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Methionine and Cysteine are the two amino acids containing sulfur. According to wikipedia, cysteine is built from methionine. However, the wikipedia also claims that methionine is synthesized from cysteine.
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?

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Microorganisms (and plants) make cysteine from serine using H2S as the sulphur source. Condensation of these two directly gives cysteine in some bacteria, but most often the reaction sequence is:

serine + acetyl-CoA >>> O-acetylserine + CoA-SH

O-acetylserine + H2S >>> cysteine + acetate + H2O

As you say, in terms of the sulphur, cysteine and methionine are interconvertible.

Animals rely upon their diet for methionine, and can make cysteine from that methionine.

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  • $\begingroup$ +1 and accept: the key info was that H2S is a sulfur source. $\endgroup$ – mart Mar 10 '14 at 8:39
  • $\begingroup$ Which plants use H$_2$S? I only found sources about microorganisms doing this and plants using soluble forms sulphur. $\endgroup$ – Chris Mar 26 '14 at 16:20
  • $\begingroup$ Apparently, a number of them can take up H$_2$S. plosone.org/article/…. $\endgroup$ – Michael S Taylor Aug 5 '14 at 15:45
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First of all, the wikipedia article you quote states that methionine is synthesized from cysteine in plants and microorganisms, not in humans. In humans, both cysteine and methionine are essential amino acids which means that they cannot be produced by the human body but need to be acquired elsewhere.

Now, this is mostly done by eating proteins that contain these amino acids. The proteins are broken down and the amino acids (or their constituents, such as sulfur) are reused. In mammals, cysteine is produced from serine but the sulfur comes from methionine via homocysteine:

enter image description here

So, methionine comes from the diet and it is then used to produce cysteine.

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