Protein Structure There are four levels of protein structure:1. Primary2. Secondary3. Tertiary4. Quaternary Primary Structure The primary structure of a protein is its linear amino acid sequence. Secondary Structure The secondary structures of a protein refer to the ways in which local segments of its primary structure fold on themselves. There are a variety of different secondary structural elements; the most common of these are alpha helices and beta sheets. Alpha HelicesThese right-handed helices are common motifs in protein secondary structure. Beta SheetsBeta sheets are pleated sheets consisting of beta strands laterally connected by at least 2-3 backbone hydrogen bonds. Tertiary Structure The tertiary structures of a protein are the local three-dimensional shapes its secondary structure folds on itself to create. Quaternary Structure Quaternary structure refers to the arrangement and number of multiple protein subunits comprising a multi-subunit complex.