Proteins have multiple and different functions in biological beings. In humans, proteins have an structural role at multiple levels, from a cellular level, like proteins in membranes, up to the tissue or body level (as collagen); they can act as enzymes, also as antibodies, as hormones, as transporters in blood, in the respiratory process (at a cellular level, like cytochromes, or as a reservoir of oxygen, like myoglobin, or as an Oxygen transporter in blood, as Hemoglobin).
Of course, in order to perform all these functions, the structure of proteins need to be very complex, and in fact, it is very complex. For facilitating its understanding, the structure of proteins has been divided in different structural levels:
– Primary structure, ( or first level of organization), defined as the sequence of amino acids linked through peptide bonds
– Secondary structure (second level of organization), the ordered arrangements of amino acids in localized regions of a protein. The main kinds of secondary structures are alpha-helix and beta-strands.
– Tertiary structure (tertiary level of organization) is the spatial conformation of protein stabilized through several interactions between the R side chains of distant amino acid residues.
– Quaternary structure, seen only in some proteins, and defined as the non covalent assemble of different peptide chains to form a functional protein.
In recent years, it has been defined other intermediary structural levels, combination of some secondary structures, that have been called Supersecondary structures.
It is important to emphasize that the above description correspond to an artificial division for facilitating the understanding and comprehension of the protein structure, since actually each protein has just one structure as a whole.