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8.1 Metabolism
Understanding
 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
 Metablic 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
 Distinguishing different types of inhibition from graphs at
specified substrate concentration
 Calculating and plotting rates of reaction from raw
experimental results
Metabolic Pathways
 Most chemical reactions in living things are pathways
 They can be linear (chains)
A
B
C
D
 Where A is the initial reactant, B and C are intermediates and
D is the final product
Linear Pathways
 Example: Glycolysis
 This is a 10 step, linear
metabolic pathway
 Glucose is the initial reactant
and lactate is the final
product (in animals)
 Every step is controlled
by an enzyme
Cycles
 Some pathways don’t have a beginning or an end. These are
called cycles.
 Example: The Krebs Cycle
 This is an 8 step pathway
 It doesn’t really have a start or a
finish
 Many “coupled reactions” occur
in conjunction with the cycle
Activation Energy
 As learned in 2.5, enzymes speed up reactions by decreasing
the activation energy required to start a reaction
Competitive vs Non-Competitive
Inhibition
 Also learned in 2.5, enzymes can be inhibited by certain
substances
 Most drugs work by inhibiting enzymes
 Inhibition can be competitive or non-competitive
Competitive vs Non-Competitive
 While both serve to slow down metabolic reactions, there
are differences
 Competitive inhibitors can be overcome by increasing the
substrate concentration but that is not the case with noncompetitive inhibitors
 With enough substrate
concentration, the rate
of reaction can be
just as high as without
an inhibitor
End Product Inhibition
 Also called Feedback Inhibition
The process of enzyme control where the end product acts as
an inhibitor for one of the enzymes involved in an earlier step
in the pathway.
End Product Inhibition
 Example:
Threonine to Isoleucine
 This is a five step pathway
that creates a new amino
acid.
 Isoleucine inhibits the
enzyme threonine
deaminase, the enzymetht
catalyses the first step in
the pathway.