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An Introduction to Metabolism
and Enzymes
Metabolism
is the sum
total of an
organism’s
chemical
reactions
Metabolism-
e.g. oxidation of sugar
Catalase
Facts to be explained
 Enzymes are proteins
 Enzymes are required in minute quantities
 Enzymes show striking specificity. They catalyze
specific reactions involving specific substrates
 Enzymes increase the speed of a chemical
reaction - Both forward and backward reaction
 The action of enzyme is greatly affected by
temperatures.
 Enzymatic activity is affected by H+ ions
concentration (pH)
 The rate of reaction is affected by substrate
and enzyme concentration
 Enzyme itself is not used up in the reaction it
catalyzes so it can be used over again and again
Specificity-lock and key hypothesis
Induced Fit model
of enzyme action
3D / tertiary structure of an
enzyme
Binding sites of an enzyme
Mechanism of enzyme reaction
The substrate
is the reactant an
enzyme acts on
The enzyme
binds to its
substrate, forming an
enzyme-substrate
complex
The catalytic cycle of an enzyme
1 Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates
Enzyme-substrate
complex
6 Active site
Is available for
two new substrate
Mole.
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
3 Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
•provide correct substrate orientation,
•stress the substrate bonds
•stabilize the transition state,
•provide a favorable microenvironment,
Enyme
5 Products are
Released.
Products
4 Substrates are
Converted into
Products.
Interaction between Active
site and substrate
fitting of a substrate brings
chemical groups of the active
site into positions that enhance
their ability to catalyze the
chemical reaction
Activation
energy-barrier to reaction
Activation energy
The active site can lower an EA
The active site can lower an EA
barrier by
Orienting substrates correctly
Straining substrate bonds
Stabilizing transition state
Providing a favorable microenvironment
Reaction pathway
Effect of Enzyme concentration
Effect of Substrate
concentration
Effect of temperature on rate
of chemical reaction
Chemical reaction takes place only if the
particles collide at the right orientation and
with sufficient energy (activation energy)
Effect of temperature on rate of chemical reaction:
http://www.absorblearning.com/media/att
achment.action?quick=w2&att=2298
Effect of temperature on rate
of chemical reactions
Effect of temperature
Are all enzymes equally susceptible to
denaturation by high temperature ?
Some
bacteria
survive in hot
spring at the
yellow stone
national park
US) at
~ 70°C
Some bacteria
survive in
deep sea
hydrothermal vent
at >
110°C
Enzyme application_thermophilic
Effects of Temperature
Each enzyme
Has an optimal temperature in which it
can function best
Optimal temperature for
typical human enzyme
Optimal temperature for
enzyme of thermophilic
Rate of reaction
(heat-tolerant)
bacteria
0
20
40
Temperature (Cº)
(a) Optimal temperature for two enzymes
80
100
pH – a measure of the conc of H+
Optimal pH for different enzymes
Optimal pH for pepsin
(stomach enzyme)
Rate of reaction
Optimal pH
for trypsin
(intestinal
enzyme)
3
4
0
2
1
(b) Optimal pH for two enzymes
5
6
7
8
9
Effect of pH on enzyme / protein
Effect of pH on enzyme / protein
Effect of pH on enzyme
Coenzymes
-- Non-protein molecules that are enzyme helpers
Some enzymes work only in the
presence of other molecules
E.g. NAD+ in dehydrogenation
Some vitamins are important because
they are co-enzymes e.g. Vitamin B
groups
Co-enzyme e.g. NAD+
NAD+ helps the enzyme dehydrogenase
accepting H removed from substrates
Denaturation and renaturation
of enzyme
Denaturation of enzyme
Denaturation of enzyme
It is the loss of the specific tertiary structure
of a protein molecule.
Lose its normal biological function.
The change may be temporary or permanent, but
the amino acid sequence (primary structure) of
the protein could remain un________ --renaturation, though rare, might occurs and
restore the enzyme function.
Factors causing protein denaturation
Factor
Explanation
Example
Heat or
radiation
(UV, infraRed)
High energy causes the atoms of the protein to
vibrate more (increased kinetic energy), thus
breaking the weak hydrogen and Ionic bonds
Coagulation of albumen (boiling eggs makes the
white more fibrous and less soluble)
Acids
Additional H+ ions in acids combine with COOgroups on amino acids and form COOH. Ionic
bonds are hence broken
The souring of milk by acid (e.g. Lactobacillus
bacterium, produces lactic acid, lowering pH and
causing it to denature the casein, making it
insoluble and thus forming curds)
Alkalis
Reduced number of H+ ions causes NH3+
groups to lose H+ ions and form NH2. Ionic
bonds are hence broken
Inorganic
chemicals
The ions of heavy metals such as mercury and
silver are highly electropositive. They combine
with COO- groups and disrupt ionic bonds.
Similarly, highly electronegative ions, e.g.
cyanide (CN-), combine with NH3+ groups and
disrupt Ionic bonds
Many enzymes are inhibited by being denatured in
the presence of certain ions, e.g. cytochrome
oxidase (respiratory enzyme) is inhibited by
cyanide
Organic
chemicals
organic solvents have a affinity for the
hydrophobic surfaces of the protein and this
results in denaturation of the protein along with
precipitation
Alcohol denatures certain bacterial proteins. This
is what makes it useful for sterilization
Mechanical
force
Physical movement may break hydrogen bonds
On stretching a hair, the hydrogen bonds in the
keratin helix are broken. The helix is extended and
the hair stretches. If released, the hair returns to its
normal length. If, however, it is wetted and then
dried under tension, it maintains its new length the basis of hair styling
Factors causing protein denaturation
Factor
Explanation
Example
Heat or
radiation
(UV, infraRed)
High energy causes the atoms of the protein to
vibrate more (increased kinetic energy), thus
breaking the weak hydrogen and Ionic bonds
Coagulation of albumen (boiling eggs makes the
white more fibrous and less soluble)
Acids
Additional H+ ions in acids combine with COOgroups on amino acids and form COOH. Ionic
bonds are hence broken
The souring of milk by acid (e.g. Lactobacillus
bacterium, produces lactic acid, lowering pH and
causing it to denature the casein, making it
insoluble and thus forming curds)
Alkalis
Reduced number of H+ ions causes NH3+
groups to lose H+ ions and form NH2. Ionic
bonds are hence broken
Inorganic
chemicals
The ions of heavy metals such as mercury and
silver are highly electropositive. They combine
with COO- groups and disrupt ionic bonds.
Similarly, highly electronegative ions, e.g.
cyanide (CN-), combine with NH3+ groups and
disrupt Ionic bonds
Many enzymes are inhibited by being denatured in
the presence of certain ions, e.g. cytochrome
oxidase (respiratory enzyme) is inhibited by
cyanide
Organic
chemicals
organic solvents have a affinity for the
hydrophobic surfaces of the protein and this
results in denaturation of the protein along with
precipitation
Alcohol denatures certain bacterial proteins. This
is what makes it useful for sterilization
Mechanical
force
Physical movement may break hydrogen bonds
On stretching a hair, the hydrogen bonds in the
keratin helix are broken. The helix is extended and
the hair stretches. If released, the hair returns to its
normal length. If, however, it is wetted and then
dried under tension, it maintains its new length the basis of hair styling
Enzyme Inhibitor_types
Inhibitor
Reversible
inhibitor
Competitive
inhibitors
irreversible
inhibitor
Non-competitive
inhibitors
Competitive inhibition
Competitive inhibition
Competitive inhibition–e.g. sulphonamide
Non competitive inhibition
Com vs non comp inhibition
End product inhibition –helps regulate
enzyme activities
End product inhibition
End product_cellular respiration
Allosteric activator
Allosteric inhibitor
Specific Localization of Enzymes
Within the cell, enzymes may be
Grouped into complexes
Incorporated into membranes
Contained inside organelles
Mitochondria,
sites of cellular respiraion
1 µm
•
Is 37 C the optimum temperature for
human enzyme?
•
Why then our body temperature is 37 C?
•
Are all enzymes denatured above 80 C?