help:

S$-$Adenosylmethionine $($SAM$)$ is a methyl donor in many biological processes.

a$)$ Propose the mechanism for methylation of cytosine to $5-$methylcytosine in the presence

of SAM.

b$)$ Name three amino acids that can be methylated in the presence of SAM? What are

products?

c$)$ Explain why has SAM a chiral center on the $S$ atom. Are both stereoisomers produced in

nature?

d$)$ SAM can be degraded under basic conditions. Which two products are formed? $($Hint:

both products are cyclic$)$

e$)$ Draw the structure of another coenzyme that is often used for methyl transfer reactions.

f$)$ The SAM analogue shown below can be used for substrate labelling studies. Draw the

structure of the labelled product.