The main sulfur amino acids are methionine, cysteine, homocysteine and taurine. Of these, the first two are proteinogenic.
This group of reactions contains all processes that 1) break down sulfur amino acids, 2) interconvert between them, and 3) synthesize them from solved sulfide which comes from sulfate assimilation and reduction. Only plants and microorganisms employ all processes. Humans cannot de novo synthesize any sulfur amino acid, nor convert cysteine to methionine (Brosnan & Brosnan, 2006).