 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 action Temperature Rate of reaction Temperature （ O C ）

Factors affecting enzyme action Temperature 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 inactive active again optimum temperature

Factors affecting enzyme action Temperature Above the optimum temperature, heat changes the _______ of enzymes and their _____________, __________ their activities. When temperature is too high (above 60 o C), most enzymes are ____________, and lose their catalytic property permanently. shape active sitesdecreases denatured

Factors affecting enzyme action pH Rate of reaction 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 action Substrate concentration Max. Rate Rate of reaction 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