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