Question

One of the following enzymes is not a protein:

• A. DNAase.
• B. Abzyme.
• C. Eco RI.
• D. Ribozyme.

Hint:

Ribozymes are catalytic RNA molecules, the one common exception to enzymes being proteins.

Answer 1

The Correct Option is D

Most enzymes in the body are proteins, but there is one well known exception that this question is testing. Let's check each option in turn.

1. DNAase: This is an enzyme that breaks down DNA, and like the great majority of enzymes, it is a folded chain of amino acids, so it is a protein.
2. Abzyme: The name is short for antibody enzyme, it refers to an antibody that also works as a catalyst. Since every antibody is built from amino acid chains, an abzyme is a protein too.
3. Eco RI: This is a restriction enzyme from bacteria that cuts DNA at a specific recognition sequence. It is a standard protein enzyme, widely used in molecular biology labs for cutting DNA at defined sites.
4. Ribozyme: This is the exception. A ribozyme is a piece of RNA that can catalyze a chemical reaction on its own, without any protein component. A well known example is the peptidyl transferase activity of the ribosome, which is carried out by ribosomal RNA, not by the ribosomal proteins. Since ribozymes are made of nucleotides, not amino acids, they are not proteins.

Three of the four options, DNAase, Abzyme, and Eco RI, are all protein based catalysts. Only the ribozyme is built from RNA instead of protein.

Let's summarize:

• Most enzymes, including DNAase, Abzyme, and Eco RI, are proteins.
• Ribozymes are catalytic RNA molecules, the key exception to "all enzymes are proteins".

So the enzyme that is not a protein is the ribozyme.