Topic 4: Chapter 5

Enzymes Enzymes – – Part 2Part 2

CharacterCharacter

istics of istics of

EnzymesEnzymes

Characteristics of Enzymes?

Enzymes 1)speed up chemical reactions

2)are required in minute amounts

3)are highly specific in their action

4)are affected by temperature

5)are affected by pH

6)Some catalyse reversible reactions

7)Some require co-enzymes

8)Are inhibited by inhibitors

(1) Enzymes Speed up chemical reactions

(1) Enzymes Speed up chemical reactions

Activation Energy with enzyme

Activation Energy without enzyme

Substrate

Products:

Energy

Time

• By lowering the activation energy needed to start the reaction.

(2) Enzymes are required in minute amounts(2) Enzymes are required in minute amounts

Sucrose Glucose + Fructose

• They remain chemically unchanged after catalysing the reactions.

• The same enzyme molecules can be reused over again.

• Therefore, only a small amount of enzyme is required to catalyse a large number of reactions

Sucrase

Chemically unchangedChemically unchanged

Maltose Glucose + GlucoseMaltose Glucose + Glucose

(3) Enzymes are highly specific(3) Enzymes are highly specific

• Each chemical reaction is catalysed by its own specific, unique enzyme.

• This is due to every enzyme’s specific 3-d configuration.

• How the shape of an enzyme affects its function can be explained by the “LOCK & KEY HYPOTHESIS”.

Maltase

Starch MaltoseAmylase

• Enzymes are proteins and each has an active site.

• Active sites are depressions on the surface of enzyme molecules with specific shape and specific charges.

• Active sites are formed due to the way the proteins are folded and held together by H-bonds.

Lock &

Key

Hypothe

sis

Lock &

Key

Hypothe

sis

• Every active site has it’s own specific shape and charges.

• It only allows substrates with a COMPLEMENTARY shape and charge to bind with it to form an Enzyme-Substrate Complex

Lock &

Key

Hypothe

sis

Lock &

Key

Hypothe

sis

Active Sites

Substrate Enzyme-Substrate Complex Products + EnzymeSubstrate Enzyme-Substrate Complex Products + EnzymeEnzyme

Anabolic Reaction - Synthesis

Catabolic Reaction - Hydrolysis

TheThe LockLock

and and KeyKey HypothesisHypothesis

• The substrate binds to a free enzyme with a complementary active site to form the enzyme-substrate complex

• The enzyme-substrate complex brings about the necessary reactions

• The product separates from the enzyme, leaving the enzyme molecule unchanged and free to combine again with more substrate molecules

(4) Enzymes are affected by temperature

(4) Enzymes are affected by temperature

• Enzymes can function over a range of temperatures.

• But all enzymes have their own optimum temperature.

Optimum temperature• The optimum temperature is the temperature at

which the enzyme is most active, catalysing the largest number of reactions per second.

• Different enzymes have different optimum temp.• Example:

most enzymes in the human body functions best at about 37-40oC, near body temperature.

Enzymes of thermophilic bacteria that live in hotsprings will have very high optimum temperatures.

How enzyme activity is affected by temperature

• At low temperature, enzymes are INACTIVE.• As temperature rises, the rate of enzyme activity

increases (usually 2x as active for every 10oC rise).

• Enzyme reaching maximum rate of activity at OT

NOTE:

Describe the enzyme activity with respect to the Rate-Temperature Graph

• Raising the temperature increases the kinetic energy supplied to the substrate and enzyme molecules.

• This increases the no. of collisions between enzyme and substrate molecules.

• Increasing the rate of formation of enzyme-substrate complex.

• Rate of formation of the products increases up till the optimum temperature

Increasing temp up to optimum temp.

How enzyme activity is affected by temperature

• Beyond the optimum temperature, rate of enzyme activity deceases.

• Until it is completely denatured by the extreme heat

NOTE:

Describe the enzyme activity with respect to the Rate-Temperature Graph

• Increase in temperature increases the vibrations of the atoms in the enzymes.

• Beyond the OT, the vibrations are so violent that they break the hydrogen bonds that hold the 3-D structrure in place.

• The enzyme loses its shape and active site• The enzyme is DENATURED.

Increasing temp beyond optimum temp.

(4) Enzymes are affected by pH(4) Enzymes are affected by pH

• Different enzymes have different optimum pH.

• Enzymes are affected by the acidity or alkalinity of the solutions

• M indicates the optimum pH when the rate of reaction is the highest

Different enzymes have different optimum pH

• Enzymes that work best at ACIDIC conditions Renin and Pepsin: Found in the stomach

• Enzymes that work best at ALKALINE conditions Intestinal Enzymes

• Enzymes that work best at NEUTRAL conditions Amylase

ExamplesExamples

Different enzymes have different optimum pH

How enzyme activity is affected by pH

At pH slightly above or below the optimumAt pH slightly above or below the optimum

• Enzyme activity reduces when the conditions are slightly more acidic or alkaline than the optimum pH

• Slight changes in the pH brings about reversible changes

• Changes can be restored by bringing enzyme back to the optimum pH

How enzyme activity is affected by pH

At extreme pH conditionsAt extreme pH conditions

• Enzymes are DENATURED

• Extreme changes in the pH causes: A change in the charges at the active sites which

repels the substrate molecules, preventing them from binding.

The irreversible alteration to the bonds that holds the shape of the enzyme. Enzyme thus loses its original 3-D structure. The active site loses its shape.

(5) Some enzymes catalyze reversible reactions

(5) Some enzymes catalyze reversible reactions

AA BB CC DD+ +

Reactants Products

Products Reactants

• Some enzymes catalyse both reactions until equilibrium is reached.

• Reactions will proceed in the direction where the products are constantly being removed

• E.g formation of glucose during photosynthesis

(6) Some enzymes require co-enzymes (6) Some enzymes require co-enzymes

•Some enzymes require co-enzymes to be bound to them before they can catalyse reactions

Co-enzymes•Small, non-protein, organic “helper” molecules

(7) Enzymes are inhibited by inhibitors(7) Enzymes are inhibited by inhibitors

Type 1 (Competitive)Substrate is prevented from binding to active site by inhibitor

Type 1 (Competitive)Substrate is prevented from binding to active site by inhibitor

Type 2 (Non-competitive)Binding of inhibitor does not prevent the binding of substrate but slows down reaction speed

Type 2 (Non-competitive)Binding of inhibitor does not prevent the binding of substrate but slows down reaction speed