edX Medicinal Chemistry - Chapter Four on enzymes
Chapter four sheds lights on enzymes, which represents a large class of targets of small-molecules.
Michaelis-Menten kinetics
This part is covered well by biochemical introductory texts. It is essential to fully understand (one form of) the Michaelis-Menten equation:
where
Once this equation is understood, its linear transformation, the Lineweaver-Burk equation, is easy to understand:
Enzynmic inhibitors
Drug-mediated enzymic inhibition is mostly reversible. We differentiate between three types of reversible inhibition:
- Competitive inhibition
- Noncompetitive inhibition
- Uncompetitive inhibition
Competitive inhibition
The inhibitor binds to the active site of the enzyme in competitive inhibition. In competitive inhibition,
Noncompetitive inhibition
The inhibitor binds to an allosteric site, namely a site other from the active site which nevertheless has a regulatory role of the enyzmic function. Note that in noncompetitive inhibition, the inhibitor can bind both the enzyme and the enzyme-substrate complex. This can modulate the enzymic conversion from the substrate to the product.
In noncompetitive inhibition, as the inhibitor’s concentration increases,
Uncompetitive inhibition
An uncompetitive inhibitor binds only to the enzymic-substrate complex, which reudces both
A short summary of reversible inhibitors
E stands for enzyme, ES stands for enzyme-substrate complex.
Type | Binding target | ||
---|---|---|---|
Competitive inhibitor | Active site of E | Unchanged | Increased |
Noncompetitive inhibitor | Allosteric site of E | Decreased | Unchanged |
Uncompetitive inhibitor | ES | Decreased | Decreased |
Irreversible inhibitors
Irreversible inhibitors will chemically react with the enzyme and covalently bind to it. Off-target effects of such inhibitors may cause side effects and other problems. However, recently attentions have also been paid to such inhibitors, especially in certain settings such as oncology and infectious disease. For instance, suicide inhibition happens when the enzyme converts the inhibitor into a reactive form in its active site.
Enzymic inhibition
Two types of measures were introduced:
IC50
Ki
where
One apparent advantage of using
Cheng-Prussoff equation
It is obvious that
Further notes on Ki and IC50
Often
Only