Properties of Enzymes
Enzymes are catalysts
What properties would
ideal catalysts have?
Enzymes are catalysts
What properties would
ideal catalysts have?
1. High degree of specificity for their substrates.
2. Accelerate chemical reactions tremendously.
3. Function in mild conditions.
4. Be recycled to participate again.
A Few Definitions
Cofactor – additional chemical component needed for catalysis.
- often an inorganic metal ion (mineral).
Coenzyme – complex organic molecule needed for catalysis.
- often a vitamin.
Prosthetic group – non amino acid portion of the enzyme
needed for catalysis. Often a coenzyme or metal ion.
Holoenzyme – complete catalytically active enzyme, with all
necessary prosthetic groups.
Apoenzyme – The protein part of the holoenzyme.
Prosthetic groups are absent.
Six Classes of Enzymes
1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases
One bond
to oxygen
Two bonds
to oxygen
Stored
electrons
Amino group transferred
Water did chemistry
to break a bond
CO2 was removed
5.
5.
Amino group switched places
6.
6.
Two substrates were ligated together
Enzyme Kinetics
E = enzyme
S = substrate
P = product
ES = enzyme-substrate complex
k = rate constant
The rate of reaction is dependent on enzyme concentration
[Enzyme] <<< [substrate]
Figure 5.2
Velocity,
or how fast
the reaction
is going
Concentration of enzyme
When measuring rates
of enzyme-catalyzed
reactions, initial
velocity (vo) is
measured.
Figure 5.3
Enzyme kinetics terminology
[S] – substrate concentration
Vo – initial velocity of a reaction. A significant amount of
substrate has not yet been converted to product.
Vmax – maximal velocity of a reaction. Addition of more
substrate will not increase the rate of the reaction.
Km – The concentration of substrate at which the rate
of the reaction is half-maximal
Michaelis-Menten equation
Page 141
The rate of reaction is dependent on substrate concentration
Figure 5.4
Km values are often just above the substrate concentration
in a cell. Rates of reaction are sensitive to small changes
in cellular substrate concentrations.
kcat is a measure of the number of substrate molecules
converted to product per second per enzyme molecule
Experimental determination
of kinetic constants
Figure 5.6
Reversible Enzyme Inhibition
An inhibitor is a compound that binds to an enzyme and
interferes with its activity. Many drugs are enzyme inhibitors.
An inhibitor is characterized by an inhibition constant (Ki).
No Inhibitor
Figures 5.8 and 5.9
No Inhibitor
Many competitive inhibitors
are substrate analogs
Benzamidine is an inhibitor of the enzyme trypsin
Many competitive inhibitors are substrate analogs.
Compound (b) designed as an inhibitor of the enzyme
purine nucleoside phosphorylase, that utilizes guanosine (a) as
a substrate. (b) is a possible drug for the treatment of arthritis.
Figure 5.13
Irreversible Enzyme Inhibition
Some inhibitors are compounds
that form a stable covalent
bond with the target enzyme.
DFP inactivates serine proteases
by covalently modifying an
active site serine residue.
Figure 5.15
F-
Regulation of enzyme activity by metabolite concentration
Activator
Regulator ADP binds at an allosteric site,
Separate from the active site
The activity of the some enzymes is
regulated by reversible phosphorylation.
Example: pyruvate dehydrogenase
Figure 5.22