FIGURE 7.06 The Alpha Helix

A) The general shape of an a-helix. B) The carbon backbone of the polypeptide chain. C) The hydrogen bonds between peptide groups.

Hydrogen bonding is responsible for the formation of alpha-helix and beta-sheet structures in proteins.

In the a-helix (Fig. 7.06), a single polypeptide chain is coiled into a right-handed helix and the hydrogen bonds run vertically up and down, parallel to the helix axis. In fact, the hydrogen bonds in an a-helix are not quite parallel to the axis. They are slightly tilted relative to the helix axis because there are 3.6 amino acids per turn rather than a whole number. The pitch (repeat length) is 0.54 nm and the rise per residue is about 0.15 nm.

The hydrogen bonds hold successive twists of the helix together and run from the C=O group of one amino acid to the NH group of the fourth amino acid residue down the chain. The a-helix is very stable because all of the peptide groups (—CO—NH—) take part in two hydrogen bonds, one up and one down the helix axis. A right-handed helix is most stable for l- amino acids. (A stable helix cannot be formed

Reverse turn

A) Flat rectangular sheet

B) Twisted sheet, saddle shape

FIGURE 7.08 The Beta Sheet Conformations

B) Twisted sheet, saddle shape

Reverse turn

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