CONTROL OF METABOLIC
PATHWAYS (2)
HIGHER HUMAN BIOLOGY
UNIT 1 - HUMAN CELLS
CONTROL BY REGULATION OF ENZYME
ACTION
• Some metabolic pathways are required to operate
continuously
• Their genes that code for their enzymes need to be
switched on continuously
• These metabolic pathways are controlled by
regulating the action of their enzymes
SIGNAL MOLECULES
• Some enzyme activity is controlled by signal
molecules
• E.g. Epinephrine (Adrenaline)
• Triggers liver cells to activate an enzyme
that converts glycogen to glucose.
• Intracellular - molecules that originate in the
cell itself
• Extracellular - molecules that originate from
the cells environment
EFFECT OF INHIBITORS
• Inhibitors decrease the rate of an enzymecontrolled reaction
• There are 2 types
• Competitive inhibitors
• Non-competitive inhibitors
COMPETITIVE INHIBITOR
• These compete with molecules of substrate for the
active sites on an enzyme
• Molecular structure similar to the substrate
• Rate of reaction is reduced
COMPETITIVE INHIBITION
EFFECT OF INCREASING SUBSTRATE
CONCENTRATION
NON-COMPETITIVE INHIBITORS
• Does not combine directly with an enzyme’s active
site
• Attaches to a non active (allosteric) site
• Changes the shape of the enzyme
• Active site becomes altered indirectly
• Prevents it combining with the substrate
ACTIVATORS AND INHIBITORS
• Some enzyme molecules are composed of several
subunits.
• Several active sites
• Several allosteric sites
• These may be active or inactive
FEEDBACK INHIBITION BY END
PRODUCT
• As the concentration of an end product builds up, some
of it binds to an enzyme in the pathway.
• This slows down the conversion of intermediate
metabolites
• Which in turn lowers the concentration of the product.
• When the concentration of the product drops, fewer
molecules of the enzyme are affected
• More end product is produced and so on……..
• Negative feedback control
• Prevents wasteful conversion and accumulation of
intermediates and final products
Metabolite W
Enzyme 1
Metabolite X
Enzyme 2
Metabolite Y
Enzyme 3
Metabolite Z
Some of end product
(Z) binds to enzyme 1
and prevents
conversion of W to X