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Metabolic reactions are regulated in response to the cell’s needs.
Understandings
Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions.
Enzymes lower the activation energy of the chemical reactions that they catalyse.
Enzyme inhibitors can be competitive or non-competitive.
Metabolic pathways can be controlled by end-product inhibition.
Applications
End-product inhibition of the pathway that converts threonine to isoleucine.
Use of databases to identify potential new anti-malarial drugs.
Skills
Calculating and plotting rates of reaction from raw experimental results.
Distinguishing different types of inhibition from graphs at specified substrate concentration.
METABOLIC PATHWAYS
Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions.
Metabolism describes the sum total of all reactions that occur within an organism in order to maintain life.
most chemical changes in a cell result from a series of reactions (pathways).
each step is controlled by a specific enzyme.
metabolic pathways are catalysed by over 5,000 different types of enzyme.
metabolic pathways allow for a greater level of regulation, as the chemical change is controlled by numerous
intermediates.
Metabolic pathways are typically organised into chains or cycles of enzyme-catalysed reactions.
Chains e.g. glycolysis (in cell respiration), coagulation cascade (in blood clotting)
Cycles e.g. Krebs cycle (in cell respiration), Calvin cycle (in photosynthesis)
,Patterns:
1. Most chemical changes happen not in one large jump, but in a sequence of small steps, together forming what
is called a metabolic pathway.
2. Most metabolic pathways involve a chain of reactions.
3. Some metabolic pathways form a cycle rather than a chain. In this type of pathway, the end product of one
reaction is the reactant that starts the rest of the pathway.
ENZYMES AND ACTIVATION ENERGY
Enzymes lower the activation energy of the chemical reactions that they catalyse.
Every chemical reaction requires a certain amount of energy in order to proceed – activation energy (EA).
enzymes speed up the rate of a biochemical reaction by lowering the activation energy.
when an enzyme binds to a substrate it stresses and destabilises the bonds in the substrate.
this reduces the overall energy level of the substrate’s transition state.
less energy is needed to convert it into a product, so the reaction proceeds at a faster rate.
the net amount of energy released by the reaction is unchanged by the involvement of the enzyme.
Types of enzymatic reactions
, Exergonic: if the reactants contain more energy than the products, the free energy is released into the system.
usually catabolic (breaking down), as energy is released from broken bonds within a molecule.
Endergonic: if the reactants contain less energy than the products, free energy is lost to the system.
usually anabolic (building up), as energy is required to synthesise bonds between molecules.
ENZYME INHIBITORS
Enzyme inhibitors can be competitive or non-competitive.
An enzyme inhibitor is a molecule that prevents the formation of an enzyme-substrate complex and hence prevent
the formation of product.
can be either competitive or non-competitive depending on their mechanism of action.
Types of enzyme inhibition
Enzyme inhibition may be either reversible or irreversible depending on the specific effect of the inhibitor.
Normal enzyme reaction
a substrate binds to an enzyme (via the active site) to form an enzyme-substrate complex.
the shape and properties of the substrate and active site are complementary, resulting in enzyme-substrate
specificity (lock and key theory).
when binding occurs, the active site conforms to optimally interact with the substrate (induced fit).
this conformation destabilises chemical bonds within the substrate, lowering the activation energy.
as a result of enzyme interaction, the substrate is converted into product at an accelerated rate.
Competitive inhibition
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