Enzymes

• The energy needed to get over the hill

• Enzymes provide alternative path involving a lower hill

• Activated complex

A. Cellular Metabolism

• Refers to all of the enzyme-mediated (chemical) reactions within a cell.

• Two main types of reactions 1. Anabolic metabolism – to build

2. Catabolic metabolism – to break down

B. Activation Energy

• The energy needed for a chemical reaction to take place.

• The energy needed to get over the hill

B. Activation Energy

• the speed of a reaction depends on the amount of activation energy required to break existing bonds

• In either kind of reaction, additional energy must besupplied to start the reaction. • This energy is the activation energy.

B. Activation Energy

• Endergonic - Refers to a chemical reaction that consumes energy. (anabolic)

• Exergonic - Describes a chemical reaction that releases energy in the form of heat, light, etc. (catabolic)

C. Catalyst

• A catalyst lowers the amount of energy required (by stressing chemical bonds)

• A substance that speeds up chemical reactions, but is not part of the products.

• Enzymes are the cell’s catalysts

C. Catalyst• Enzymes provide alternative path involving a

lower hill

D. Enzymes

• Cells contain many different enzymes, each of which catalyzes a different reaction.

• They cannot speed up reactions that would not normally occur on their own.

• A given enzyme interacts with a set of reactants (called substrates) or occasionally with a few closely related ones.

D. Enzymes• Enzymes are mostly

globular (tertiary) proteins with one or more invaginations on their surface called the active site

Lock and Key Theory:

In order for the catalysis to occur the substrate must fit perfectly into this depression.

D. Enzymes

Lock and Key Theory:

D. Enzymes• Induced FitInduced Fit:

• Proteins are not rigid, so the enzyme may give a little allowing an induced fit.

D. Enzymes

• Denaturing of an enzyme is the “loss of the active site.”

Factors that affect Enzyme Activity:

1. Temperature: • Human enzymes work best between 35o and

40oC• The rate of a chemical reaction is reduced

by half for every 10oC drop.• Below this temperature the protein is not

flexible to allow induced fit and becomes deactivated. • This is not the same as denaturation, as

deactivation is reversible.

Factors that affect Enzyme Activity:

1. Temperature: • Above this temperature, the H-bonds are too weak • The upper limit of enzyme activity before being

denatured is 40oC• Damage caused by mild heating may in some cases

be reversible• But continued warming would continue to denature

more and more of the enzyme until no active enzyme remains

• Ex: amylase would be completely denatured at 80oC

Factors that affect Enzyme Activity:

2. pH:– Optimum pH is between 6 and 8 (except

pepsin which prefers a pH of 2)– Straying from these pH values denatures

protein by disrupting bond charges, especially H-bonds between R-groups

– The result is a lost of the active site.

Factors that affect Enzyme Activity:

3. Inhibitors:

a) Competitive Inhibitors –block active sitesA competitive inhibitor binds reversibly to the enzyme, preventing the binding of the substrate.

On the other hand, binding of substrate prevents binding of the inhibitor, thus substrate and inhibitor compete for the enzyme.

Factors that affect Enzyme Activity:

3. Inhibitors:

b) Non-competitive Inhibitor:

binds to the enzyme and alters the shape of the active site, so the substrate no longer fits.

Ex: heavy metal poisoning

(lead or mercury)

Factors that affect Enzyme Activity:

4. Cofactors: Often enzymes use additional chemical components to aid catalysts.

a) Metal ions: they draw electrons from substrate molecules.

– Ex: carboxypeptidase has a zinc ion that draws electrons from the bonds joining amino acids.

– This is why we need trace elements (minerals) for good health

Factors that affect Enzyme Activity:

4. Cofactors: Often enzymes use additional chemical components to aid catalysts.

b) Coenzymes: non-protein organic molecules used as cofactors• Ex: vitamins

Note: coenzymes shuttle energy in the form of an atom form one place in the cell to another.