Enzymes Objectives Flow of energy through life. Metabolism meaning. Chemical reactions & energy. Activation energy meaning. Catalyst meaning.

L.Noha SolimanL.Noha Soliman

BiochemistryBiochemistry(BMS)(BMS)

EnzymesEnzymes

Objectives

Flow of energy through life. Metabolism meaning. Chemical reactions & energy. Activation energy meaning . Catalyst meaning . What Are Enzymes?What Are Enzymes? Function of enzymes. How do enzymes Work?How do enzymes Work? Properties of enzymes. Mechanisms of Enzymes. Factors affecting enzyme function.

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Flow of energy through life

• Life is built on chemical reactions.– transforming energy from one form to another.

organic molecules ATP & organic molecules

organic molecules ATP & organic molecules

sun

solar energy ATP & organic molecules

Metabolism

• Metabolism can be described as a set of biochemical reactions on which the entire life of living organisms depend.

• Chemical reactions of life: forming bonds between molecules

• dehydration synthesis

• synthesis

• anabolic reactionsbreaking bonds between molecules

• hydrolysis

• digestion

• catabolic reactions

Chemical reactions & energy

Some chemical reactions release energy.

– exergonic

– digesting polymers

– hydrolysis = catabolism

Some chemical reactions require input of energy.

– endergonic

– building polymers

– dehydration synthesis = anabolism

digesting molecules= LESS organization=lower energy state

building molecules= MORE organization=higher energy state

Energy & life Organisms require energy to live

– where does that energy come from?

• Coupling exergonic reactions (releasing energy) with

endergonic reactions (needing energy).

+ + energy

+ energy+

digestion

synthesis

Activation energy

• To start any chemical reaction, energy is required.

• The minimum amount of energy required to start a reaction is known as activation energy.

Catalyst

• A substance that lowers activation energy of a reaction so the reaction occurs more quickly but is NOT used up by the reaction.

What Are Enzymes?What Are Enzymes?

Most enzymes are Proteins.Proteins.

Kind of catalysts found only in living things.

Act as biological Catalysts.

Enzymes vocabulary

Enzyme:• helper protein molecule.

Substrate: • reactant which binds to enzyme.• enzyme-substrate complex.• Temporary association.

substrate

enzyme

productsactive site

Enzymes vocabulary

Product: • end result of reaction.

Active site • part of enzyme that substrate molecule fits into.• enzyme’s catalytic site.

substrate

enzyme

productsactive site

Types of enzymes

1. Intracellular enzymes:• Enzymes that are produced in certain cells and remain

to react in the cell (work inside the cells).

• These enzymes could exist either in cytoplasm (in organelles) or nucleus.

2. Extracellular enzymes:• Some enzymes are produced by cell but then

transported out of the cell for action outside the cell.

• Enzymes that are produced inside

cells but work outside them.

Function of enzymes

Increase the rate of chemical reaction by lowering activation energy.

Enzymes change only the speed of the reaction.

They help the reactions to occur, but they are not changed or used up in the reaction, so they can be used over and over again (Reusable).

Enzymes allow digestion to occur faster; otherwise the hamburger you ate last week might still be in your stomach!

By using enzymes to break chemical bonds in food molecules, organisms release energy for life processes.

How do enzymes Work?How do enzymes Work?

FreeEnergy

Progress of the reaction

Reactants

Products

activation energy

Without Enzyme

With Enzyme

Enzymes work by weakening bonds which lowers activation energy.

Enzyme(sucrase)

Activesite

1

2

3

Substrate(sucrose)

Enzyme available with empty active site

Substrate binds to enzyme

Substrate is converted to

products

4

Products are released

Glucose Fructose

The enzyme is unchanged and can repeat the process

How an enzyme works?

Substrate(sucrose)

• Enzymes are recycled and reused many times for the same type of reaction (never used up).

Example:• Sucrase converts sucrose glucose + fructose

»Sucrase remains unchanged

How an enzyme works?

Enzymes are specific for the reaction they catalyze.• Each enzyme is the specific helper to a specific reaction

– each enzyme needs to be the right shape for the job– enzymes are named for the reaction they catalyze.

sucrase breaks down sucrose proteases breakdown proteins lipases breakdown lipids DNA polymerase builds DNA adds nucleotides to DNA strand

Oh, I get it!They end in -ase

Properties of enzymes

Properties of enzymes

Reaction specific:– Each enzyme works with a specific substrate

• Chemical fit between active site & substrate:– H bonds & ionic bonds.– The enzymes are usually bigger than the molecules that they

act upon.

Not consumed in reaction– Single enzyme molecule can catalyze thousands or more reactions

per second.• Enzymes unaffected by the reaction.

Affected by cellular conditions– Any condition that affects protein structure.

Which is the enzyme and which is the substrate? How do You know? What kind of reaction is this?

The enzyme does not get changed in this reaction - therefore it is the green one. Another clue is that it is bigger than the substrate.

Hydrolysis reaction because the substrate is being broken down.

Learning check Learning check

Hydrolysis with enzyme action

Dehydration synthesis of a product with enzyme

Mechanisms of Enzymes

There are 2 main hypothesis explaining the mechanism of enzyme action :

The lock-and-key hypothesis

The induced-fit hypothesis

Lock-and-key hypothesis

• The hypothesis proposed that the active site and substrate are exactly complementary.

• The shape of the substrate (‘key’) fits into the active site of the enzyme (‘lock’) forming an enzyme-substrate complex.

• Reaction takes place and products are formed and released.

A lock has a specific shape. Only one specific key willopen that lock.

More accurate model of enzyme action. The hypothesis suggested that active site is flexible and is not exactly complementary to the shape of the substrate. The substrate binds to the active site. The binding induces the enzyme to change shape leading to a tighter fit.

Induced-fit hypothesis

Factors affecting enzyme function

Enzyme concentrationSubstrate concentrationTemperature pHActivators Inhibitors

Enzyme concentration

The rate of an enzyme-catalyzed reaction is directly proportional to the concentration of the enzymes if substrates are present in excess concentration and no other factors are limiting.–as enzyme = reaction rate

• more enzymes = more frequently collide with substrate

Enzyme concentration

After a while the rate of reaction becomes constant.

At this point , the rate of the reaction can be increased only by increasing the concentration of the substrate.

Reaction rate levels offsubstrate becomes limiting factornot all enzyme molecules can find substrate.

Substrate concentration

If enzyme molecules are available in a reaction, increase in substrate concentration causes increase in the rate of reaction.–as substrate = reaction rate

• more substrate = more frequently collide with enzyme

If the ratio of enzymes is less than the ratio of substrates then the enzymes enter a state called saturation of active sites.So there are no vacant active sites available for reaction so instead of increased rate of reaction, the rate remains stagnant.•reaction rate levels off:all enzymes have active sites saturated.

Faster reaction but it reaches a saturation point when all the

enzyme molecules are occupied

Substrate concentration

Optimum temperature for maximum enzyme function is usually about 35-40C.

Reactions proceed slowly below optimal temperatures.

Above 45 C most enzymes are denatured (change in their shape so the enzyme active site no longer fits with the substrate and the enzyme can't function).

Temperature

The effect of pH

• Each enzyme has a pH range within which it can work best. • This is called for that particular enzyme.

• Most enzymes act at a pH in range of 5 – 9, and reactions most efficiently at a pH of 7.

• There are exceptions for certain enzymes such as:pepsin (stomach)= pH 2-3trypsin (small intestines)= pH 8

7

The effect of pH

pH

reac

tio

n r

ate

20 1 3 4 5 6 8 9 10

pepsin trypsin

11 12 13 14

pepsin

trypsin

Activators

Compounds which help enzymes. They increase enzyme activity. Cofactors can be either inorganicor organic compounds.

There are three kinds of cofactors:a) Prosthetic group: Organic molecule that is permanently attached to

an enzyme. (tightly bound)b) Coenzymes: organic molecules are not permanently attached to the

enzyme molecule. (bound and easily released) (vitamins)c) Metal cofactors: Inorganic metal ions. (Mg, K, Ca, Zn, Fe, Cu)

Inhibitors

Molecules that reduce enzyme activity. They block the enzyme but they do not usually destroy it. Many drugs and poisons are inhibitors of enzymes in the nervous

system.

Competitive inhibitors

Noncompetitive inhibitors

Enzyme Inhibitors

Competitive inhibitors:

are chemicals that resemble an enzyme’s normal substrate and compete with it for the active site.

(b) Competitive inhibition

A competitiveinhibitor mimics thesubstrate, competingfor the active site.

Competitiveinhibitor

A substrate canbind normally to the active site of an enzyme.

Substrate

Active site

Enzyme (a) Normal binding

Competitive inhibition

Succinate Fumarate + 2H++ 2e-

Succinate dehydrogenase

CH2COOH

CH2COOH CHCOOH

CHCOOH

COOH

COOH

CH2

Malonate

Enzyme Inhibitors

A noncompetitiveinhibitor binds to theenzyme away fromthe active site, alteringthe conformation ofthe enzyme so that itsactive site no longerfunctions.

Noncompetitive inhibitor

(c) Noncompetitive inhibition

Noncompetitive inhibitors:

Bind to another part of the enzyme causing the enzyme to change its shape, which in turn alters the active site.

Classification of Enzymes

• Enzymes are classified according to the type of reaction they catalyze:

Class Reactions catalyzedOxidoreductases Oxidation-reductionTransferases Transfer of functional groups Hydrolases Hydrolysis Lyases -Formation of double bonds by removing chemical groups from a substrate without

hydrolysis -Addition of chemical groups to double bond

Isomerases Rearrange atomsLigases Use ATP to combine molecules

Classification of Enzymes

Classification of Enzymes

Home Work

The digestion of

Carbohydratesstarts in

the mouth but it stops in the stomach,

Why?