy to drive endothermic reactions
• Very important in respiration
• Is usually involved in oxidation reactions
Lecture 4 – Respiration
Stages in Catabolism
• Glucose à Glycolysis à Krebs Cycle à Respiratory e- Transport
Chain à 6CO2 + Energy
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