2.4.3 Sulfur (S)
Most proteins contain about 1% sulfur, which occurs in the side-chains (R groups) of two of the protein-forming amino acids, methionine and cysteine.
Cysteine is particularly important in proteins such as collagen (found in bone, tendons, cartilage and skin) and keratin (found in hair and nails, as well as skin).
What is the common feature of these two proteins that may be linked to the presence of cysteine in them?
Collagen and keratin are both found in parts of the body that are relatively tough and strong. They are known as ‘structural proteins’. Links between the sulfur atoms of the cysteines in adjacent protein (polypeptide) molecules link the molecules firmly together, providing that strength.
After injury, there is a particular need for sulfur to repair and build new structural proteins. However, a diet containing sufficient protein almost certainly provides sufficient sulfur for the body. Sulfur is also found in the vitamins biotin and thiamin and in some enzymes that play important roles in metabolism.