Download EnzymesLect1 2014

Enzymes
Prof. Dr. Hedef Dhafir El-Yassin
2014
Enzymes Chaecteristics and
Regulations
Objectives:
1. To define enzymes
2. To state the major functions and characteristics of enzymes
3. To describe enzyme specificity and selectivity
4. To describe enzyme-substrate interaction
5. To list the major factors that regulate enzyme activity
and describe how.
Definition:
An enzyme is a protein that catalyzes, or speeds up, a
chemical reaction.
Enzyme activity can be affected by other molecules:
a. Inhibitors are naturally occurring or synthetic
molecules that decrease or abolish enzyme
activity;
b. activators are molecules that increase activity.
Certain RNAs also have catalytic activity, but to differentiate them from protein enzymes,
they are referred to as RNA enzymes or ribozymes.
Major functions and charecteristcs
Enzymes are essential to life because most chemical reactions in biological cells would
occur too slowly, or would lead to different products without enzymes.
A malfunction (mutation, overproduction, underproduction or deletion) of a single enzyme
can lead to a severe disease.
For example, the most common type of phenylketonuria is caused by a single amino acid
mutation in the enzyme phenylalanine hydroxylase, which catalyzes the first step in the
degradation of phenylalanine. The resulting build-up of phenylalanine and related products
can lead to mental retardation if the disease is untreated.
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
Characteristics:
•
Like all catalysts, enzymes work by providing an alternate pathway of lower
activation energy of a reaction, thus allowing the reaction to proceed much faster.
Enzymes may speed up reactions by a million times.
•
Like any catalyst, an enzyme remains unaltered by the completed reaction and can
therefore continue to function.
•
Because enzymes do not affect the relative energy between the products and
reagents, they do not affect equilibrium of a reaction.
•
The advantage of enzymes compared to most other catalysts is their sterio-, regioand chemoselectivity and specificity.
3D Structure:
In enzymes, as with other proteins, function is determined by structure. An enzyme can be:
•
A monomeric protein, i.e., containing only one polypeptide chain, typically one
hundred or more amino acids; or
•
An oligomeric protein consisting of several polypeptide chains, different or
identical, that acts together as a unit.
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
As with any protein, each monomer is actually produced as a long, linear chain of amino
acids, which folds in a particular fashion to produce a three-dimensional product. Individual
monomers may then combine via non-covalent interactions to form a multimeric protein.
Many enzymes can be unfolded or inactivated by heating, which destroys the threedimensional structure of the protein.
Active site:
Most enzymes are larger than the substrates they act on and only a very small portion of
the enzyme come into direct contact with the substrate(s). This region is known as the
active site of the enzyme.
Some enzymes contain sites that bind cofactors, which are needed for catalysis. Certain
enzymes have binding sites for small molecules, which are often direct or indirect products
or substrates of the reaction catalyzed. This binding can serve to increase or decrease the
enzyme's activity (depending on the molecule and enzyme), providing a means for
feedback regulation.
Specificity:
Enzymes are usually specific to the reactions they catalyze and the substrates that are
involved in these reactions. Factors responsible for this specificity are:
1. Shape,
2. charge complementarity's, and
3. Hydrophilic/hydrophobic character of enzyme and substrate.
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
Mode of Enzyme Action
There are two different views to explain the mode of enzyme action the lock and key
hypothesis and the induced-fit hypothesis.
1. Lock and Key Hypothesis
It was suggested by Emil Fischer in 1894. According to this view, the enzyme molecule
operates by chemically uniting with the substrate molecule, forming an enzyme-substrate
complex. The enzyme molecule provides a uniquely structured template on which the
substrate molecules can become attached and interact subsequently. This brings about an
interaction between the specific active sites in the enzyme molecule and the reactive sites
in the substrate molecule. The enzyme now breaks down the substrate into products. The
products initially remain attached to the enzyme for a short while forming an enzyme
product complex. The products get released from the enzyme molecule subsequently. The
enzyme is now ready to receive another substrate molecule again. Thus, the same
enzyme can be used again and again.
Example:
Lock and Key Hypothesis
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
2. Induced Fit Hypothesis
It was proposed by Daniel Koshland in 1959. According to this hypothesis, there is an
intermediate condition called transition state between the substrate and the products. It is
highly unstable.
When the substrate molecules bind to the enzyme molecule, a change is brought about in
the active site to precisely fit the transition state (induced fit). This induced fit hold the
substrates at the correct angle for the reaction to take place.
The fact that an active site may also have a conformation to fit the product helps in
explaining the role of enzymes in catalysing reversible reactions.
Induced Fit Hypothesis
Isoenzymes
There are some enzymes which have slightly different molecular structure but exert similar
catalytic action. Such enzymes are called isoenzymes or isozymes. More than 100
isozymes have been identified. The enzyme lactic dehydrogenase (LDH) in human
skeletal muscle has five isozymes.
Another kind of posttranslational modification is the cleavage and splicing of the
polypeptide chain. Chymotrypsin, a digestive protease, is produced in inactive form as
chymotrypsinogen in the pancreas and transported in this form to the stomach where it is
activated. This prevents the enzyme from harmful digestion of the pancreas or other
tissue. This type of inactive precursor to an enzyme is known as a zymogen.
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
REGULATION OF ENZYMES
FIRST.
pH. A change in
pH can alter the rates
of
enzyme-catalyzed
reactions, with many
enzymes exhibiting a
bell-shaped
curve
when enzyme activity
is plotted against pH
(see Figure).
Examples:
Changes in pH can alter the following:
a. The ionization state of the substrate or the enzyme-binding site for substrate
b. The ionization state at the catalytic site on the enzyme
c. Protein molecules so that their conformation and catalytic activity change
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
SECOND.
Temperature. The rate of an enzyme-catalyzed reaction usually
increases with increasing temperature up to an optimum point (see Figure below),
and then it decreases because enzymes are thermolabile.
THIRD. Product
inhibition. If the product accumulates, it can inhibit some
enzymes. This form of control limits the rate of formation of the product when the
product is underused.
FOURTH.
Covalent modification
1. Phosphorylation
a. Effect on enzyme activity. In certain enzymes, the addition of a phosphate
group to a specific amino acid residue [usually serine (Ser), tyrosine (Tyr), or
threonine (Thr)] by specific protein kinases dramatically enhances or
depresses activity.
b. This modification is reversible. The phosphorylated enzyme may be
dephosphorylated by specific phosphatases.
2. Nucleotidylation
a. Effect on enzyme activity. The activities of certain enzymes are regulated by
the reversible addition of a nucleotide (e.g., adenosine) to a specific amino
acid.
b. This modification is reversible. For example, an adenylated enzyme may be
deadenylated by a specific enzyme.
3. Proteolytic cleavage.
Certain enzymes are synthesized as proenzymes, or zymogens, which are inactive
forms of enzymes that become active only after being cleaved at a specific site in their
polypeptide chain by specific proteases.
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
a. Many digestive enzymes that hydrolyze proteins (e.g., trypsin, pepsin)
are synthesized as zymogens in the stomach and pancreas.
b. Blood clotting is mediated by a series of proteolytic zymogen activities
of several serum enzymes.
FIFTH. Allosteric regulation of metabolic pathways. .
A. Key enzymes that catalyze rate-limiting steps of metabolic pathways or that are
responsible for major cellular processes must be regulated to maintain homeostasis
of individual cells and the organism overall.
B. Allosteric regulation refers to binding of a molecule to a site on the enzyme other
than the active site and induces a subsequent change in shape of the enzyme
causing an increase or decrease in its activity.
C. Many allosteric enzymes have multiple subunits whose interaction accounts for their
unusual kinetic properties.
i. Enzymes that are subject to allosteric regulation by either positive or negative
effectors exhibit cooperativity.
ii. In the presence of
positive cooperativity,
a plot of v versus [S]
shows sigmoidal kinetics,
ie, is S-shaped (Figure).
a. This kinetic
behavior
signifies that the
enzyme's affinity
for the substrate
increases as a function of substrate loading.
b. This is analogous to O2 binding by hemoglobin, in which O2 loading to
one subunit facilitates O2 binding to the next subunit, and so on.
Figure: Relationship between V; and [S] for a reaction catalyzed by an allosteric enzyme,
showing the effects of positive and negative effectors.
D. Feedback inhibition occurs when the end product of a metabolic pathway
accumulates, binds to and inhibits a critical enzyme upstream in the pathway, either
as a competitive inhibitor or an allosteric effector.
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014
Conclusions:
1. Enzymes are proteins that catalysis biological reactions. Some have slightly different
molecular structure but exert similar catalytic action such enzymes are called isoenzymes.
2. Enzymes have specific characteristics and properties that make them similar to catalyst yet
differ from them.
3. Enzymes are stereo specific and stereo selective
4. Several theories are discussed to explain the enzyme –substrate interactions among them
are:
a. Lock and key model
b. Induced fit hypothesis
5. enzyme activities are regulated by several factors:
1 pH
2 Temperature
3 Product inhibition
4 Covalent modifications
5 Allosteric regulations of metabolic pathways
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Professor Dr.H.D.El-Yassin
Ph.D. Post Doctorate
2014