Sulfur-Containing Amino Acids: The Conversion Process from Product to Substrate

Methionine and cysteine are the principal sulfur-containing proteinogenic amino acids, playing pivotal roles in protein structure, function, and cellular metabolic regulation. The biosynthetic machinery of these amino acids is intricate and exhibits distinct evolutionary divergence across species. This review comprehensively summarizes the biosynthesis of cysteine and methionine in model organisms ranging from Escherichia coli and yeast to plants and Homo sapiens. Specifically, we examine the metabolic interconversion and the transition of roles between these two amino acids during de novo synthesis. Furthermore, we dissect the physiological significance of the transsulfuration pathway in mammals, which utilizes methionine as a precursor for cysteine biosynthesis.