Enzymes

1
Enzymes

• Module 16

2
Enzymes

• Enzymes are proteins that speed up the rate of
  biological reactions i.e. - enzymes are
  catalysts
• Using enzymes, carbohydrates (polysaccharides),
  lipids (fat), and proteins are broken down by
  cells at a very rapid rate and under mild
  conditions
• In the laboratory, they are broken down slowly,
  and in the presence of an acid or a base and heat

3
Activation Energy of Enzymes and Enzyme
Specificity

• Activation energy
• Activation energy is the energy needed for a
  chemical reaction to take place
• This energy is used to BREAK bonds in the
  reactants and FORM bonds in the products
• An enzymes speeds up a specific reaction by
  providing an alternative pathway for that
  reaction, thus reducing the activation energy for
  the reaction
• Enzyme Specificity
• An enzyme only reacts with one reactant i.e.
  enzymes are SPECIFIC catalysts

4
The Lock and Key Model

• The Lock and Key Model explains the action
  enzymes take to catalyze reactions
• Steps in the Lock and Key Model
• The reactant or substrate (S) is combined with
  the enzyme (E) in the active site E S ? E-S
• The substrate fits tightly into the enzyme, just
  like a key fits into its lock
• The active site is the pocket where the substrate
  and enzyme combine
• E-S is referred to as the enzyme-substrate
  complex
• The reaction takes place, thus forming the
  products (P) E-S ? E-P
• The product dissociates from the enzyme, leaving
  the enzyme free to be used again E-P ?
  E P

5
A Diagram of the Lock and Key Model
6
An Example of the Lock and Key Model Hydrolysis
of Maltose

• The enzyme (E) maltase catalyzes the hydrolysis
  of maltose
• In this reaction, the reactant or substrate (S)
  is maltose, and the products (P) are two
  molecules of glucose
• Steps in the Lock and Key Model of the hydrolysis
  of maltose
• Maltose (S) combines with maltase (E)
• Maltose is hydrolyzed, thus forming two molecules
  of glucose (P)
• The two molecules of glucose dissociate from
  maltase, thus freeing maltase to catalyze another
  molecule of maltose

7
Naming and Classifying Enzymes

• Naming
• Enzymes are named according to the substrate they
  combine with and with the ending -ase
• Maltase is the enzyme to hydrolyze maltose
• Lactase is the enzyme to hydrolyze lactose
• Lipases are the enzymes that hydrolyze lipids
• Classifying
• Enzymes that consist of a single polypeptide
  chain are called simple enzymes
• More complex enzymes are called conjugated
  proteins
• These enzymes are activated by a nonprotein
  component called a cofactor
• A cofactor can by an organic compound, which is
  called a coenzyme, or a metal ion

8
Factors Affecting Enzyme Activity

• There are four factors that affect the rate at
  which an enzyme catalyzes a reaction
• Substrate Concentration
• Enzyme Concentration
• Temperature
• pH

9
Factor 1 Substrate Concentration

• The activity of an enzyme (i.e. the reaction
  rate) increases if the amount of substrate
  present increases
• The maximum activity level is achieved when all
  the enzyme molecules are combined with substrate
  molecules
• After this level is achieved, no matter how much
  substrate is added, the activity of the enzyme
  will not increase
• At the maximum activity level, the enzyme
  molecules are saturated with substrate molecules

10
Factor 2 Enzyme Concentration

• The activity of an enzyme (i.e. the reaction
  rate) increases if the amount of enzyme present
  increases

11
Factor 3 Temperature

• The activity of an enzyme (i.e. the reaction
  rate) increases if the temperature increases
• The optimum temperature is the temperature at
  which an enzyme reaches its maximum activity.
• For most enzymes in the body, the optimum
  temperature is 37 C.
• ABOVE and BELOW the optimum temperature, the
  reaction rate decreases.
• At temperatures above 60 C, most enzymes
  denature, thus destroying the structure of their
  active site. At these temperatures, the activity
  of enzymes is zero.

12
Factor 4 pH

• The optimum pH is the pH at which an enzyme
  reaches its maximum activity
• Each enzyme has a specific optimum pH
• Pepsin, an enzyme in the stomach, has an optimum
  pH of 2
• Trypsin, an enzyme in the small intestine, has an
  optimum pH of 8
• The STRUCTURE and FUNCTION of enzymes are very
  dependent on pH
• Since enzymes are proteins, changes in pH can
  cause changes in the side chains of the amino
  acid groups, which causes changes in the tertiary
  structure (i.e. the three-dimensional structure)

13
An Example of How Changing Factors Affects the
Reaction Rate

• Identify if the reaction rate increases or
  decreases when the following changes are made for
  an enzyme whose optimum pH is 4.5 and optimum
  temperature is 37 C
• Decrease the substrate concentration __decreases
  rate__
• Increase the substrate concentration __increases
  rate__
• Decrease the enzyme concentration __decreases
  rate__
• Increase the enzyme concentration __increases
  rate__
• Decrease the temperature to 25 C __decreases
  rate__
• Increase the temperature to 45 C __decreases
  rate__
• Increase the pH to 8.2
  __decreases rate__
• Decrease the pH to 2.3
  __decreases rate__

14
Enzyme Inhibition

• An inhibitor is a compound that slows down or
  stops enzyme activity
• There are two types of enzyme inhibition
• Competitive Inhibition
• Noncompetitive Inhibition

15
Enzyme Inhibition Competitive Inhibition

• Competitive inhibition involves the inhibitor
  competing with the substrate for the active site
  on the enzyme.
• The inhibitor can compete with the substrate
  since its structure is similar to the structure
  of the substrate.
• While the inhibitor is bound to the enzyme, the
  substrate cannot combine with the enzyme to
  react.
• Competitive inhibition is reversed by adding
  large amounts of the substrate.
• Medicine uses competitive inhibition a lot
• Some antibiotics fight bacterial infection by
  being a competitive inhibitor, thus interfering
  with the growth of bacteria. They do not
  interfere with the formation of human cell
  membranes, but do interfere with the formation of
  bacterial cell membranes. The antibiotic binds
  to the active site of the bacterial enzyme, thus
  stopping the growth of the bacteria and
  controlling the infection.

16
A Diagram of Competitive Inhibition
17
Enzyme Inhibition Noncompetitive Inhibition

• Noncompetitive inhibition involves the inhibitor
  binding to the enzyme at a site other than its
  active site
• The binding of the inhibitor to the enzyme alters
  the structure of the enzyme and thus changes the
  shape of the active site
• While the inhibitor is bound to the enzyme, the
  substrate cannot combine with the enzyme to react
  since it does not fit into the enzyme anymore
• Competitive inhibition cannot be reversed by
  adding large amounts of the substrate since the
  inhibitor does not bind to the enzyme at its
  active site

18
A Diagram of Noncompetitive Inhibition