Enzymes are proteins that act on substrates, catalyzing chemical reactions within the cell. Enzymes are specific in the sense that each enzyme only reacts with a few closely related substrates. Some enzymes require cofactors (biotin, lipoamide, cobalamin) to function properly. Enzymes can become denatured by changes in temperature or pH. Enzymes are classified as oxidoreductases, transferases, hydrolases, lyases, ligases, and isomerases, based on the type of reaction they catalyze. Enzyme kinetics is the study of enzyme reaction rates, which are determined using the Michaelis-Menten and Lineweaver-Burk equations. These equations can be also used to evaluate how different types of enzyme inhibitors affect the reaction rate. Enzyme deficiencies can result in severe diseases such as Lesch-Nyhan syndrome, Gaucher disease, and phenylketonuria.
Enzymes are complex proteins that catalyze chemical reactions. Enzymes act on substrates that can either be cleaved or joined to form a new product (e.g., carbonic anhydrase enzyme → CO2 + H20 ⇄ H2CO3). They are essential for life, if enzymes did not exist, cellular reactions would not occur fast enough to sustain life. Thus, enzyme deficiencies can result in severe diseases (e.g., ).
The name of enzymes is usually based on the reaction catalyzed plus the suffix -ase. For example, the name of the enzyme that adds hydroxyl groups (OH-) formed as follows: hydroxyl + -ase → hydroxylase.
- Active site: binding site for a specific substrate on a specific enzyme
- Specificity: Enzymes are highly specific for their substrate and product.
- Rate: enzymes catalyze reactions by a factor of 106–1011
Coenzymes: many enzymes require coenzymes (e.g., biotin) that allow them to perform their action on a substrate.
- Usually small organic molecules derived from metal ions or vitamins
- Enzymes do not affect the energy level of substrates or products (free energy released remains the same).
- Enzymes are able to decrease the energy of activation required to start a reaction.
- The velocity of enzymatic reactions increases with temperature (up to 37o C in humans).
- Temperatures > 37o C slow down enzymatic reactions and can result in denaturation of enzymes.
- pH: Each enzyme has a specific pH at which it can achieve maximum velocity (Vmax).
Energy (∆G) for enzymatic reactions usually comes from the break down of ATP or GTP bonds (hydrolysis). Enzymatic reactions can occur spontaneously or nonspontaneously. The following are relationships between energy and enzymatic activity.
|Oxidoreductases|| || |
|Hydrolases|| || |
|Isomerases|| || |
|Base molecule||Transferred group||Carrier of energy||Released energy||Metabolic site|
|ADP||Phosphate||-31 KJ/mol|| |
|Thiamine pyrophosphate (TPP)||Vitamin B1||Oxidative decarboxylation|
|FMN/FMNH2||Vitamin B2||Electron transfer|
|FAD+/FADH2||Vitamin B2||Electron transfer|
|NAD+/NADH||Vitamin B3||Electron transfer|
|NADP+/NADPH||Vitamin B3||Electron transfer|
|Coenzyme A||Vitamin B5||Acyl group transfer|
|Pyridoxal phosphate||Vitamin B6||Transamination, dehydration|
|Biotin||Vitamin B7||Carboxyl group transfer|
|Tetrahydrofolate||Vitamin B9||Methyl group transfer|
|Cobalamin||Vitamin B12||Alkyl group transfer|
|S-Adenosylmethionine (SAM)||Methyl group transfer|
|Ascorbic acid||Vitamin C|| |
Electron transfer and hydroxylation
|Phylloquinone||Vitamin K||Electron and carboxyl group transfer|
Electron and oxygen atom transfer
|ATP||Phosphate group transfer|
For more information, see.
|Gluconeogenesis|| || |
|Pyrimidine synthesis|| |
|Urea cycle|| |
|Fatty acid synthesis|| |
| || |
|Cholesterol synthesis|| |
[E] = enzyme, [S] = substrate, [P] = product, [V] = velocity
- E + S ⇄ ES → E + P
- Maximum velocity (Vmax): maximum rate at which an enzyme can catalyze a reaction
- Michaelis constant: (Km): the substrate concentration at which half of the active sites of the enzymes are bound to the substrate
- Michaelis-Menten equation: v = Vmax [S] / (Km + [S])
Lineweaver-Burk equation and plot
The Lineweaver-Burk equation is a double reciprocal of the Michaelis-Menten equation; , where V = Vmax [S] / Km + [S] (if [E] remains constant), becomes 1 / v = Km / Vmax× 1/[S] + 1 / Vmax. It represents enzyme kinetics in a linear graph rather than a hyperbola. This equation is particularly important to determine the effect of drugs on enzymes.
- Intercept with y-axis: 1/Vmax: the further from zero, the lower Vmax
- Intercept with x-axis: 1/-Km : the closer to zero, the lower the affinity
- Slope: Km/Vmax
The details of pharmacodynamics are explained in the learning card on the .
|Parameter||Uncompetitive inhibitors|| |
Competitive inhibitors (reversible)
Competitive inhibitors (irreversible)
|Similar to the substrate|| || || || |
|Effect of increased [S]|| || || || |
|Binding site|| || |
|Effect on Km|| || || || |
|Effect on Vmax|| || || || |
|Pharmacodynamic effect|| || || || |
For more information on the effects of inhibitors andsee .
Uncompetitive inhibitors are enzyme inhibitors that bind to the enzyme-substrate complex, decreasing Km and Vmax.