Download Discovery of Enzymes

Enzymes
L. Scheffler
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Enzymes
Enzymes are catalysts. They increase
the speed of a chemical reaction without
themselves undergoing any permanent
chemical change.
Enzymes are neither used up in the
reaction, nor do they appear as reaction
products.
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Enzymes
Enzymes are protein molecules that catalyze
biochemical reactions
The substance on which the enzyme acts is
known as the substrate
Models of some proteins – tertiary and quaternary structure.
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Discovery of Enzymes
1825 Jon Jakob Berzelius
discovered the catalytic effect of
enzymes.
1926 James Sumner isolated the
first enzyme in pure form.
1947 Northrup and Stanley together
with Sumner were awarded the
Nobel prize for the isolation of the
enzyme pepsin.
Berzelius
Sumner
Northrup
Stanley
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Enzyme Characteristics
High molecular weight
proteins with masses
ranging from 10,000 to as
much as 2,000,000 grams
per mole
Substrate specific
catalysts
Highly efficient, increasing
reaction rates by a factor as
high as 108
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Enzyme Nomenclature
The earliest enzymes that were
discovered have common names:
i.e. Pepsin, Renin, Trypsin, Pancreatin
The enzyme name for most other
enzymes ends in “ase”
The enzyme name indicates the substrate
acted upon and the type of reaction that it
catalyzes
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Enzyme Names
Examples of Enzyme Names
Glutamic Oxaloacetic Transaminase (GOT)
L-aspartate: 2-oxoglutarate
aminotransferase.
Enzyme names tend to be long and
complicated. They are often abbreviated
with acronyms
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Types of Enzyme Specificity
1.
2.
3.
4.
Enzyme specificity may be characterized as
Absolute: The enzyme catalyzes on one
reaction
Group Specific: The enzyme acts only on
molecules having a particular functional group
Linkage Specific: The enzyme acts on a
particular type of chemical bond
Stereo-chemical Specific: The enzyme acts
on a particular stereo or optical isomer
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Enzyme Specificity
The action of an enzyme depends
primarily on the tertiary and quaternary
structure of the protein that constitutes the
enzyme.
The part of the enzyme structure that acts
on the substrate is called the active site.
The active site is a groove or pocket in
the enzyme structure where the substrate
can bind.
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Cofactors
Cofactors are other compounds or ions
that enzymes require before their catalytic
activity can occur.
The protein portion of the enzyme is
referred to as the apoenzyme.
The enzyme plus the cofactor is known as
a holoenzyme.
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Cofactors
Cofactors may be one of three types
1. Coenzyme: A non protein organic
substance that is loosely attached to the
enzyme
2. Prosthetic Group: A non protein organic
substance that is firmly attached to the
enzyme
3. Metal ion activators: K+, Fe2+, Fe3+,
Cu2+, Co2+, Zn2+, Mn2+, Mg2+, Ca2+, or
Mo2+,
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Types of Cofactors
Enzymes have varying degrees of specificity.
One cofactor may serve many different
enzymes.
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Enzymes and Cofactors
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Enzyme Mechanisms
Enzymes lower the activation energy for reactions
and shorten the path from reactants to products
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Enzyme Mechanisms
The basic enzyme reaction can be
represented as follows:
E + S  ES  E + P
Enzyme
Substrate
Enzyme substrate
Enzyme
Product(s)
complex
The enzyme binds with the substrate to form the
Enzyme-Substrate Complex. Then the substrate is
released as the product(s).
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Enzyme Mechanisms
Diagram of
the action
of the
enzyme
sucrase on
sucrose.
E+SES
E+P
Enzyme Mechanics
An enzyme-substrate complex forms when
the enzyme’s active site binds with the
substrate like a key fitting a lock.
The shape of the enzyme must match the
shape of the substrate.
Enzymes are therefore very specific; they
will only function correctly if the shape of
the substrate matches the active site.
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Induced Fit Theory
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Induced Fit Theory
The substrate molecule normally
does not fit exactly in the active
site.
This induces a change in the
enzymes conformation (shape)
to make a closer fit.
In reactions that involve breaking
bonds, the inexact fit puts stress
on certain bonds of the
substrate.
This lowers the amount of
energy needed to break them.
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Induced Fit Theory
The enzyme does not actually
form a chemical bond with the
substrate. After the reaction, the
products are released and the
enzyme returns to its normal
shape.
Because the enzyme does not
form chemical bonds with the
substrate, it remains unchanged.
The enzyme molecule can be
reused repeatedly
Only a small amount of enzyme is
needed
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Enzymes and Reaction Rates
Factors that influence reaction rates of
Enzyme catalyzed reactions include
1. Enzyme and substrate
concentrations
2. Temperature
3. pH
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Enzymes and Reaction Rates
At low concentrations,
an increase in substrate
concentration increases
the rate because there
are many active sites
available to be occupied
At high substrate
concentrations the
reaction rate levels off
because most of the
active sites are occupied
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Substrate concentration
The maximum
velocity of a reaction
is reached when the
active sites are
almost continuously
filled.
Increased substrate
concentration after
this point will not
increase the rate.
Vmax is the maximum
reaction rate
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Substrate concentration
Vmax is the maximum
reaction rate
The MichaelisMenton constant ,
Km is the substrate
concentration when
the rate is ½ Vmax
Km for a particular
enzyme with a
particular substrate
is always the same
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Effect of Temperature
Higher temperature
increases the number
of effective collisions
and therefore
increases the rate of a
reaction.
Above a certain
temperature, the rate
begins to decline
because the enzyme
protein begins to
denature
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Effect of pH
Pepsin is most efficient at
pH2.5-3 while Trypsin is efficient
at a much higher pH
Each enzyme has an
optimal pH at which it is
most efficient
A change in pH can alter
the ionization of the R
groups of the amino acids.
When the charges on the
amino acids change,
hydrogen bonding within
the protein molecule
change and the molecule
changes shape.
The new shape may not be
effective.
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Inhibitors
Enzyme inhibitors are substances
which alter the catalytic action of the
enzyme and consequently slow down or
stop catalysis.
There are three common types of
enzyme inhibition
1. Competitive inhibitors
2. Non-competitive inhibitors
3. Substrate inhibition.
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Competitive Inhibitors
Competitive inhibition
occurs when the
substrate and a
substance resembling
the substrate are both
added to the enzyme.
The inhibitor blocks the
active site on the
enzyme stopping its
catalytic action
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Non-competitive Inhibitors
Non-competitive
inhibitors deactivate
the active site of the
enzyme.
They alter the
enzyme so that it
can no longer bind to
the substrate
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Effect of inhibitors on the
Reaction Rate
For non-competitive
inhibitors Vmax is
lower but Km is the
same.
For competitive
inhibitors, Vmax is
the same but Km is
increased.
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Substrate Inhibitors
Substrate inhibition occurs
when excessive amounts
of substrate are present.
Additional substrate
sometimes interferes with
the ability of substrate
molecules to find active
sites on enzymes
In these cases the reaction
velocity decreases after
the maximum velocity has
been reached.
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