Question

Among the following, which one is incorrect?

• A. All coenzymes are cofactors, but all cofactors are not coenzymes.
• B. All enzymes, without exception, use cofactors for their activities.
• C. In metalloenzymes, metal ions form coordination bonds with side chains at the active site and at the same time form one or more coordinate bonds with the substrate.
• D. Cofactors are non-protein constituents bound to the enzyme to make the enzyme catalytically active.

Answer 1

Answer: (B)

Almost all the enzymes consist of proteins. They catalyze the biological reactions without themselves being used up.
Enzymes are colloidal in nature. They have a high molecular weight (1000–5,00,000). As far as the mode of enzyme action is concerned, the enzyme lowers the activation energy of the reaction. A non-catalyzed reaction requires higher activation energy than that of a catalyzed reaction. However, there is no difference between the free energies in catalyzed and non-catalyzed reactions. Enzymes show a 3D structure containing 'R' (alkyl) group of amino acid that form active sites during the folding of the polypeptide chain. Usually, 3-12 amino acids form an active site. An enzyme reacts with a substrate only at these active sites.
Enzymes are of two types: (1) Simple enzyme, which is made up of only protein (e.g., pepsin, trypsin). A simple enzyme does not involve a non-protein part or cofactor. (2) Conjugated enzyme (holoenzyme), which is made up of protein and non-protein parts.
The protein part of the enzyme is called an apoenzyme, while the non-protein part associated with the polypeptide chain is known as a cofactor. Cofactor activates the enzyme by binding with it. Cofactors are of three types: (1) Coenzyme, (2) Prosthetic group and (3) Metal ion.
A coenzyme is an organic molecule loosely bound to the enzyme's active site. All coenzymes are cofactors, but cofactors may be coenzymes or prosthetic groups or metal ions.
A prosthetic group is an organic molecule tightly bound to the enzyme's active site.
Enzymes containing metal ion are also known as metalloenzymes. Metal ions are associated by coordinate bonds with the side chain of the active site of the enzyme, e.g., Zn²⁺ is associated with carbonic anhydrase, alcohol dehydrogenase and carboxypeptidase.