Transcript LECT23 Enz1

ENZYMES
ENZYMES ARE PROTEINS
Polymers of amino acids
Heat-sensitive
Absorb light at 280 nm
ENZYMES ARE CATALYSTS
Reaction accelerators
Lower energy of activation of reactions
Stabilize the transition state
Not consumed in the reaction
There are two components to a chemical reaction: (1) its kinetic or rate component, and
its thermodynamics, or energy component. When we deal with enzymes, we are
concerned primarily with the rate component.
Do’s and Don’t’s for Enzymes
Do
1. Lowers energy of activation of forward and
reverse reactions
2. Speeds up the time required to reach equilibrium
or completion
3. Stabilizes the Transition State
4. Conducts a specific reaction with no side products
DON’T’S
1. Changes the DG of a Reaction
2. Changes the Equilibrium Position of a Reaction
3. Changes the Energy Yield (or requirement)
of a Reaction
STOP
ENZYME TALK
1. Enzyme: A protein or RNA molecule that has the property of a catalyst, sometimes
called a biocatalyst.
2. Substrate: The target of the enzyme’s action. The molecule that will undergo
chemical change as a result of the enzyme
3. Enzyme activity: A measure of the enzymes catalytic effectiveness as manifested by
the rate of the reaction catalyzed.
4. Cofactor: A component that works with the enzyme in effecting catalysis. Literally,
any chemical factors the assists the activity of an enzyme-catalyzed reaction.
5. Coenzyme: Related to cofactor, but generally used to describe molecules that are
derived from B-vitamins.
6. Enzyme kinetics: A branch of enzymology that deals with mechanism as studied by
factors that affect the rate of enzyme reactions.
7. Active Site: That region on the enzyme surface or within a crevice where the
substrate binds and catalysis is performed.
HA-HB + B
+
HA + HB-B
+
Transition State
Free
Energy
DG
HA-HB + B
Free energy of
Activation
HA + HB-B
Reaction Coordinate
No change in
free energy
of reaction
HA-HB + B
Free energy of
activation of
forward reaction
HB-B + HA
DGf
DGf
Free energy change
of reaction
DG
DGr
Free energy of
activation of
reverse reaction
DGr
Reaction Coordinate
Enzymes low the energy of activation of both forward and
reverse reactions. They do not affect the overall free energy
Multiple Step Reactions (multi-transition States)
A+B
D-glucose + CH3OH
(open chain)
[A*]
A + B
[-D-glucose]
Methyl-D-glucoside
(hemiacetal)
Two Steps
Slow step
(Rate-determining)
A+B
DG
A*
(hemiacetal)
A-B
(methyl-D-glucoside)
A
B
Transition state
Energy of activation
of forward uncatalyzed reaction
Energy
DG
A
B
A
Equilibrium
B
position
Reaction Coordinate
A reaction at equilibrium requires energy
to move the reaction off the equilibrium position
ENZYME CLASSIFICATION
1. TRIVIAL
2. REACTION TYPE AND SUBSTRATE
3. SYSTEMATIC NOMENCLATURE
TRIVIAL (Oldest)
Digestive Fluids, Bacteria, Plants
Trypsin
Chymotrypsin
Pepsin
Cathepsin
Lysozyme
Papain
Thermolysin
Appearance
Old Yellow Enzyme - Otto Warburg
Kuhn’s “enzyme” meaning “in yeast”
SUBSTRATE - REACTION TYPE
1. Enzymes given “ase” suffix
2. Substrate first, then reaction type
Example
CH3CH2OH
CH3CHO
Product is an aldehyde
Substrate is alcohol
Reaction removes hydrogen atoms
ALCOHOL DEHYDROGENASE
PROBLEM: TOO MANY REACTIONS TYPES
Dehydrogenases
Proteases
Lipases
Esterases
Acyltransferases
Deaminases
Reductases
Glycosidases
Synthetases
Isomerases
Synthases
Epimerases
DNAses
Amylases
RNAses
Peptidases
Nucleosidases
ENZYME CLASSIFICATION
Systematic Nomenclature
1.0
OXIDOREDUCTASES
2.0
TRANSFERASES
3.0
HYDROLASES
4.0
LYASES
5.0
ISOMERASES
6.0
LIGASES
SYSTEMATIC NOMENCLATURE
CH3CH2OH
NAD+
CH3CHO + NADH + H+
EC1.1.1.1. To see why, study the Table below.
1.
Oxidoreductases (oxidation-reduction reactions of all types)
1.1
1.1.1
1.1.1
acting on CH-OH group of substrates
requires NAD+ or NADP+ as hydrogen acceptor
specific substrate is ethyl alcohol
1.0 Oxidoreductases: (Add or remove electrons)
2.0 Transferases
(Transfer group to substrate)
CH2OH
HO-C-H
CH2OH
+ ATP
CH2OH
Glycerol
Glycerol phosphate
3.0 Hydrolases
(Cleave bonds with H2O)
CH2OH
CH2OH
HO-C-H
CH2OPO3=
+ ADP
HO-C-H
CH2OPO3=
+ H2O
Glycerol phosphate
HO-C-H
CH2OH
Glycerol
+ HPO4-
4.0 LYASE (split C-X without water; reverse forms
bond without a need for energy )
CH2OPO3
C=O
CH2OPO3
C=O
HO-C-H
split
HO-C-H
H-C-OH
H-C-OH
CH2OPO3
unite
Dihydroxyacetone phosphate
H
O
H
H-C-OH
C
Glyceraldehyde-3-phosphate
CH2OPO3
Fructose 1,6 bisphosphate
Split product of the forward or one substrate
of the reverse reaction must have a double bond
5.0 ISOMERASES (change groups around)
COOH
H C OH
CH2OPO3
3-phosphoglycerate
CH2OPO3
OH
H
HO
OH H
OH
H OH
glucose 6-phosphate
COOH
H C OPO3
CH2OH
2-phosphoglycerate
CH2OPO3
O CH2OH
H HO
H
H
HO H
fructose 6-phosphate
6.0 LIGASES (tie together…need energy)
CH3 C S CoA + HCO3 + ATP
O
acetyl-CoA
Synthetases
Synthases
HOOC CH2 C S CoA + ADP + HPO4
O
malonyl-CoA
Use ATP
No ATP
Caution: Synthases could be Lyases