Tropocollagen synthesis

The first step in the synthesis of type I collagen in bone is the production of alpha-1 and alpha-2 polypeptides. After many of the prolines and lysines in the collagen chains are hydroxylated to hydroxyproline and hydroxylysine, one alpha-2 (shown in red) and two alpha-1 polypeptides (shown in blue) wind around each other to form a helical structure known as procollagen (top). Amino- and carboxy-terminal propeptides of type I collagen are cleaved off the procollagen molecule, resulting in the formation of tropocollagen (bottom).