Point Power to Accompany
Principles and Applications of
Inorganic, Organic, and Biological
Chemistry
Denniston, Topping, and Caret
4th ed
Chapter 20
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
20-1
Introduction
The proteins which serve as enzymes, Mother
Natures catalysts, are globular in nature.
Because of their complex molecular
structures, they often have exquisite
specificity for their substrate molecule and
can speed up a reaction by a factor of
millions relative to an uncatalyzed reaction.
This presentation will describe how
enzymes function.
20-2
20.1 Nomenclature and Classification
Oxidoreductases catalyze redox reactions.
Eg. Reductases or oxidases
-
COO
COO
+ Lactate
CO
HO C H + NAD
dehydrogenase
CH3
CH3
+ NADH + H+
Transferases transfer a group from one
molecule to another. Eg. Transaminases
catalyze transfer of an amino group, kinases
a phosphate group
HO
HO
CH CH2NH2
OH
PNMT
HO
HO
CH CH2NH CH3
OH
20-3
Enzyme Classes, cont.
Hydrolases cleave bonds by adding water.
Eg. Phosphatases, peptidases, lipases
Protein + H2O
peptidase amino acids
Lyases catalyze removal of groups to form
double bonds or the reverse. Eg.
decarboxylasaes or synthases
O C O+H O H
carbonic
anhydrase
OH
O C2O4 H
0-
Enzyme Classes, cont.
Isomerases catalyze intramolecular
rearrange-ments. Eg. epimerases or
phosphoglycerate
mutases
CO O
HC O H
2CH2O PO3
mutase
CO O
2HC O PO3
CH2OH
Ligases catalyze a reaction in which a C-C,
C-S, C-O, or C-N bond is made or broken.
O
DNA strand-3'-OH + - O P O -5'-DNA strand
DNA ligase O
O
DNA strand-3'-O- P O -5'-DNA strand
-O
20-5
Nomenclature
With some historical exceptions, the name for
an enzyme ends in ase.
The common name for a hydrolase is derived
from the substrate. E. g.
Urea: -a + ase = urease
Lactose: -ose + ase = lactase
Other enzymes may be named for the reaction
they catalyze. E. g.
Lactate dehydrogenase, pyruvate
decarboxylase
But: catalase, pepsin, chymotrypsin, tripsin
20-6
20.2 Enzymes and Activation Energy
Transition state
Activation energy
(Ea) for the reaction
Free
Reactants
Energy
Energy change
( H) for the reaction
Products
Reaction progress
An enzyme speeds a reaction by lowering the
activation energy. It does this by changing
the reaction pathway.
20-7
20.3 Substrate Concentration
Rates of uncatalyzed reactions increase as
the concentration increases.
Rates of enzyme catalyzed reactions behave
as shown below. The first stage is the
formation of an enzyme-substrate complex
followed by slow conversion to product.
Rate is limited by enzyme availability.
20-8
20.4 Enzyme-Substrate Complex
The following reversible reactions represent
the steps in an enzyme catalyzed reaction.
The first step involves formation of an
enzyme-substrate complex, E-S.
E-S* is the transition and E-P is the enzymeproduct complex.
Step I
Step II
E+S
E-S
Step IV
E+P
E-P
E-S*
Step III
20-9
Enzyme-Substrate Complex, cont.
The part of the enzyme combining with the
substrate is the active site. Active sites are:
Pockets or clefts in the surface of the
enzyme. R groups at active site are called
catalytic groups.
Shape of active site is complimentary to the
shape of the substrate.
The enzyme attracts and holds the enzyme
using weak noncovalent interactions.
Conformation of the active site determines
the specificity of the enzyme.
20-10
Enzymes Models
In the lock-and-key model, the enzyme is
assumed to be the lock and the substrate
the key. The two are made to fit exactly.
This model fails to take into account the fact
that proteins can and do change their
conformations to accommodate a substrate
molecule.
The induced-fit model of enzyme action
assumes that the enzyme conformation
changes to accommodate the substrate
molecule.
20-11
Enzymes Models, cont.
Insert Fig 20.3
20-12
20.5 Specificity of the E-S Complex
Absolute: enzyme reacts with only one
substrate.
Group: enzyme catalyzes reaction involving
molecules with the same functional group.
Linkage: enzyme catalyzes the formation or
break up of only certain bonds.
Stereochemical: enzyme recognizes only one
of two enantiomers.
20-13
20.6 Transition State and Product
As the substrate interacts with the enzyme,
its shape changes and this new shape is
less energetically stable. This transition
state has features of both substrate and
product and falls apart to yield product
which dissociates from the enzyme.
1. The enzyme might put stress on a bond.
2. The enzyme might bring two reactants into
close proximity and proper orientation.
3. The enzyme might modify the pH of the
microenvironment, donating or accepting
a H+.
20-14
20.7 Cofactors and Coenzymes
Polypeptide of portion enzyme (apoenzyme)
and nonprotein prosthetic group (cofactor)
make up the active enzyme (holoenzyme).
Cofactors may be metal ions, organic
compounds, or organometallic compounds.
A coenzyme, an organic molecule temporarily
bound to the enzyme, is required by some
enzymes. Most coenzymes carry electrons
or small groups.
20-15
Vitamin
Coenzyme
Process
Thiamine(B1)
TPP
decarboxylation
Riboflavin(B2)
FMN, FAD
carry H atoms
Niacin(B3)
NAD(P)+
hydride carrier
Pyradoxine(B6)
Pyridoxal P
amino group
transfer
Folic acid
tetrahydrofolate one-carbon
transfer
Vit A
retinal
vision, growth
Biotin
biocytin
CO2 fixing
20-16
Coenzymes, cont
Nicotinic acid
nicotinamide
H
(niacin) is
involved in
O
redox
+
OPO
N
O
reactions.
O
OPO
O
NAD+
OH OH
O
O
C NH
2
NH2
N
N
N
N
(NADP+)
OH OH (PO32-)
20-17
Coenzymes, cont.
The nicotinamide part of NAD+ accepts a
hydride (H plus two electrons) from the
alcohol to be oxidized. The alcohol loses a
proton to the solvent.
H
O
C NH2
H
+
N + H O C R1
R
H
Ox form
HO
H
ox
red
C NH2
N
R
+
O C R1
H
+ H+
Red form
20-18
Coenzymes, cont.
Flavin
coenzymes
also serve in
redox
reactions
Flavin
adenine
dinucleotide
Flavin
mononucleotide
FAD
FMN
H 3C
H 3C
O
OPO
O
OPO
O
CH2
H COH
H COH
H COH
CH2
N
N
O
OH
Adenine
OH
O
NH
N
O
20-19
Coenzymes, cont,
The flavin coenzymes accept electrons in the
flavin ring system.
H3C
H 3C
H HO
_O
_
OCC CCO
HH
R
N
N
O
NH
N
O
FAD
O
OCC
H
R
N
HO
_
CCO
_
H3C
H3C
H
N
NH
N
H
O
O
FADH2
20-20
20.8 Environmental Effects
An enzyme has an
optimum temperature
that is usually close to
the temperature at which
it normally works, ie.
37 oC for humans.
Excessive heat can
denature a protein.
20-21
Environmental Effects, cont.
Enzymes work best
at the correct
physiological pH.
Extreme pH
changes will
denature the
enzyme. Pepsin
(stomach) and
chymotrypsin
(small intestine)
have different
optimum pHs.
pepsin
Chymotrypsin
20-22
20.9 Regulating Enzyme Activity
Some methods that organisms use to
regulate enzyme activity are:
1. Produce the enzyme only when the
substrate is present.
2. Allosteric enzymes
3. Feedback inhibition
4. Zymogens
5. Protein modification
20-23
Allosteric Enzymes
Effector molecules change the activity of an
enzyme by binding at a second site.
Some effectors speed up enzyme action
(positive allosterism)
Some effectors slow enzyme action
(negative allosterism)
E. g. The third reaction of glycolysis places a
second phosphate on fructose-6-phosphate.
ATP is a negative effector and AMP is a
positive effector of the enzyme
phosphofructokinase.
20-24
Allosteric Enzymes
Insert Fig 20.11
20-25
Regulation, cont.
With feedback inhibition, a product in a series
of enzyme-catalyzed reactions serves as an
inhibitor for a previous allosteric enzyme in
the series.
A zymogen is a preenzyme. It is coinverted to
its active form, usually by hydrolysis, at the
active site in the cell. E. g. Pepsinogen is
synthesized and transported to the stomach
where it is converted to pepsin.
The most common form of protein
modification is addition or removal of a
phosphate group.
20-26
20.10 Inhibition of Enzyme Activity
Irreversible inhibitors bind tightly, sometimes
even covalently, to the enzyme and thereby
prevent formation of the E-S complex.
Reversible competitive inhibitors often
structurally resemble the substrate and bind
at the normal active site
Reversible noncompetitive inhibitors usually
bind at someplace other than the active site.
Binding is weak and thus inhibition is
reversible.
20-27
20.11 Proteolytic Enzymes
Proteolytic enzymes cleave the peptide bond
in proteins.
They depend on a hydrophobic pocket.
Chymotrypsin cleaves the peptide bond at the
carboxylic end of methionine, tyrosine,
tryptophan, and phenylalanine.
HO
HO
HO
+
H3N C C N C C N C C O
HH
H
CH3
Chymotrypsin
cleaves here
20-28
Proteolytic Enzymes, cont.
Chymostypsin: just seen
Trypsin cleaves on the carboxyl side of basic
amino acids.
Elastase cleaves on the carboxyl side of
glycine and alanine.
These enzymes have different pockets but the
catalytic sites remain unchanged during
evolution and the mechanism of proteolytic
action is the same for all these serine
proteases.
20-29
20.12 Enzymes in Medicine
Diagnostic
Heart attack: uses levels of lactate
dehydrogenase, creatine phosphate, and
serum glutamate-oxaloacetate
transaminase
Pancreatitis: elevated amylase and lipase
Analytical Reagents
Urea converted to ammonia via urease and
then blood urea nitrogen (BUN) measured.
Replacement Therapy
Administer genetically engineered glucocerebrosidase for Gauchers
disease.
20-30
THE END
Enzymes
20-31