Alpha-Helix and Beta-Sheet Collection (AH1) TEACHING POINTS The linear amino acid sequence defines the primary structure of a protein. Regions of the linear polypeptide chain fold into the stable alpha-helix and beta-sheet structures to form the protein secondary structure. The tertiary protein structure is the overall three-dimensional Beta Structure: Parallel and antiparallel beta strands are much more extended than alpha helices (phi/psi of -57,-47) but not as extended as a fully extended polypeptide chain (with phi/psi angles of +/- 180). The beta sheets are not quit so extended (parallel -119, +113 ; antiparallel, -139, +135), and can be envisioned as rippled sheets.
Nevertheless, it was clear from inspection of Ramachandran plots of twisted beta sheets that the polypeptide chains generally had phi,psi angles corresponding to chains with a local left-handed helical twist, which for essentially extended polypeptide chains, produces sheets with a right-handed twist. Know the classes of secondary structure (helices, sheets, the value of amide planes in secondary structure) Know the main difference between an alpha-helix and beta-pleated sheet Know what are the main causes of sterically forbidden regions (phi, and psi) Know what kinds of bonds stabilize secondary structures Know how to use the helical wheel