What is the only essential sulfur-containing amino acid?

Methionine is the only essential sulfur-containing amino acid because it cannot be synthesized by the human body and must be obtained through the diet. Essential amino acids are critical for various biological processes, including protein synthesis, and must be consumed from food sources to meet the body's needs.

Methionine plays a significant role in numerous metabolic processes, including serving as a precursor for other important molecules. It is involved in the synthesis of cysteine, another sulfur-containing amino acid, though cysteine itself is classified as a non-essential amino acid since the body can produce it from methionine and other sources.

In contrast, tyrosine and phenylalanine are amino acids that do not contain sulfur in their structure, and while phenylalanine is an essential amino acid, tyrosine can be produced from phenylalanine and is therefore not essential when adequate amounts of phenylalanine are consumed.