Rate of enzyme mediated reactions

The rate of enzyme mediated reactions is the rate of chemical reactions mediated by enzymes.

Role of enzymes

Enzymes can increase reaction rate by favoring or enabling a different reaction pathway with a lower activation energy, making it easier for the reaction to occur.

Diagram of a catalytic reaction, showing the energy needed (E) against time (t).

The substrates (A and B) need a large amount of energy (E₁) to reach the transition state A^...B, which then reacts to form the end product (AB). The enzyme (E) creates a microenvironment in which A and B can reach the transition state (A^...E^...B) more easily, reducing the amount of energy needed (E₂). As a result, the reaction is more likely to take place, thus improving the reaction speed.

Factors

The overall rate of enzyme mediated reactions depends on many factors including:

Temperature

All chemical reactions speed up as temperature is raised.

Extremes of temperature can denature an enzyme so that it can no longer function.

Enzyme concentration

De novo synthesis (the production of more enzyme molecules) increases catalysis rates.

pH

Extremes of pH can denature an enzyme so that it can no longer function.

Salt concentration

Extremes of salt concentration can inactivate an enzyme.

Posttranslational modification

More specific regulation of reaction rate is possible by posttranslational modification (e.g., phosphorylation) of the enzyme or by cofactors like metal ions or organic molecules (e.g., NAD⁺, FAD, CoA, or certain vitamins) that interact with the enzyme.

Allosteric enzymes

Allosteric enzymes are composed either of effector binding sites or mulitple protein subunits that interact with each other and thus influence catalytic activity.

Inhibition

Enzymes reaction rates can also be regulated by competitive inhibitors (Fig. 4) and uncompetitive inhibitors and activators (Fig. 5). Inhibitors and activators are often used as medicines, but they can also act as poisons.

Figure 3: Diagram of reaction speed and Michaelis-Menten constant. The speed V means the number of reactions per second that are catalyzed by an enzyme. With increasing substrate concentration [S], the enzyme is asymptotically approaching its maximum speed V_max, but never actually reaching it. Because of that, no [S] for V_max can be given. Instead, the characteristic value for the enzyme is defined by the substrate concentration at its half-maximum speed (V_max/2). This K_M value is also called Michaelis-Menten constant.

Figure 4: Competitive inhibition.
A competitive inhibitor (I) fits the enzyme (E) as well as its real substrate (S), sometimes even better. The inhibitor (I) takes the place of the substrate (S) in the active center, but cannot undergo the catalytic reaction, thus inhibiting the enzyme (E) from binding with a substrate (S) molecule. Some inhibitors (I) form covalent bonds with the enzyme (E), inactivating it permanently (suicide inhibitors).

File:Ncomp inhib.png
Figure 5: Uncompetitive inhibition.
Uncompetitive inhibitors/activators (I) do not bind to the active center, but to other parts of the enzyme (E) that can be far away from the substrate (S) binding site. By changing the conformation (the three-dimensional structure) of the enzyme (E), they disable or enable the ability of the enzyme (E) to bind its substrate (S) and catalyze the desired reaction.