Proteins and Enzymes

ID #1495

In our text book, it states that "the presence of amino acid with large R groups that distort the coil or otherwise prevent the formation of the necessary hydrogen bonds will keep an alpha helix from forming." Does it mean that a polypeptide chain which contains amino acids with large R groups can't make alpha helix? Do they just make beta pleated sheet in a secondary structure?

Not every region in a polypeptide chain adopts secondary structure elements like alpha helices or beta sheets.  And as your book points out, although R-groups do not directly participate in the formation of secondary structure, the presence of particular R-groups can certainly influence what secondary structure is adopted. So chances are if an R-group is bulky enough to keep an alpha helix from forming, it will probably also keep a beta sheet from forming.  That region of the protein simply wouldn't have any secondary structure, but will typically still participate in the tertiary structure of the polypeptide chain.

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