Lecture 29 Summary

One-carbon carriers: S-adenosyl methionine (SAM); folates. Homocysteinemia. The shikimate pathway (plants and bacteria). Tryptophan synthase in E. coli.

Catabolic pathway of methionine and synthesis of S-adenosylmethionine (SAM)

The first step in the catabolic pathway of methionine is its reaction with ATP to form S-adenosylmethionine, or SAM. SAM is the usual donor of activated, reduced carbon atoms - i.e. methyl groups - in methyltransferase reactions. The enzyme catalyzing SAM synthesis, methionine adenosyltransferase [EC 2.5.1.6], is unusual in that all three phosphates are lost from ATP. Note the loss of the 5' oxygen from adenosine.

Study questions

• Explain why SAM is an "activated" methyl donor.

Page updated 12-27-06

References

1. Berg, Tymoczko, and Stryer. Biochemistry (BTS): 6th edition (2007, Freeman) Ch.24, pp.685-697.