Download Enzyme Optimum pH - Sir Sabir Hussain

Biology Notes
Class; F.Sc 1st Year
By: Sabir Hussain Rana, M.D.(Biochemistry)
-E N Z Y M E SEnzymes are defined as the most important group of globular proteins which are Biologically
Active. OR as “Biocatalysts” which speed up the biochemical reactions (more specifically as
catalysis) occurring in the living body by lowering their activation
energy
Importance of Enzyme:
1. They tremendously increase the efficiency of a biochemical
reaction and are specific for each type of reaction
2. Without these enzymes the reaction would proceed at a very
slow speed making life impossible
3. Many enzymes are simply dissolved in the cytoplasm while
others are tightly bound to certain subcellular organelles
4. They are produced by living cells for use in or near the site of their production
5. Thus, enzymes important for photosynthesis are found in chloroplasts and enzymes
involved in cellular respiration are found in the mitochondria
6. Some of the enzymes which are involved in the synthesis of proteins are integral part of
ribosomes
SOME IMPORTANT TERMS RELATED TO ENZYMES:
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Chemical Processes and Reactions occurring in the cell/organism is called Metabolism
Metabolism is of two types, namely; catabolism and anabolism
Break down of large molecules into smaller molecules is called Catabolism e.g. Respiration
Small molecules may combine to form large molecules and the process is called Anabolism
e.g. Photosynthesis
 The reactant of the enzyme or the substance at which an enzyme works is called its
Substrate
 The site/part of the enzyme where normally a substrate attach or react is called Active Site*
(see pg # 3)
Enzymes
Dr. Sabir Hussain
(Chapter # 3) F.Sc Biology
 The non-protein part of an enzyme which is essential for its proper functioning and acts as a
“bridge” between an enzyme and its substrate is called a Co-factor e.g. Mg2+, Fe2+, Cu2+,
Zn2+ etc
 An inorganic detachable cofactor is known as Activator
 If the co-factor is covalently bonded to the enzyme, it is known as a Prosthetic Group
 If a co-factor is loosely attached to the protein part of enzyme, it is called Co-enzyme. Coenzymes are mostly vitamins which can also be used again and again like enzyme
 An enzyme with its co-enzyme or prosthetic group removed is designated as Apoenzyme
 An activated enzyme consisting of polypeptide chain and cofactor (co-enzyme and
prosthetic group) is called Holoenzyme
CHARACTERISTICS OF ENZYMES:
Enzymes, the biochemical catalysts, possess the following important characteristics:
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All enzymes are globular proteins (except nucleases)
They increase the rate of reaction without being used up
They do not affect the nature or properties of end products
Small amount of enzyme can accelerate chemical reactions
They are very specific in their action; a single enzyme catalyzes a single chemical reactions
or group of related reactions
They are very sensitive to even the minor change in pH, temperature and substrate
concentration
Some enzymes require a co-factor for their proper functioning
They lower the activation energy of the reactions
Mechanism of Enzyme Action (CATALYSIS):
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An enzyme is a three dimensional globular protein that has specific chemical composition
due to its component amino acid and a specific shape
Every enzyme by virtue of its specificity recognizes and reacts with a special chemical
substance called Substrate
Any enzyme reacts only with its specific substrate and transforms it into the product(s)
It is released unaltered after the catalysis and thus can be used again and again
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Enzymes
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Dr. Sabir Hussain
(Chapter # 3) F.Sc Biology
An enzyme and its substrate react with each through a specific charge bearing site of an
enzyme called active site
The charge and shape of the active site is formed by some amino acids present in the
polypeptide chain of the active site of the enzyme
These amino acids are brought closer and are arranged in a specific way by coiling and
folding of the polypeptide chain within the globular symmetry of the enzyme
The active site* of the enzyme is made up of two definite regions i.e. the binding site and
the catalytic site
The binding site helps the enzyme in the recognition and binding of a proper substrate to
produce ES complex and catalytic site catalyzes the transformation of the substrate into
product(s)
Enzyme detaches after catalysis from its products unchanged
Enzyme requires aqueous medium for its activity
Emil Fisher (1890) proposed Lock and Key Model to visualize
substrate and enzyme interaction.
o According to this model; as one specific key can open only a specific lock in the same
manner a specific enzyme can transform only one substrate into product(s)
o According to this model, the active site is a rigid structure and there is no
modification or flexibility in the active site before, during or after the enzyme action
and it is used as a template
o Later study did not support this model in all reactions
 On the basis of new evidences Koshland (1959) proposed a
modified form of enzyme and substrate interaction called
Induce Fit Model
o He argued that when substrate combines with an enzyme, it induces changes in the
enzyme structure.
o The change in structure enables the enzyme to perform its catalytic activity more
effectively
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Enzymes
Dr. Sabir Hussain
(Chapter # 3) F.Sc Biology
FACTORS AFFECTING THE RATE OF ENZYME ACTION:
Any factor that can alter the chemistry and shape of an enzyme can affect its rate of Catalysis.
Some of the important factors that can affect the rate of enzyme action are:
1)
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Enzyme Concentration
Substrate Concentration
Temperature
pH of the medium
1. Enzyme Concentration:
 The rate of reaction depends directly on the amount of enzyme present at a specific
time at unlimited substrate concentration
 If the amount of enzyme is increased by two fold the reaction rate is doubled
No substrate is free for
enzyme
V is directly proportional to E
 By increasing the enzyme molecules an increase in the number of active sites take place
 More active sites will convert the substrate molecules into product(s) in a given period
of time
 After a certain limiting concentration, the rate of reaction will no longer depend upon
this increase
2. Substrate Concentration:
 At low concentration of substrate,
the reaction rate is directly proportional to the
substrate available
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Enzymes
Dr. Sabir Hussain
(Chapter # 3) F.Sc Biology
 If the enzyme concentration is kept constant and the amount of substrate is increased, a
point is reached where further increase in the substrate does not increase the rate of
reaction any more
 The reason behind this constant rate of reaction is that at high substrate level all the
active sites of the enzymes are occupied and further increase in the substrate does not
increase the reaction rate
3. Temperature:
 The rate of enzyme controlled reaction may increase with increase in temperature but
up to certain limit
 All enzymes can work at their maximum rate at a specific temperature called as
optimum temperature
 For example, for human body optimum temperature is 37oC
 The reason behind this increase in rate of reaction is that heat provides activation
energy and also supplies kinetic energy to the reacting molecules, causing them move
rapidly due to which chances of their collision with each other increase
 However, further increase in heat also increases vibrations too violently of atoms which
make up the enzyme molecule thus destroying the globular structure of enzyme (called
denaturization) and reaction rate thus ultimately starts decreasing as shown below:
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Enzymes
Dr. Sabir Hussain
4. pH Value:
 Every enzyme
(Chapter # 3) F.Sc Biology
functions most effectively over a narrow range of pH known as the
optimum pH
 A slight change in pH can change the ionization of the amino acids at the active site
 Moreover it may affect the ionization of substrate
 Under these Changed conditions enzyme activity is either retarded or blocked
completely
 Extreme changes in pH causes bonds in the enzyme to break, resulting in the enzyme
denaturation
Enzyme
Optimum pH
Pepsin
Sucrase
Enterokinase
Salivary Amilase
Catalase
Chymotrypsin
Pacreatic Lipase
2.00
4.50
5.50
6.80
7.60
7.00-8.00
9.00
Enzyme Inhibitors
An Inhibitor is a chemical substance other than substrate which can react with Enzyme in place
of substrate but is not converted to the product(s) thus blocking the active site temporarily or
permanently.
For example, poisons like cyanide, antibiotics, anti-metabolites and some drugs.
Types of Inhibitors:
a) Irreversible Inhibitors
b) Reversible Inhibitors
Irreversible Inhibitors:
 They check the reaction rate permanently by occupying the active sites or destroying
the globular structure
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Enzymes
Dr. Sabir Hussain
(Chapter # 3) F.Sc Biology
 They block the enzyme’s active site physically or chemically by making covalent bond
Reversible Inhibitors:
 They form weak linkages with enzyme
 An increase in substrate concentration can completely or partly neutralize their effect
They are further of two major types:
a) Competitive Inhibitors
b) Non-competitive Inhibitors
Competitive Inhibitors
Because of structural similarity with substrate
they may be selected by the binding sites but
not able to activate active sites. Thus
product(s) are not formed
Non-competitive Inhibitors
They form enzyme inhibitor complex at a point
other than active site. They alter the structure
of the enzyme in such a way that even if
genuine substrate binds the active site,
catalysis fails to take place
The end
These notes/handouts have been written keeping in view the course content of F.Sc premedical. All the necessary information accordingly has been provided omitting any unnecessary detail at this level.
With Thanks,
Sabir.H.N.
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