In biosynthesis of cysteine and methionine, where does the sulfur come from?

Question

Methionine and Cysteine are the two amino acids containing sulfur. [According to wikipedia, cysteine](https://en.wikipedia.org/wiki/Cysteine) is built from methionine. However, the wikipedia also claims that [methionine is synthesized from cysteine](https://en.wikipedia.org/wiki/Methionine#Biosynthesis). So, a chicken and egg problem (esp. since eggs are a rich source for those proteins). Where do living things get their sulfur to build these amino acids in the first place, in what forms is sulfur available biologically?

Answer 1

This is an example of miscommunication on the part of Wikipedia that needs to be clarified.The biosynthesis of cystein and methionine differs from plants to animals. And I would assume there are differences between species within the animal kingdoms.

For the purpose of this question, I will keep the answer brief and only focus on biosynthesis of animals and plants.

Mammals acquire methionine, the only sulfur-containing essential amino acid., from their diet such as beans, eggs, fish, poultry, onions, garlic, seeds, yogurt, etc. This methionine is used in the biosynthesis of cystein.

In plants, the sulfur is obtained for inorganic phosphage and fixed to cystein in a series of biochemical reactions involving enzymes. From here, the cystein is used in the biosynthesis of methionine.

Please take note that this information does not consider recent publications. If you are interested in this kind of research, contact me for further details as this would be a terrific research project.

Resources will be provided in the comments. Please feel free to update the Wikipedia article with this information.