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4.5.4. Regulation of enzyme activity

Influence of temperature

If we supply energy in the form of heat to an enzymatic reaction, when it is captured by the molecules it is transformed into kinetic energy. In this way, the movement of the molecules and the number of intermolecular encounters increases. Thus, increasing the temperature increases the speed of chemical reactions.

If the temperature is too high, the enzyme loses its tertiary structure, becomes denatured and ceases to be functional. There is an optimal temperature for which the enzymatic activity is maximum.

If you decrease the temperature, the activity also decreases, but the enzyme is not destroyed, and the process is reversible.

Influence of pH

All enzymes have an optimal pH at which they are most effective, although they can function between two pH limit values. If the pH is outside these limits, the enzyme denatures  and stops working.

Influence of substrate concentration

If you increase the substrate concentration, keeping the enzyme concentration constant , the rate of the reaction increases. As there are more substrate molecules, the enzyme-substrate encounter is more likely to occur.

This increase in speed has a limit, since if the substrate concentration is excessive, all enzymes will be in the ES complex form, they will be saturated, and the reaction speed will not increase.

If the substrate concentration is decreased, the rate of the chemical reaction will decrease.

Activators effect

There are some ions that favor the binding between the enzyme and the substrate. For example, the enzyme phosphorylase is activated by the presence of magnesium ions Mg2+, forming ATP from ADP and a phosphate group (H3PO4).

Effect of inhibitors

Inhibitors are substances that decrease the activity and effectiveness of an enzyme or completely prevent its action. Inhibition can be of two types: irreversible and reversible.

  • The irreversible inhibition or poisoning of the enzyme occurs when the inhibitor is permanently fixed to the active site of the enzyme by altering their structure and thus making it unusable.
  • The reversible inhibition occurs when temporarily prevents normal operation of the enzyme without disabling the active center. There are two ways: competitive and non- competitive.
    • The reversible inhibition competitive occurs when the inhibitor is similar to the substrate, so that both can bind to the active center of the enzyme.
      • If the substrate is attached to the enzyme, the products are formed.
      • If the inhibitor is fixed, the enzyme cannot act until it is released from the inhibitor.
    • The reversible inhibition noncompetitive is due to an inhibitor that binds to the enzyme by a different site to which the substrate does, by altering its conformation. It can also act on the enzyme-substrate complex, making it fixed.

A special case of inhibition is uncompetitive inhibition, in which the inhibitor does not bind to the free enzyme, but to the enzyme-substrate complex to prevent product formation.


Fundamental insights on the regulation of enzyme activity

The speed of enzymatic reactions depends on several factors:

  • Influence of temperature.

    As the temperature increases to an optimum temperature, the molecules move faster and the number of encounters between molecules is greater, thus increasing the speed of the reaction. If the temperature continues to increase, the formation of the enzyme-substrate complex becomes more difficult and the reaction speed decreases, in addition to the denaturation of the enzyme and the loss of its functionality, since it loses its secondary,  tertiary structures (mainly) and  quaternary .

    The denaturation is usually irrerversible, although again lower the temperature, the enzyme does not recover its native three - dimensional structure. Sometimes, if it has been subjected to denaturing factors and low intensity for a short time, denaturation can be reversible.

  • Influence of pH .
    • Each enzyme has its maximum activity (maximum reaction rate) at a certain pH called the optimum pH. Outside this pH, the structure of the enzyme is altered and its catalytic capacity is less effective.
  • Influence of the substrate concentration .
    • As the concentration of substrate increases, if the concentration of the enzyme does not vary, it is more likely that encounters between enzymes and substrates will occur, thus the reaction rate increases.
    • If the substrate concentration is excessive, all the enzymes will be saturated forming the enzyme-substrate complex and the reaction speed will not increase, since no free enzymes would remain.
    • If the substrate concentration decreases, the number of encounters in enzymes and substrates will be less and the rate of the chemical reaction will decrease.
  • Activators effect .
    • Some ions promote the binding between the enzyme and the substrate.
  • Effect of inhibitors .
    • Inhibitors are substances that decrease the effectiveness of an enzyme. The inhibition can be of two:
      • The irreversible inhibition when the inhibitor is permanently fixed to the active site of the enzyme by altering their structure and thus making it unusable.
      • The reversible inhibition when temporarily prevents normal operation of the enzyme without disabling the  active center. There are two ways: 
        • The reversible inhibition  competitive occurs when the inhibitor is similar to the substrate, so that both can bind to the  active center  of the enzyme.
        • The reversible inhibition noncompetitive is due to an inhibitor that binds to the enzyme by a different site to which the substrate does, by altering its conformation. 
      • A special case of inhibition is uncompetitive inhibition, in which the inhibitor does not bind to the free enzyme, but to the enzyme-substrate complex to prevent product formation.