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Enzymes are catalystsWhat properties wouldideal 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
The rate of reaction is dependent on enzyme concentration[Enzyme] <<< [substrate]
Figure 5.2
Velocity,or how fastthe reaction
is going
Concentration of enzyme
[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 moresubstrate will not increase the rate of the reaction.
Km – The concentration of substrate at which the rate of the reaction is half-maximal
Enzyme kinetics terminology
Km values are often just above the substrate concentrationin a cell. Rates of reaction are sensitive to small changes
in cellular substrate concentrations.
kcat is a measure of the number of substrate moleculesconverted to product per second per enzyme molecule
Reversible Enzyme Inhibition
An inhibitor is a compound that binds to an enzyme andinterferes with its activity. Many drugs are enzyme inhibitors.
An inhibitor is characterized by an inhibition constant (Ki).
Many competitive inhibitors are substrate analogs.Compound (b) designed as an inhibitor of the enzyme
purine nucleoside phosphorylase, that utilizes guanosine (a) asa substrate. (b) is a possible drug for the treatment of arthritis.
Figure 5.13
F-
DFP inactivates serine proteasesby covalently modifying anactive site serine residue.
Figure 5.15
Irreversible Enzyme InhibitionSome inhibitors are compounds
that form a stable covalentbond with the target enzyme.
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