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

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.