The four levels of protein structure are primary, secondary, tertiary, and quaternary.
Specific order of amino acid linked by peptide bond forms a primary structure.
Polypeptides bend and fold to form a three-dimensional shape that can either look like a spiral (an alpha helix) or a sort of accordion shape (a beta-pleated sheet). These regular structure taken up by these polypeptides form a secondary structure.
Tertiary protein structure describes a final form of a functional protein comprised of its secondary structure components. Tertiary structures form in relation to their environment, with hydrophobic amino acids on the interior of a protein and hydrophilic ones on the exterior.
The quaternary structure of a hemoglobin molecule includes four tertiary structure protein chains, two of which are alpha helices and two of which are beta-pleated sheets.
catabolite gene activator protein (CAP) is a DNA binding protein involved with the transcription of several genes. It operates in lac operon. CAP can activate or repress transcription initiation upon binding of cAMP. The cAMP-CAP complex binds to promoters on the DNA.