Enzymes

Objectives

3.6.1 – Define enzyme and active site.

C.2.3 – Explain that enzymes lower the activation energy of the chemical reactions that they catalyze.

3.6.2 – Explain enzyme - substrate specificity.

3.6.3 – Explain the effects of temperature, pH, and substrate concentration on enzyme activity.

3.6.4 – Define enzyme denaturation.

Introduction to metabolismSum of all chemical reactions in a body is

metabolism.Metabolic pathways alter molecules in a series of steps.

Catabolic pathways release energy by breaking down complex mole- cules to simpler compounds.Anabolic pathways consume energy to build complicated molecules from simpler compounds.Enzymes selectively accelerate each step.

Metabolic pathways alter molecules in a series of steps.

Enzymes selectively accelerate each step.Catabolic pathways release energy by breaking down

complex molecules to simpler compounds.Anabolic pathways consume energy to build

complicated molecules from simpler compounds.

Metabolic pathway

Introduction to metabolismOrganisms transform energy.

Energy is the capacity to do work - to move matter against opposing forces. Energy is also used to rearrange matter.

Kinetic energy is the energy ofmotion - ex: photons, heat.

Potential energy is the energy matter possesses because of

its location or structure.Chemical energy is a form of potential energy in mole-

cules because of the arrangement of atoms.

ATP

Introduction to metabolismEnergy can be converted from one form to

another.Ex: as a girl climbs a ladder to the top of the slide she is converting her kinetic energy to potential energy.

As she slides down, potential energy is converted back to kinetic energy.

It was the potential energy in the food she had eaten earlier that provided

the energy that permitted her to climb up initially.

Introduction to metabolismCellular respiration and other catabolic pathways unleash energy stored in sugar and other complex molecules, which were created during photosyn- thesis, an anabolic path-

way.

CO2 + H2O ⇄ C6H12O6 +O2

Respiration

←←← Respiration

Photosynthesis → → →

Anabolism

Catabolism

Introduction to metabolismAnabolic reactions (building molecules) are endergonic

(or endothermic) – ones that absorb energy. Ex: the overall reaction of photosynthesis:

6CO2 + 6H2O → C6H12O6 + 6O2

Through this reaction, 686 kcal of energy from (fro the sun) have been put into the chemical bonds of a sugar molecule. The pro-

ducts have 686 kcal more energy than the

reactants.

Introduction to metabolismCatabolic reactions (breaking molecules) are

exergonic (or exothermic) – ones that release energy.

Ex: the overall reaction of cellular respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O

Through this reaction 686 kcal have been

made available to do work in the cell. The products have 686

kcal less energy than the reactants.

Introduction to metabolismExergonic vs. endergonic reactions

Respiration Photosynthesis -686 kcal (released) +686 kcal from the sun

Introduction to metabolismThe energy created through respiration is used to

do work. A cell does three main kinds of work:

Mechanical work: beating of cilia, muscle contraction …Transport work: pumping substances across membranesChemical work, driving ender-gonic reactions such as the synthesis of polymers from monomers.

Introduction to metabolismIn most cases, the immediate source of energy

that powers cellular work (coupling exergonic reactions to endergonic reactions) is ATP (adenosine triphosphate).

Introduction to metabolismEnergy from respiration (burning food with O2) is

used to add a PO4- group to ADP.

When energy is needed by a cell, the PO4- group

is removed, and the energy is released.The energy traveled from the sun, to the plant, to the animal.

Exergonic → ← Endergonic

Enzyme kineticsAn enzyme is a catalytic protein.

A catalyst is a chemical agent that changes the rate of a reaction without being consumed by the reaction.Enzymes speed up metabolic reactions by lowering energy barriers.

Ex: In a match head,

S + O2 → SO2 + energy, but the reaction is not

spontaneous – friction must be applied to

give some initial energy for combustion.In a match head: S + O2 → SO2 + energy

friction

Enzyme kineticsActivation energy (EA) is the amount of energy

that is necessary to push the chemical reactants over an energy barrier. This energy makes the reactants unstable. Ex: friction makes a match head hot enough to ignite.

Enzymes lower

activation energy.

Enzyme specificityEnzymes are substrate specific.

A substrate is a reactant that binds to an enzyme.Enzymes catalyze

the conversion of a substrate to a product.

Ex: sucrase: binds to sucrose and breaks this disac- charide into fruc- tose and glucose.

Enzymes end in –ase.

Enzyme specificityThe active site of an enzyme is a pocket or groove on the surface of the protein into which the substrate fits.

The specificity of an enzyme is due to the fit between the

active site and that of the substrate.

As substrate binds, enzyme changes shape, bringing

chemical groups into a closer posi-tion.

Enzyme specificityThe active site is an enzyme's catalytic center.

Substrates are held in the active site by weak hydrogen bonds and ionic bonds.

Within the active site, chemical bonds are stressed, and ATP provides the little energy needed to cause the chemical reaction.

Enzyme-catalyzed reactions are reversible.

Enzyme activityA cell's physical & chemical environment affects

enzyme activity (remember, enzymes are proteins).Temperature has a major impact on reaction rate.

As temperature increases, collisions between substrates and active

sites occur more fre-quently.

At some point, the enzyme is destroyed (becomes denatured).

Detergents & high salt also destroy enzymes.

Enzyme activityWhen an enzyme is denatured, it is

destroyed.The protein structure is degraded.

Think what happens to fresh egg white when it is cooked – it goes from colorless liquid to white solid

→

Egg white is the protein albumin.

S |S

H |H

Enzyme activityA cell's physical & chemical environment

affects enzyme activity (remember, enzymes are proteins).pH influences shape of enzymes & reaction rate.

Each enzyme has an optimal pH. This falls between pH 6 - 8 for

most enzymes.At some point, all the

H+ destroys the shape of the enzyme, and it

won’t function.

Enzyme activityReaction rate also depends on enzyme and

substrate concentrations.At low substrate concentrations, an increase in sub-strate speeds binding to available active sites (below left)But at high substrate concentrations, the active sites on all enzymes are engaged, and the enzyme is saturated (far right).

Substrate Concentration