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Enzymes and Vitamins
http://www.wiley.com/college/pratt/0471393878/student/animations/enzyme_inhibition/index.html
http://www.wiley.com//legacy/college/boyer/0470003790/animations/animations.htm
Catalysis by Enzymes
• Unlike inorganic chemistry reactions in living organisms
take place in a very narrow
range of temperature and ph
conditions
• Fireflies must produce light
without raising the temperature
beyond physiological limits
Luciferinase, an enzyme found in firefly tails,
yields light as a reaction product.
Enzymes
Enzymes catalyze reactions essential for life. Many of
these life sustaining reactions may not occur in the
absence of enzyme action under physiological
conditions. Therefore, the critical nature of enzymes
quickly becomes apparent
The enzyme lactate dehydrogenase
acts upon the substrate L-lactate
Substrate: A reactant in an enzyme-catalyzed
reaction
Enzyme: A protein or
other molecule that
acts as a catalyst for a
biological reaction.
Enzymes have active sites that
provide Specificity
1.
2.
3.
A pocket in an enzyme with the
specific shape and chemical
makeup necessary to bind a
substrate
The active site has an specific
shape and chemical reactivity
needed to catalyze the reaction
It holds one or more substrates in
place by attractions to groups that
line the pocket
Link
4. Within the folds of an
enzyme’s protein
chain is the active
site— the region
where the reaction
takes place
Specificity is a matter of fit
1. Because of the need to “fit”
into a fold, active sites are
very specific – and enzymes
work on only one
enantiomer
2. The enantiomer at the top
fits the reaction site like a
hand in a glove, but the
enantiomer at the bottom
does not.
– The enzyme lactate
dehydrogenase catalyzes the
removal of hydrogen from Llactate but not from D-lactate.
Turnover Number
Maximum number of substrate molecules acted upon per
enzyme per unit time.
Most enzymes turn over 10–1000 molecules per second.
7
Enzyme Cofactors
• The dehydrogenation of L- lactate shown requires a coenzyme
which acts as an oxidizing agent that makes the reaction
possible.
– (NAD+ accepts the H ; “donating electrons”, so it is reduced, this makes
it an oxidizing agent – the L-lactate is oxidized)
What is a Cofactor?
• Cofactor: A nonprotein
part of an enzyme
essential to catalytic
activity; often a metal
ion or a coenzyme.
• Coenzyme: An organic
molecule that acts as an
enzyme cofactor.
The functional groups in proteins are limited to those of the amino
acid side chains. By combining with cofactors, enzymes acquire
chemically reactive groups not available as side chains
Oxidoreductases
1. Catalyze redox reactions of substrates
2. Addition or removal of oxygen or hydrogen.
3. These enzymes require coenzymes that are
reduced or oxidized as the substrate is
oxidized or reduced.
Transferases
Catalyze the transfer of a group from one molecule to
another.
Kinases transfer a phosphate group from ATP; producing
to give ADP and a phosphorylated product.
Hydrolases
• Catalyze the breaking of bonds with addition of
water.
• The digestion of carbohydrates and proteins by
hydrolysis requires these enzymes.
Ligases
Catalyze the bonding together of two substrates.
• These reactions are generally not require energy
from ATP hydrolysis.
Isomerases
Catalyze the rearrangement of atoms
of a substrate in reactions that have
but one substrate and one product.
Lyases
Catalyze the addition of a molecules to a double
bond or the reverse reaction in which a molecule
is eliminated from a double bond.
Enzyme Classification
Enzymes are divided into six main classes according to the
general kind of reaction they catalyze.
1.
2.
3.
4.
5.
6.
Oxidoreductases: addition or removal of oxygen
Transferases: move a group
Hydrolases: Breaking by adding water
Ligases: Bonding together
Isomerases: rearrangement
Lyases: addition to a double bond or the reverse
Classification expanded
Nomenclature
• Enzymes have the family-name ending -ase.
– (Exceptions occur for enzymes such as papain and
trypsin, which are still referred to by older common
names).
• Modern systematic names always have two
parts:
– the first identifies the substrate on which the
enzyme operates
– the second part is an enzyme subclass name
• Example: Pyruvate carboxylase is a ligase that
acts on the substrate pyruvate to add a
carboxyl group.
How Enzymes Work: Two Models
• lock-and-key model (Historical) The substrate is
described as fitting into the active site as a key fits
into a lock.
• Induced-fit: (Current Model) As an enzyme and
substrate come together, their interaction induces
changes in the shape of the active site, that results in
exactly the right fit for catalysis of the reaction.
How Enzymes Work: Two Models
• Induced-fit: (Current Model) As an enzyme and
substrate come together, their interaction induces
changes in the shape of the active site, that results in
exactly the right fit for catalysis of the reaction.
• Link
• Link :
http://www.chem.ucsb
.edu/~molvisual/ABLE/i
nduced_fit/index.html
• Link
• http://www.stolaf.edu/
people/giannini/flasha
nimat/enzymes/chemic
al%20interaction.swf
• Link names of enzymes
• http://scholar.hw.ac.uk
/site/biology/activity5.
asp?outline=no
Hydrolysis of a peptide bond by chymotrypsin.
a) The polypeptide
enters the active site
(b) Hydrogen transfer
allows formation of a
strained intermediate
(c) The peptide bond is broken.
Copyright © 2010 Pearson
Education, Inc.
Enzymes act as catalysts because of
their ability to:
– Bring substrate(s) and catalytic sites together (proximity
effect).
– Hold substrate(s) at the exact distance and in the exact
orientation necessary for reaction (orientation effect).
– Provide acidic, basic, or other types of groups required for
catalysis (catalytic effect).
– Lower the energy barrier by inducing strain in bonds in the
substrate molecule (energy effect).
– Link :
http://www.chem.ucsb.edu/~molvisual/ABLE/induced_fit/in
dex.html
Concentration & Enzyme Activity
• Rate of an enzyme catalyzed reaction is
controlled by the amount of substrate and the
overall efficiency of the enzyme.
• If the enzyme–substrate complex is rapidly
converted to product, the rate at which enzyme
and substrate combine to form the complex
becomes the limiting factor.
• Enzyme and substrate molecules moving at
random in solution can collide
with each other no
8
more often than about 10 collisions per
(mole/liter) per second.
• One of the most efficient enzymes is catalase, this
enzyme
can break down H2O2 at a rate of up to
7
10 catalytic events per second.
At Low Concentrations of substrate
• Not all enzyme is bound by substrate
the substrate and enzyme must first
“find” each other in solution
– Increasing S or E will impact the rate.
Therefore, both E and S appear in the
rate law
• The reaction cannot go any faster than
the rate at which E and S come
together
• Reaction rate is directly
proportional to substrate
concentration.
  
Rate  k E S
1
1
At Saturating conditions
• Enzyme is saturated with
Substrate.
• Increasing [S] will not
impact the rate.
• At saturated condition,
the rate of the reaction is
determined by the Kcat
That is – the rate of
catalytic turnover
  
Rate  k E S
1
0
The Michaelis-Menten equation (curve) can be converted to a
linear form that matches the general formula y = mx+b.
• In the presence of
excess substrate, the
concentration of an
enzyme can vary
according to our
metabolic needs.
– This is why enzymes are
considered metabolic
regulators
•
If the concentration of
substrate does not
become a limitation, the
reaction rate varies
directly with the enzyme
concentration.
The Michaelis-Menten equation (curve) can be converted to a
linear form that matches the general formula y = mx+b.
• In the presence of
excess substrate, the
concentration of an
enzyme can vary
according to our
metabolic needs.
– This is why enzymes are
considered metabolic
regulators
•
If the concentration of
substrate does not
become a limitation, the
reaction rate varies
directly with the enzyme
concentration.
Effect of Temperature and pH on
Enzyme Activity
• Enzyme catalytic activity is highly dependent on pH
and temperature.
• Optimum conditions vary slightly for each enzyme
but are generally near normal body temperature
and the pH of the body fluid in which the enzyme
functions.
– Pepsin, which initiates protein digestion in the highly
acidic environment of the stomach, has its optimum
activity at pH 2.
– Trypsin, which acts in the alkaline environment of the
small intestine, has optimum activity at pH 8.
(a) The rate increases with increasing temperature
until the protein begins to denature; then the rate
decreases rapidly. (b) The optimum activity for an
enzyme occurs at the pH where it acts.
30
Enzyme Regulation:
• A variety of strategies are utilized to adjust the
rates of enzyme-catalyzed reactions.
• Any process that starts or increases the action
of an enzyme is an activation.
• Any process that slows or stops the action of
an enzyme is an inhibition.
• Feedback and Allosteric Control are two
strategies for enzyme regulation.
Feedback control:
• Regulation of an enzyme’s activity by the product of
a reaction later in a pathway.
• If a product near the end of a metabolic pathway
inhibits an enzyme that functions near the beginning
of that pathway, then no energy is wasted making
the ingredients for a plentiful substance.
• Allosteric control: An interaction in which the
binding of a regulator at one site on a protein affects
the protein’s ability to bind another molecule at a
different site.
• Allosteric control can be either positive or
negative.
• Binding a positive regulator changes the active
sites so that the enzyme becomes a better
catalyst and the rate accelerates.
• Binding a negative regulator changes the active
sites so that the enzyme is a less effective catalyst
and the rate slows down.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
33
Enzyme Regulation: Inhibition
• Enzyme inhibition can be reversible or
irreversible.
• In reversible inhibition, the inhibitor can leave,
restoring the enzyme to its uninhibited level of
activity.
• In irreversible inhibition, the inhibitor remains
permanently bound and the enzyme is
permanently inhibited.
• The inhibition can also be competitive or
noncompetitive, depending on whether the
inhibitor binds to the active site or elsewhere.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
34
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
35
A competitive inhibitor can eventually be
overcome by higher substrate concentrations.
With a noncompetitive inhibitor the maximum
rate is lowered for all substrate concentrations.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
36
• Hg+2 and Pb+2 ions are irreversible inhibitors
that bond to the S atoms in cysteine residues.
• Organophosphorus insecticides such as
parathion and malathion, and nerve gases like
Sarin are irreversible inhibitors of the enzyme
acetylcholinesterase.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
37
19.9 Enzyme Regulation: Covalent Modification
and Genetic Control
• There are two general modes of enzyme
regulation by covalent modification, removal of a
covalently bonded portion of an enzyme, or
addition of a group.
• Some enzymes are synthesized in inactive forms
known as zymogens or proenzymes, activation
requires a chemical reaction that splits off part of
the molecule.
• Genetic (enzyme) control: Regulation of enzyme
activity by hormonal control of the synthesis of
enzymes is especially useful for enzymes needed
only at certain times.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
38
Enzymes that cause blood clotting or digest
proteins are examples of enzymes that must
not be active at the time and place of their
synthesis.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
39
Glycogen phosphorylase , the enzyme that
initiates glycogen breakdown, is more active
when phosphorylated. When glycogen stored in
muscles must be hydrolyzed to glucose for quick
energy, two serine residues are phosphorylated.
The groups are removed once the need to break
down glycogen for quick energy has passed.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
40
19.10 Vitamins
• Vitamin: An organic molecule, essential in
trace amounts that must be obtained in the
diet because it is not synthesized in the body.
• Scurvy, pellagra, and rickets are caused by
deficiencies of vitamins.
• Vitamins are grouped by solubility into two
classes: water-soluble and fat-soluble.
• Some vitamins are valuable as antioxidants.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
41
Water soluble vitamin C is biologically active without any
change in structure, biotin is connected to enzymes by an
amide bond at its carboxyl group but otherwise
undergoes no structural change from dietary biotin.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
42
Other water-soluble vitamins incorporate into coenzymes.
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Chapter Nineteen
43
• The fat-soluble vitamins A, D, E, and K are stored
in the body’s fat deposits.
• Although the clinical effects of deficiencies of
these vitamins are well documented, the
molecular mechanisms by which they act are not
nearly as well understood as those of the watersoluble vitamins. None has been identified as a
coenzyme.
• The hazards of overdosing on fat-soluble vitamins
are greater than those of the water-soluble
vitamins because of their ability to accumulate in
body fats.
• Excesses of the water-soluble vitamins are more
likely to be excreted in the urine.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
44
• An antioxidant is a substance that prevents
oxidation.
• In the body, we need protection against active
oxidizing agents that are by-products of
normal metabolism.
• Our principal dietary antioxidants are vitamin
C, vitamin E, b-carotene, and the mineral
selenium.
• They work together to defuse the potentially
harmful action of free radicals, highly reactive
molecular fragments with unpaired electrons.
Copyright © 2010 Pearson
Education, Inc.
Chapter Nineteen
45
• http://www.wiley.com/college/pratt/0471393
878/student/animations/enzyme_inhibition/i
ndex.html
• http://www.wiley.com//legacy/college/boyer/
0470003790/animations/animations.htm
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