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corey-walton
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Enzymes
• Catalyze (speed up) reactions
• Recognize and bind specific substrates
• Act repeatedly
• Most are proteins
• Reaction does not alter them
Activation Energy
• For a reaction to occur, an energy barrier must be surmounted
• Enzymes make the energy barrier smaller
activation energywithout enzyme
activation energywith enzyme
energyreleased
by thereaction
products
starting substance
Fig. 4-6, p.62
reactants
products
energybarrier
with noenzyme to
promotereaction
energybarrier with
an enzyme’sparticipation
Activation Energy
Factors Influencing Enzyme Activity
Coenzymes and cofactors
Allosteric regulators
Temperature
pH
Salt concentration
Allosteric Control
• Activator or inhibitor binds to an enzyme
• Binding changes enzyme shape
• Change hides or exposes active site
allosteric activator
allosteric binding site vacant
active site altered, substrate can bind
substrate cannot bind
enzyme active site
Allosteric activation
allosteric inhibitor
allosteric binding site vacant; active site can bind substrate
active site altered, can’t bind substrate
Allosteric inhibition
Stepped Art
Fig. 4-8, p.64
enzyme 1
enzyme 2 enzyme 3 enzyme 4 enzyme 5
endproduct
(tryptophan)
substrate
Excess molecules of endproduct bind to moleculesof an enzyme that catalyzesthis pathway’s first step.The greater the excess,the more enzyme moleculesare inhibited, and the lesstryptophan is synthesized.
Feedback Inhibition
• Product of pathway binds to and inhibits enzyme in the pathway
Effect of Temperature
• Small increase in temperature increases molecular collisions, reaction rates
• High temperatures disrupt bonds and destroy the shape of active site
Enzymes and the Environment