Download Nature of Enzymes

BIOLOGY CHAPTER NO.3
Enzyme (Organic Catalysts)
(GK ; EN = with in , zyme = leavevn – living)
A complex protein produced within a living cell that promotes a specific chemical
reaction by acting as a catalyst : ( chemical that promote chemical reaction).
Nature of Enzymes
An enzyme is a partly or entirely a protein that can tremendously increase the
efficiency of a biochemical reaction.
Enzyme are generally specific for a particular reaction.
Chemical Components of Enzymes
Every enzyme is protein wholly or has a protein as major part of its structure.
Enzyme
Holo enzyme
(Protein)
Apoenzyme)
Non Protein
(Cofactor)
Protein organic Cofactor
Covalently bonded to Protein Part
(Prosthetic group)
Inorganic metal ions Activator
Loosely attached
(Co-enzyme)
Cofactor :- ( Co = together , factor = component)
A substance, for example , a coenzyme or metal ion that acts with enzyme and is essential
to the activity of an enzyme.
Role of Cofactor
Cofactor act as “ links” between the enzyme and its substrate.
Cofactor directly participate in the chemical reaction.
Cofactor some times supply chemical energy to start chemical reaction.
Types of Cofactors:- There are three types of cofactors.
Activator ( To make something reactive)
The removable inorganic ions as cofactor is called activator For example Mg +2, Fe+2
Cu+2 Zn+2, etc.
Prosthetic Groups :-
(Gk ; Prosthetic = to add )
Prosthetic group are non protein organic molecules added to protein part through covalent
bond.
For example : flavin is attached to dehydrogenase enzyme involved in respiration as
prosthetic group.
Co-enzymes :- = (enzyme partner)
Non- protein organic compound or molecule that is not tightly held (loosely attached) to
protein part of enzyme.
For example :- Vitamins (Latin; vita = life , amine = Amino Acids) are source of coenzymes.
Vitamin Nicotinic Acid produce NAD ( co-enzyme)
(Nicotinamide adenine dinucleotide)
Apoenzyme :- (Gk ; apo = detached)
The protein component of an enzyme ( that determines enzyme’s specific function )
detached or separated from its proper co-enzyme or prosthetic group.
Holoenzyme :-
(Gk ; holos = entire or whole )
(enzyme cofactor complex)
An active enzyme consisting of a protein part and cofactor.
Physical Nature of enzymes :Enzymes have globular structure
( L ; glubulus = little globe ) , Composes of hundreds of amino acids.
Active site :- The part of an enzyme that react with the substrate ( the substance on
which enzyme acts).
Active site consisting of only a few amino acids (between 3-12 amino acids)
Active site consist of two known regions
Binding site :- It recognizes the specific substrate and form ES complex , it activates
catalytic site.
Catalytic site :- ( Greek ; Kata = apart , lutikos =able to loose)
The part active site, when get stimulated by binding site convert substrate in to products.
Active site has specific shape and charge matching to its substrate.
Location and distribution of enzymes in a cell :Enzymes produce with in living cells called = Intracellular enzymes.
Many enzymes are found dissolved in cytosol.
Some enzyme are present attached to the cell membrane or membranes of cell organelles.
For example :- Chloroplast have
Enzyme used in photosynthesis
Mitochondria have enzymes involved in respiration.
Enzymes used in protein synthesis are essential part of ribosome..
Characteristics of enzymes :German Physiologist Willelm Kuhne firstly used the name enzyme in 1867, due theit
typical catalytic nature with living cells.
Enzymes have the following characteristic:
1) High molecular weight :- Enzymes are made up of large number of amino acids
making its protein part.
2) Globular proteins:- All enzymes are globular proteins.
3) Bio-catalyst:- They increase the rate of biochemical reaction without being used in it.
4) Water soluble :- All enzymes are soluble in water and perform its catalytic in aquas
5) (of water) medium.
6) Active in small amounts:- Even small amount of an enzyme can bring about the
change in a large amount of substrate.
7) Specificity: - Enzymes are very specific in then action; Normally an enzyme
catalyzes single substrate or one group of related substrates.
8) Nature of end products:- presence of enzymes in a biochemical’s reaction never affect
the nature and properties of end products.
9) Enzyme – Cofactor Complex:- Some enzyme need a cofactor for their proper
functioning.
10) Lowers action energy :- Enzymes lowers he amount of ( energy need to make
molecules of a substrate take part in chemical reaction ) activation energy.
11) Sensitive to inhibitors :- Catalytic activity of enzymes can be slowed down or stopped
by certain chemicals.
12) Denaturation :- Enzymes are sensitive to even a minor change in pH, temperature and
substrate concentration.
13)
Mechanism of Enzyme action :-
(CATALYSIS)
Specificity of enzymes:A enzyme is a three dimensional (three directional) globular protein
Specificity of enzymes is due to particular chemical composition , specific Amino Acids
special shape.
This character enables an enzyme to react with a particular chemical substance (substrate)
or group of related chemicals.
Substrate :A substance on which an enzyme acts in to a biochemical reaction.
Enzyme –Substrate Complex:
Any enzyme reacts with its specific substrate and change it into products. In the end, the
enzyme releases without any change.
Thus enzymes can be used again and again.
E + S
Enzyme
Substrate
ES
Enzyme – Substrate
complex
E + P
Enzyme
(unchanged)
Products
Active Site
Every enzyme has one small part with definite charge through which it binds the
substrate. Or The part of enzyme that reacts with substrate is called active site.
Properties of Active Site:
It is a three dimensional groove or pocket with the chemical and electrical properties of
Amino Acids.
The shape of active site is formed by folding pattern of polypeptide chain with in
globular symmetry of the enzyme.
Particular shape of active site determines the specificity of enzyme.
Parts of Active sites:Active site has two Regions
(1) Binding Region:The binding site helps the enzyme in the identification and recognition of
activate.
substrate
(2) Catalytic region.
Activated catalytic region changes the substrate into product.
It works by lowering activation energy barrier
Metabolic path ways: (Gk; metabolikos = changeable)
The formation complex bio chemicals with in a cell occurs in step wise fashion, such
ordered series of reactions catalyzed by set of enzymes is called metabolic path way.
For Example = Photosynthesis , respiration, etc.
Every metabolic path way have following participants:
Precursor :-Substances that enters at the start of metabolic pathway.
Intermediate Substances: Any chemical substance produced between start and end
pathway.
End pathway:- Those substances which are produced at the final step of the metabolic
pathway.
Regulation of Metabolic pathway:Feed back inhibition:When end product of pathway is in substance , it binds to first enzyme of the pathway at
allosteric and stops, so no more product is formed.
Allosteric :- (Gk;Allo = other, steric = site)
The site other than active site.
Precursor activation :-
(L; Precursor = to run before)
As the products of metabolic pathway get utilized, precursor substance attaches to final
enzyme and act as activator to speed up the chemical reaction.
Models For Enzyme Action
Lock and key Model :Emil Fischer presented the lock and key model in 1890 to explain the interaction of
enzyme and substrate. According to this model, “one specific key can open only a
specific lock, similarly a specific enzyme can transform only specific substrate in to
products”.
Lock & key model describe active site rigid (inelastic)
Active site act as model (template) and shows no flexibility during any step of
biochemical reaction.
Further research does not support this model in all reactions.
Induce – Fit Model :Daniel Koshland proposed his induce fit theory to explain mechanism of enzyme Action
(1959) “ According to this theory when a substrate combines with an enzyme, it induce
(produces ) changes in the enzyme structure.”
The change in structure allows the enzyme to perform its catalytic activity more
effectively.
Koshland’s induce-fit model of Enzyme Action.
Factors influencing Enzyme Activity
Enzyme’s functional specificity depends its particular chemistry and structural
arrangement.
All the factors which change their chemistry and shape can also affect their activity rate.
Some important factors are as following.
(1) Enzyme Concentration :“The rate of reaction depends directly on the amount of active enzyme present at a
particular time when substrate availability is unlimited”.
High substrate concentration: Rate of reaction (directly proportional)
availability of enzyme
Double the available Enzyme = Double the rate of reaction.
Increased the supply of enzyme provide large number of available active site to convert
more substrate into products.
As substrate becomes in short supply, the reaction rate can not be further increased by
increasing supply of active enzyme.
(2) Substrate Concentration :At low concentration of substrate the reaction rate is directly proportional to the amount
of available substrate.
Low substrate concentration
Reaction rate (directly proportional ) substrate available
Keeping enzyme concentration constant at a particular point, further increase in substrate
can not increase rate of reaction. As the substrate molecules fill all the active sites of all
the available enzymes at high substrate concentration.
Temperature :-
As the temperature rises, enzymes activity increases but only up to a certain point.
All enzymes show their maximum activity rate at a specific temperature = Optimum
temperature
Human body enzymes = optimum temp. 37C
Rate of chemical reaction is increased by increasing temperature as available heat
provides activation energy.
Heat increase Kinetic energy of reactions, this increases the chances of their effective
collision with each other and with enzyme.
Above optimum, further supply of heat energy increases the vibrations of atom making
enzyme molecules.
Such strong vibrations destroy globular structure of enzymes.
Enzyme Denaturation :- Any change in molecular structure or chemical composition
of enzyme by which it loses its catalytic properties.
PH VALUE :- Every enzyme has optimum PH value.
Optimum pH:- It is a narrow range of pH at which enzyme work most effectively
At minor change in pH of medium can change the ionization of the active site and
reduces or blocks the enzyme activity.
Extreme changes in pH cause the bonds in the enzyme to break, causing denaturation.
Inhibtors :- (L;Inhibere = to stop) (A chemical
that interferes with)
“A substance that slows or stops a metabolic process. By blocking the enzyme activity”
An inhibitor is chemical substance which can react (in place of substrate) with the
enzyme but is not transferred into product and blocks the active site temporary or
permanently.
For example = cyanide, antibodies, anti metabolites
Poisons
(Sulfanilamide)
and sum drugs ( Sulfa drugs)
Types of Inhibitors :- ( Anti enzymes)
They occupy or destroy the active sites of the enzymes and decrease the rate of
biochemical reaction.
They capture active site by forming covalent bonds or may physically block it.
Irreversible Inhibitor:- These inhibitors from weak linkages with enzymes such as
hydrogen bonds.
Their effect can be cancelled out completely or partly by increasing substrate
concentration.
These are the following types
Competitive Inhibitor :- “ It resembles the enzyme’s normal substrate and competes
with substrate active site on the enzyme”
Competitive Inhibitor when bind to active site, it is not converted into products and
blocks the active site.
Non Competitive Inhibitor :It does not enter the active site.
It finds to enzyme at a point other than active site calling allosteric site and changes the
shape of enzyme so that active site no longer fits the substrate.