Review What Is an Enzyme?

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

“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. Recall: All chemical reactions must overcome an initial amount of Activation energy

The presence of enzymes does not change the amount of chemical energy stored in the reactants, or in the products. MODELS OF ENZYME ACTIVITY 1. Lock and Key model 2. 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  One substrate molecule binds weakly  The enzyme’s active site changes shape so that a second (specific) substrate molecule can bind  The second substrate bind  Chemical process occurs FACTORS AFFECTING ENZYME ACTIVITY Enzymes are very diverse Variety reflects specificity – they have evolved to function best in the environments/conditions in which they work

Temperature  Will see an increase in activity with increase in temperature (molecules have more energy)... but only to a certain point.  Too much heat will disrupt the 3° and 4° protein structure  Denaturation & loss of function

 Every enzyme has a temperature at which it functions best – this is the optimal temperature  Optimal temperature of human enzymes is usually 37°C (body temperature) pH  Enzymes also have optimal pHs  These will vary according to the location in which they function o E.g. pepsin, found in stomach – optimal temperature is pH 2 o trypsin, found in small intestine – optimal temperature pH8 REGULATION OF ENZYME ACTIVITY

Inhibition

Some substances can inhibit enzyme function Can be either competitive or noncompetitive

Competitive inhibitors are VERY similar in shape to an enzyme’s substrate. These can bind to the active site, and physically block the correct substrate from binding.

Noncompetitive inhibitors do not attach to the active site. Bind to another part of the enzyme  Conformational shape. The enzyme loses its affinity for the substrate as a result. OR The inhibitor directly affects the enzyme so that it cannot carry out its function. Allosteric regulation

Some enzymes have allosteric sites. Allosteric sites are NOT the same as active sites. They bind substances that can either: Inhibit activity (allosteric inhibitor) Stimulate activity (allosteric activator)

Both work by stabilizing protein conformations that alter access to the active site.

Allosteric activator All active sites are kept available for substrate Enzyme is in active form

Allosteric inhibitor All active sites are unavailable to substrate Enzyme is in inactive form Feedback inhibition

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