ENZYMES

Outline Review – What is an enzyme? Models of enzyme activity ◦ Lock and key ◦ Induced fit model Factors affecting enzyme activity ◦ Temperature ◦ pH Regulation of enzyme activity ◦ Inhibition ◦ Allosterically ◦ Feedback inhibition

What is an enzyme? “Biological catalysts”: Speed up reactions in the body

Type of protein ◦ Polymer of amino acids Often globular (spherical) in shape Contain “active sites” – depressions or grooves on the surface where molecules can bind

Binding to an enzyme is very selective: ◦ Active sites interact with specific molecules to catalyze specific reactions The molecule that binds to an enzyme is called its “substrate”

Enzymes lower the activation energy required for a reaction to occur. ◦ Stabilizes the transition state by stretching the bonds of the substrate

Some enzymes need to bind to additional molecules to function: ◦ Cofactors (usu. metal ions), or ◦ Coenzymes (act like shuttles btw. enzymes) Apoenzyme – The enzyme, without its cofactors/coenzymes Holoenzyme – The active form, bound to cofactors/coenzymes

MODELS OF ENZYME ACTIVITY Lock and Key model Induced fit model

The “Lock and Key” model The enzyme has an active site that is unchanging Substrates bind  chemical process occurs

Induced fit model The enzyme can change its shape 1. One substrate molecule binds weakly 2. The enzyme’s active site changes shape so that a second substrate molecule can bind 3. The second substrate binds  Chemical process (rx) occurs

FACTORS AFFECTING ENZYME ACTIVITY 1. Temperature 2. pH

Temperature

Increase in activity with temperature (molecules have more energy)... but only to a certain point. Too much heat will disrupt the 2°, 3° and 4° structure ◦ Denaturation & loss of function

Every enzyme has a temperature at which it functions best – this is the optimal temperature For human enzymes it is usually 37°C (body temp.)

pH Enzymes also have optimal pHs These will vary according to the location in which they function LocationOptimal pH PepsinStomach2 TrypsinSmall intestine8

REGULATION OF ENZYME ACTIVITY 1. Inhibition 2. Allosterically 3. Feedback inhibition

Competitive inhibition Are very similar in shape to an enzyme’s substrate. Bind to the active site, and physically block the correct substrate from binding.

Competitive Inhibition

Enzyme Efficiency / Rate of Rx

Noncompetitive inhibition May be allosteric OR The inhibitor directly affects the enzyme so that it cannot carry out its function. (chemicals that alter the enzyme’s shape and therefore function)

Noncompetitive inhibition

Comparing the 2 inhibitions

Allosteric Regulation Allosteric site – Site on an enzyme that can bind with an effector. NOT the same as active site. Effector: ◦ Activator, or ◦ Inhibitor

Inhibition vs Activation Inhibitors vs Activators Some substances can inhibit enzyme function – Inhibitors Some substances can enhance enzyme function – activators Inhibition can be either competitive or noncompetitive

Allosteric activator Stabilize the active-form Allosteric inhibitor Stabilize the inactive form See pg. 73, Figure 7

Feedback inhibition Feedback inhibition: The product of a series of enzyme- catalyzed reactions acts to allosterically inhibit an enzyme that acts earlier in the series. Often biological reactions occur in a series.

WHY IS FEEDBACK INHIBITION important? 5 minutes discussions: WHY DO CELLS NEED FEEDBACK? WHY DO YOU NEED FEEDBACK? “TWO-WAY” WORLD ANYONE?

SOME APPLICATIONS Cheese-making Cow enzyme rennet

Insulin for Type 1 diabetics

Lactase to digest lactose

Homework Pg. 77 #1-8