What is the universal methyl donor which is used by DNA methyltransferase enzymes to methylate?

Question

What is the universal methyl donor which is used by DNA methyltransferase enzymes to methylate DNA (and is utilised to methylate other biological molecules including histones)?

1. Homocysteine

2. Methionine

3. S-adenosylhomocysteine (SAH)

4. S-adenosylmethionine (SAM)

Answer 1

95% of the methyltransferases that include DNA methyltransferases and histone methyltransferases use S-adenosyl methionine (SAM) as a methyl donor. However, there are some instances where non-SAM methods of methyl transfer takes places.

In your multiple choice question, methionine helps to transfer methyl groups during the process, so methionine itself is not a methyl donor. During the methyl transfer process, SAM is converted into SAH. therefore, its easy to be confused with the choices.

Therefore, the most appropriate choice for your question is, SAM.

I hope this helps.