Enzymes

Characteristics of Enzymes

• Proteins

• Catalysts– Speed up chemical

reactions without being used up

Remember that proteins have a very specific structure

• Primary through quaternary structure must be maintained for the protein to function properly

• What happens if it is not? – an enzyme’s shape is changed so that it is no

longer able to catalyze reactions, we call it…

DENATURED

Denaturation

Disruption of secondary, tertiary and quaternary protein structure byheat

Break apart H bonds and disrupt hydrophobic attractions acids/ bases

Break H bonds between polar R groups andionic bonds

heavy metal ions React with S-S bonds to form solids

agitation Stretches chains until bonds break

Applications of Denaturation

• Hard boiling an egg• Wiping the skin with alcohol swab for

injection• Cooking food to destroy E. coli.• Heat used to cauterize blood vessels• Autoclave sterilizes instruments• Milk is heated to make yogurt

How do enzymes work?

• Lower Activation Energy to speed up rates of reaction (of chemical reactions)– Reactions require energy to begin…enzymes

lower the amount of energy required.

Am

ount

of

ener

gy

Reaction Rate

Without an enzyme

With an enzyme

Activation energy

Naming

• Often end in “–ase”

• The name usually relates to the reactants they are associated with or the reaction they help start

A specific enzyme catalyzes each cellular reaction

A unique 3-D shape of an enzyme determines which chemical reaction it catalyzes.

Important Vocab:– SUBSTRATE: A specific reactant that an

enzyme acts on– ACTIVE SITE: region of the enzyme where the

substrate fits (lock and key)

The enzyme and substrate form a complex…

substrate

enzyme

Active site

Enzyme-substrate complex

Lock and Key Model

The enzyme remains the same before/after the rxn, while the substrate changes to new products after the rxn

+ +

E + S ES complex E + P

S

P

P

S

The cellular environment affects enzyme activity.

• An enzyme is most effective under certain conditions.

TemperatureTemperature affects molecular motion • an enzyme’s optimal temperature

produces the highest rate. -(Most human enzymes work

best at 35-40 ºC.)

WATCH OUT!!!If the temperature gets too high, the enzyme may be

denatured!

Factors Affecting Enzyme Action

Optimum temperature

ReactionRate

Low High

Temperature

IonsSalt concentration and pH influence enzyme activity.• SALT: Salt ions can interfere with the chemical bonds

that maintain protein structure.• pH: The same is true of the extra hydrogen ions at

very low pH.-The optimal pH for most enzymes is near

neutral.

Where is the charged area on the following R groups?

Factors Affecting Enzyme Action: Substrate Concentration

• Increasing substrate concentration increases the rate of reaction (enzyme concentration is constant)– Why?

• Maximum activity reached when all of enzyme combines with substrate

Factors Affecting Enzyme Action

Maximum activity

Reaction

Rate

substrate concentration

Enzyme Inhibition

Inhibitors • Cause a loss of catalytic activity

– May change the protein structure of an enzyme

Competitive Inhibition

A competitive inhibitor• Has a structure similar to

substrate• Occupies active site

– “Competes” with substrate for active site

• Effects can be reversed by increasing substrate concentration

Competitive Inhibitor

Competitive Inhibitor Examples• Methanol poisoning occurs because methanol

is oxidized to formaldehyde and formic acid which attack the optic nerve causing blindness. Ethanol is given as an antidote for methanol poisoning because ethanol competitively inhibits the oxidation of methanol. Ethanol is oxidized in preference to methanol and consequently, the oxidation of methanol is slowed down so that the toxic by-products do not have a chance to accumulate.

• Ethylene glycol (found in antifreeze) if ingested, can be poisonous. Ethylene glycol is oxidized by the same enzymes used in the previous examples by ethanol and methanol.

Noncompetitive Inhibition

A noncompetitive inhibitor• Does not have a structure like substrate• Binds to the enzyme (not at active site) and

changes the shape of enzyme & active site– Substrate cannot fit altered active site

• No reaction occurs• Effect is not reversed by adding substrate

Noncompetitive Inhibitor