Structure of enzymes

With the exception of ribozymes, all enzymes are globular proteins, being able to be formed only by polypeptide chains or contain, in addition, another non-protein group.

According to their structure, two types of enzymes can be differentiated:

• Holoproteins, enzymes made only of polypeptides.
• Holoenzymes, enzymes formed by the association of a polypeptide or apoenzyme part and a non-polypeptide or cofactor part.

Distinguish in a holoenzyme :

• The apoenzyme, a protein part that is responsible for providing the specific spatial structure that allows binding to substrates, molecules on which enzymes act in chemical reactions.
• The cofactor, non-protein part that forms the enzymatic components that carry out the reaction. The cofactors can be:
  • Inorganic cofactors, such as metal ions (Fe²⁺, Cu²⁺, Mn²⁺, Zn²⁺,…), which are found in small quantities (less than 0.1%, so they are trace elements). For example, Zn in carboxypeptidases, Mg in kinases, K in pyruvate kinase, etc.
  • Organic cofactors, such as:
    • Coenzymes, molecules that act weakly associatedwith enzymes, such as, for example, FAD⁺, NAD⁺, vitamins, etc. Thus, trace elements and vitamins are essential for organisms, as they are cofactors of various enzymes.
    • The prosthetic groups, molecules strongly bound by covalent bonds to the polypeptide chain, such as the group heme in cytochromes group hemino in peroxidases, etc.

Some enzymes need a non-protein component, the cofactor, in order to carry out their activity. If that enzyme does not have the cofactor, it is not active and is called an apoenzyme. When the enzyme is bound to its cofactor it is called a holoenzyme and it is active.