Protein and secondary st

Protein: Secondary, Tertiary, and Quaternary Structures


Learning Objective:

? Describe the secondary, tertiary, and quaternary structures for a protein;
? Describe the denaturation of a protein.

The secondary structure of a protein describes the type of structure that forms when amino acids form hydrogen
bonds within a polypeptide or between polypeptide chains. The three most common types of secondary
structure are the alpha helix, the beta-pleated sheet, and the triple helix
In an alpha helix (? helix), hydrogen bonds form between the oxygen of the C=O groups and the hydrogen of
N¬H groups of the amide bonds in the next turn of the ? helix (see Figure 2.4). The formation of many hydrogen
bonds along the polypeptide chain gives the characteristic helical shape of a spiral staircase. All of the R groups
of the different amino acids extend to the outside of the helix.

Another type of secondary structure found in proteins is the beta-pleated sheet (?-pleated sheet) . In a bpleated
sheet, hydrogen bonds form between the oxygen atoms in the carbonyl groups in one section of the
polypeptide chain, and the hydrogen atoms in the N¬H groups of the amide bonds in a nearby section of the
polypeptide chain. A beta pleated sheet can form between adjacent polypeptide chains or within the same
polypeptide chain. The tendency to form various kinds of secondary structures depends on the amino acids in a
particular segment of the polypeptide chain.