Proteins are macromolecules formed by the polymerization of amino acids. Structurally, proteins are divided into four levels.
Primary structure – It is the linear sequence of amino acids in a polypeptide chain.
Secondary structure – The polypeptide chain is coiled to form a three-dimensional structure.
Tertiary structure – The helical polypeptide chain is further coiled and folded to form a complex structure.
Quaternary structure – More than one polypeptide chains assemble to form the quaternary structure.
Milk has many globular proteins. When milk is converted into curd or yoghurt, these complex proteins get denatured, thus converting globular proteins into fibrous proteins. Therefore, by the process of denaturation, the secondary and tertiary structures of proteins are destroyed.