Contractile and structural are two examples of proteins. Contractile proteins are responsible for movement in the body. Examples of these include myosin and actin, which regulate muscle movement. Structural proteins provide support and include collagen and elastin found in connective tissues.
There are four protein structure levels. Any given protein may contain one or more of the structure types. These types include primary, secondary, tertiary and quaternary. Primary structure is determined by the order of the amino acids linked together in a protein. Secondary structure refers to the folding of the polypeptide chain to form a 3-D structure and may be classified as alpha helix or beta pleated. Tertiary structure is a more complex 3-D shape held together by different bonds and forces. Quaternary structure is formed when multiple polypeptide chains interact.