The Alpha Helix

The polypeptide backbone of an a helix is twisted by an equal amount about each a-carbon with a phi angle of approximately -57 degrees and a psi angle of approximately — 47 degrees. A complete turn of the helix contains an average of 3.6 aminoacyl residues, and the distance it rises per turn (its pitch) is 0.54 nm (Figure 5-2). The R groups of each aminoacyl residue in an a helix face outward (Figure 5-3). Proteins contain only L-amino acids, for which a right-handed a helix is by far the more stable, and only right-handed a helices

- . M-.--


" , , 1 - ,.L, 1 , 1 *, - 1 , ,

■ ■ 1 . . 1 . . 1 . . 1 . . I . . "

Figure 5-1. Ramachandran plot of the main chain phi (®) and psi (¥) angles for approximately 1000 nonglycine residues in eight proteins whose structures were solved at high resolution. The dots represent allowable combinations and the spaces prohibited combinations of phi and psi angles. (Reproduced, with permission, from Richardson JS: The anatomy and taxonomy of protein structures. Adv Protein Chem 1981;34:167.)

occur in nature. Schematic diagrams of proteins represent a helices as cylinders.

The stability of an a helix arises primarily from hydrogen bonds formed between the oxygen of the pep-tide bond carbonyl and the hydrogen atom of the pep-tide bond nitrogen of the fourth residue down the polypeptide chain (Figure 5-4). The ability to form the maximum number of hydrogen bonds, supplemented by van der Waals interactions in the core of this tightly packed structure, provides the thermodynamic driving force for the formation of an a helix. Since the peptide bond nitrogen of proline lacks a hydrogen atom to contribute to a hydrogen bond, proline can only be stably accommodated within the first turn of an a helix. When present elsewhere, proline disrupts the conformation of the helix, producing a bend. Because of its small size, glycine also often induces bends in a helices.

Many a helices have predominantly hydrophobic R groups on one side of the axis of the helix and predominantly hydrophilic ones on the other. These amphi-pathic helices are well adapted to the formation of interfaces between polar and nonpolar regions such as the hydrophobic interior of a protein and its aqueous envi-

Figure 5-2. Orientation of the main chain atoms of a peptide about the axis of an a helix.

ronment. Clusters of amphipathic helices can create a channel, or pore, that permits specific polar molecules to pass through hydrophobic cell membranes.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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