Question

Describe the primary, secondary, tertiary, and quaternary structures of collagen.

Short answer

The primary structure of collagen is the repeating unit of three amino acids, Gly-Pro-Hyp. The secondary structure is the left-handed helical conformation. The collagen does not have such tertiary structure due to the presence of only one type of secondary structure. The quaternary structure of collagen is the right-handed triple helix formed from the twisting of three left-handed helical polypeptide chains.

Step-by-step solution

Collagen protein.

Collagen is the most abundant protein present in vertebrates, and forms strong and insoluble fibers as the major component of the connective tissues such as teeth, bones, etc.

Collagen represents the structure of a right-handed triple helix, where three left-handed helices are twisted and joined together.

Primary structure of collagen.

The primary structure of collagen is the amino acid sequence, which is the monotonously repeating unit of triplets. The triplets mostly contain the sequence: Gly-X-Y. Here, X is often Pro and Y is often 4-hydroxyprolyl (Hyp).

Secondary structure of collagen.

The repeating amino acid sequence present in collagen typically representsa left-handed helical conformation of the collagen polypeptide chains. This helical conformation represents the secondary structure of collagen.

Tertiary structure of collagen.

Due to repeating amino acid triplets, polypeptide chains of collagen exhibits only one type of structure i.e., the left-handed helical conformation.

Thus, the collagen protein does not exhibit a tertiary structure as such, or the secondary structure of the collagen protein could be considered as the tertiary structure of collagen.

Quaternary structure of collagen.

Three of those polypeptide chains are twisted together to form the right-handed triple-helix conformation of collagen, which is the quaternary structure of collagen.

Every third amino acid residue present in each chain goes through the center of the triple helix. The residue Gly is the only amino acid that can fit into the crowded environment at the center of the triple helix.