Download Biochemistry: A Short Course

Enzymes are good catalysts
Why???
1. Enzymes have reaction specificity for a particular substrate
(Many enzymes also have stereo specificity)
2. Reactions with enzymes are typically 103 to 1020 times
faster than without enzymes.
3. Enzymes are biologically relevant because they function
in mild conditions (aqueous solution, pH~7, etc.)
4. Enzymes are good at coupling different reactions.
Hydrolyzes peptide bonds on carboxyl side
of aromatic and aliphatic residues
What do we want to know about enzymes??
1.
What reaction do they catalyze?
2.
How does the enzyme work. (mechanism)
3.
Is the enzyme regulated or controlled?
4.
What happens if the enzyme stops functioning?
5.
As in chapter 4, what are the structural features of the enzyme.
(e.g., complex or simple, 1o, 2o, 3o, 4o etc.)
6. Does the enzyme require a cofactor for activity?
Apoenzyme + Cofactor
(Protein only)
Inactive
Holoenzyme
(Active)
Note: not all enzymes are proteins, some are RNA based
Cofactors and the vitamin in supplements
The six major classes of enzymes
1. Oxidoreductases
2. Transferases
3. Hydrolyases
4. Lyases
5. Isomerases
Distribution of known enzymes
6. Ligases
1. Oxidoreductases catalyze oxidation-reduction reactions
oxidized
C is more reduced
(1 H and 1 O bonded)
reduced
Electron are stored
C is more oxidized
(2 bonds to oxygen)
Recall: Carbon is more reduced when more H’s are bonded,
while carbon is more oxidized with more O’s bonded.
Compare CH4 vs CO2
When something is oxidized, something else must be reduced
Electrons MUST be transferred. (NAD+ is reduce to NADH)
NAD+ - nicotinamide adenine dinucleotide
2. Transferases catalyze group transfer reactions
-These enzymes usually require a coenzyme
be present.
-coenzyme: complex organic molecule (often a vitamn)
needed for catalysis
ammonium group transfer
3. Hydrolyases catalyze hydrolysis.
-A special class of transferase- water is the key acceptor
of the group transferred.
Water is added to break a one of the
-P-O-P- bonds.
4. Lyases catalyze lysis of a substrate. A double bond is
generated.
-The reverse reaction is called an addition reaction
the enzymes are synthases.
CO2 was removed (a double bond was formed)
5. Isomerase catalyze structural changes within a single molecule
-Simply an isomerization reaction
Ammonium group switched places
6. Ligases catalyze ligation or joining of two substrates
-These reactions are usually coupled with a second reaction.
(chemical potential energy, or stored energy, from the second
reaction is used to drive the first reaction.)
Stored chemical energy
Two substrates
were ligated
The second reaction: ATP ADP + Pi
DGo’-The Standard Free Energy at pH = 7
- A measure of where equilibrium lies for a reaction
- If DGo’ < 0, [products] > [reactants]
- If DGo’ > 0, [reactants] >[products]
- If DGo’ = 0, [products] = [reactants]
- Many biochemical reactions are near equilibrium.
- Some biochemical reactions are irreversible or
“spontaneous” and is largely dependent on conc.
- THE ENZYME DOES NOT ALTER FREE ENERGY
DG╪ IS a measure of Enzymatic “Speed”
The energy of activation
Enzymes help to
lower the barrier
as the reaction
progresses
Active sites may include distant amino acid residues
Figure 6.4
The transition state is
energetically more favorable
once the enzyme-substrate
complex is formed.
- Active site is small
- Active sites are unique
environments
- Substrate binds via noncovalent interactions
Figure 6.6- Induced-fit model of enzyme-substrate binding
The enzymes structure
changes as the substrate binds
Specificity of binding depends
on arrangement and type of
atoms in active site
Glucose binding to the
enzyme hexokinase
The Induced Fit
Assignment
Read Chapter 6
Read Chapter 7
Topics not covered:
Details of ΔG equations on pages 97 and 98