Question

What is a coenzyme?
A. Always protein
B. Often metal
C. Always inorganic compound
D. Often a vitamin

Answer 1

Hint: The coenzyme is a type of cofactor which helps in the reaction completion by enhancing the enzyme activity. It is mostly present in the enzyme catalyzed reactions and is a non-protein part of the enzyme.

Complete answer:
The organic non-proteinaceous group present in the enzyme is called coenzyme. The cofactors that are composed of organic constituents are called coenzymes. Their attachment with the apoenzyme is very loose. They function as catalysts in different reactions. They mostly contain vitamins as their essential components. For example: coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the vitamin niacin in them.
Other types of cofactors are prosthetic groups which are organic compounds and they are attached very tightly to the apoenzyme, which is the proteinaceous part of the enzyme. Examples of a prosthetic group can be seen in enzymes peroxidase and catalase which are enzymes for breaking down the hydrogen peroxide into water and oxygen, in this haem is present as the prosthetic group.
Other types of cofactor except coenzymes and prosthetic groups are metal ions. Metal ions are responsible for forming coordination bonds with the side chains at the active and also they form one or more coordination bonds with the substrate. For example in the protein breaking enzyme carboxypeptidase, zinc metal ion is present as the cofactor.
If the cofactors are removed from the enzymes, enzymes cannot catalyze the reaction as their catalytic activity is lost.

So, the correct answer is (D).

Note: The difference between prosthetic groups, coenzymes, apoenzymes should be studied carefully as they have different composition and the method of attachment is also different.