Enzymes are biomolecules that act as catalysts, being fundamental for the regulation of metabolism. Most enzymes are proteins.
In order to speed up a reaction, enzymes must bind to reactants, which are known as substrates. For a long time, it was believed that this connection occurred in a very rigid way, a model known as lock and key. Currently , however, the model known as induced fit is accepted, which assumes that slight changes occur in the shape of the enzyme as the substrate enters the active site.
What are enzymes?
Enzymes are biomolecules that act as catalysts, that is, they are substances capable of accelerating the speed of chemical reactions that occur in living beings without being consumed during these reactions. Without the action of enzymes, some reactions would be very slow, which would harm metabolism. Enzymes selectively accelerate reactions and are therefore very specific catalysts.
Enzymes are able to speed up a reaction by decreasing the activation energy, that is, they reduce the amount of energy that must be added for a reaction to start.
Is every enzyme a protein?
Despite being often defined as biological catalysts of a protein nature, not every enzyme is a protein. There are some RNAs that function as enzymes, called ribozymes. Most enzymes, however, are proteins and are therefore made up of amino acids . The amino acid composition of these biomolecules defines the three-dimensional structure that it will acquire.
What are Enzyme-substrate complex
Substrate is the reagent on which an enzyme acts. When an enzyme binds to its substrate, the enzyme-substrate complex is formed. This binding takes place in a specific region, called the active site.
When we talk about protein enzymes, the active site corresponds to just a few amino acids, with the rest of the molecule being responsible for determining the configuration of the active site. The shape of the active site as well as the shape of the substrate are related to enzyme specificity, as they must be complementary.
lock and key model
The key-lock model , proposed by Emil Fischer, is widely used to explain the interaction between enzyme and substrate. According to this model, there is a rigid complementarity between the enzyme and the substrate, just like a key and a lock. The enzyme’s active site would have a shape complementary to the substrate, which would fit perfectly. Other molecules, therefore, would not have access to this site, which would guarantee the enzyme’s specificity. Just as a key only opens a lock, an enzyme would only bind to one substrate. Today we know, however, that this model is not correct , since enzymes are not rigid structures as previously thought.
Induced fitting model
Currently, the most accepted model to explain the link between an enzyme and its substrate is the induced fit , initially proposed by Koshland et al. The active site and substrate do not function rigidly like a lock and key. Research shows that, as the substrate enters the active site, the enzyme undergoes a slight modification, which favors the adjustment between the active site and the substrate. To better understand this model, we can think of the enzyme and substrate interaction as a handshake, which becomes firmer after the first contact.
What are enzymes – cofactors
Most enzymes need auxiliary molecules to carry out their catalytic action, called cofactors. Cofactors may be permanently bound to the enzyme or may be weakly and reversibly bound to the substrate. They can also be inorganic or organic . When the cofactors are organic molecules, they are called coenzymes .
Some vitamins act as coenzymes, this being the case, for example, with riboflavin, also known as vitamin B2. As examples of inorganic cofactors, we can mention iron and zinc in their ionic form.
Enzymes can be classified into six groups , using the type of reaction they catalyze as a criterion.
- Oxidoreductases: enzymes related to oxir – reduction reactions .
- Transferases: catalyze the transfer of groups from one compound to another.
- Hydrolases: catalyze hydrolysis reactions.
- Lyases: act by adding groups to double bonds or removing groups to form a double bond.
- Isomerases: catalyze isomerization reactions.
- Ligases: enzymes that cause the degradation of the ATP molecule , using the energy released in this reaction to form new compounds.
Factors that regulate enzyme activity
The activity of an enzyme is influenced by factors, the main ones being temperature and pH . Temperature generally plays a positive role in chemical reactions by increasing the rate of an enzymatic reaction. However, when the temperature increases above the optimal conditions, the reaction rate drops considerably. This is because the denaturation of proteins is observed. Most human enzymes have an optimal temperature between 35 and 40 °C. In addition to temperature, pH also influences enzymatic activity, and there is also an optimal value. For most enzymes, the optimal pH value is in the range of 6 to 8.