1.4 Enzymes

What is an Enzyme?

• Enzymes are specialized proteins that act as catalysts; they speed up chemical reactions by lowering the activation energy required.

• Enzymes are NOT used up during a reaction.• Enzymes are specific to a particular substrate

(reactant).

What is an Enzyme?

• All reactions require an Activation Energy (EA) that must be overcome in order for the rxn

to occur. Catalysts reduce the EA, but does not

affect the free energy change ΔG.

• Catalysts can only speed up a reaction that would occur normally.

• Substrate: the reactant that an enzyme works on. It binds to a particular site (active site) on the enzyme.

• Enzyme name usually ends in – ase.

• Ex. amylase

Amylose + H2O maltose maltase

Maltose + H2O α-glucose

The catalyst cannot change an endergonic reaction into an exergonic reaction, it can only decrease the potential energy level of the transition state.

Enzyme-Substrate Complex

• Substrates bind to a depression on the surface of enzymes known as the active site, to form an enzyme-substrate complex.

• The active site undergoes a slight conformation change to better accommodate the substrate (induced fit).

Animation: Enzyme Action and the Hydrolysis of Sucrose

Enzyme Activity • Active Site: Where the substrate binds to the enzyme.

(Lock and Key model)• Induced Fit: Protein changes shape to accommodate

substrate. Caused by interaction of functional groups. • Enzyme-Substrate Complex: Substrate attached to

enzyme.• Factors that can affect the rate of enzyme activity

include:        Temperature        pH        Concentration

Enzyme activity

http://www.kscience.co.uk/animations/model.swf

Factors that Affect Enzyme Activity

Factors that Affect Enzyme Activity

Every enzyme has an optimal temperature and pH at which it works best. Most human enzymes work

best around 37ºC.

Factors that Affect Enzymes

• What happens at cooler temperatures?

1. Temperature– Reaction rates

increase as temperature increases

– Peaks at ~ 37 - 40°C then drops rapidly

– Why?• E.g. egg frying

Factors that Affect Enzymes2. pH

– Enzymes function within an optimal pH range• Stomach pH

• Small intestine pH

Factors that Affect Enzymes

3. Concentration of Substrate Molecules– Reaction rate increases as the substrate

concentration increases up to a point

– Animation link

– The limiting factor in the reaction may be the amount of substrate or the amount of enzyme available

Cofactors and Coenzymes(required by some enzymes to function)

Cofactors: inorganic substances that aid in enzyme activity. (Ex. Zn2+, Mn2+)

 

Coenzymes: organic substances that aid in enzyme activity. (Ex. NAD+)

Animations

Enzyme Inhibition• Competitive Inhibitors

– Compete with substrate for enzymes active site.– Block active site– Reversible, overcome by increasing substrate concentration– E.g. Drugs: CO, Cyanide

Non-competitive Inhibitors bind to the enzyme at an allosteric site (not the active site) and cause a conformation change in the enzyme, preventing the normal substrate from binding. loss of enzyme activity (e.g. DDT)

Mouse Party

Drug Design• Acetylcholine is the neurotransmitter produced by neurons referred

to as cholinergic neurons. In the peripheral nervous system acetylcholine plays a role in skeletal muscle movement, as well as in the regulation of smooth muscle and cardiac muscle. In the central nervous system acetylcholine is believed to be involved in learning, memory, and mood.

• Acetylcholinesterase: This enzyme hydrolyzes acetylcholine into acetic acid and choline. If acetylcholinesterase activity is inhibited, the synaptic concentration of acetylcholine will remain higher than normal. If this inhibition is irreversible, as in the case of exposure to many nerve gases and some pesticides, sweating, bronchial constriction, convulsions, paralysis, and possibly death can occur. Although irreversible inhibition is dangerous, beneficial effects may be derived from transient (reversible) inhibition. Drugs that inhibit acetylcholinesterase in a reversible manner have been shown to improve memory in some people with Alzheimer's disease.

Allosteric Regulation

• Cells control enzyme activity to coordinate cellular activities.

• Allosteric Sites: Receptor sites that bind substances that inhibit or stimulate enzyme activity.

• Activators may bind to allosterically controlled enzymes to stabilize its shape and keep all active sites available.

• Allosteric inhibitors: Binds to allosteric site and stabilizes inactive form of the enzyme.

Essential Biochemistry - Enzyme Inhibition

Tutorial 6.1 Enzyme Catalysis

Tutorial 6.2 Allosteric Regulation of Enzymesl

Feedback Inhibition

• A method used by cells to control metabolic pathways involving a series of reactions.

• A product formed later in a sequence of reactions allosterically inhibits an enzyme that catalyzes the reaction earlier on.

Feedback Inhibition of Biochemical Pathways