The active site have a rigid shape. Only substrates with the matching

shape can fit. The substrate is a key that fits the

lock of the active site.

The active site is flexible, not rigid.

The shape of the enzyme active site and substrate adjust to maximum the fit, which improve catalysis.

There is a greater range of substrate specificity.

Temperature. pH. Enzyme & Substrate

concentration. Time.

Factors affecting enzyme actionTemperature

Rate

of

react

ion

Temperature （OC ）

0 10 20 30 40 50

Factors affecting enzyme actionTemperature

• Each enzyme works best at a particular temperature - _____________________.

• When temperature is lower than optimum temperature, activity of enzyme become ________. At low temperature, enzymes become _____________. They become _________________ when the temperature is raised.

lower inactiv

eactive again

optimum temperature

Factors affecting enzyme actionTemperature

• Above the optimum temperature, heat changes the _______ of enzymes and their _____________, __________ their activities.

• When temperature is too high (above 60oC), most enzymes are ____________, and lose their catalytic property permanently.

shapeactive sites decreas

es

denatured

Factors affecting enzyme actionpH

Rate

of

react

ion

pH

Enzyme concentration: * Low enzyme concentration great

competition for the active sites low rate. * Enzyme concentration increases, more

active sites, faster rate. * Increasing the enzyme concentration beyond a

certain point has no effect.

Substrate concentration: * Low substrate concentration many active

sites not occupied. rate is low. * More substrate added more enzyme-

substrate complexes formed. rate of reaction increases.

* Increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more enzyme-substrate complexes can be formed.

Factors affecting enzyme actionSubstrate concentration

Max. RateR

ate

of

react

ion

Substrate conc.

Enzyme Inhibition

Inhibitors • cause a loss of catalytic activity• Change the protein structure of an enzyme• May be competitive or noncompetitive• Some effects are irreversible

COMPETITIVE INHIBITION

NON-COMPETITIVE INHIBITION

Regulation of the metabolism, feed-back inhibition by the final product.

Simple feed-back inhibition. The final product (E) inhibits the step from A to B.

)usually rate-limiting(

Terms used in enzymology

Substrate: The molecule acted upon by the enzyme to

form product

Product: The substance that is produced by the action of the enzyme

Apoenzyme: The protein portion of the enzyme which is catalytically inactive

Cofactor: usually a metal ion or small organic molecule that is needed to activate the apoenzyme

Prosthetic group: coenzyme or cofactor covalently linked or bound non-covalently very tightly to an enzyme

Coenzyme: organic or organo-metallic molecule that assists an enzyme

Holoenzyme: The apoenzyme + cofactor or prosthetic group

Allosteric site: a region of enzyme molecules not at the active site where small molecules bind and effect a change in the activity of the active site by change in the conformation of the enzyme. This cause the active site to become either more active or less active by increasing or decreasing the affinity of enzyme for substrate

Proenzyme (Zymogen): inactive precursor form of some enzymes (e.g. many digestive enzymes) that will be activated by cleavage of a specific peptide in its structure