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?