Test: Secondary Structure of Protein - MCAT MCQ

Secondary structure is the special arrangement of the polypeptide chain in three-dimension. It may form some special structures such as helices, sheets, and turns, etc. These structures are very common in a protein.

The most commonly found secondary structures in a protein are the alpha-helix and beta-sheets. Both of these secondary structures are very common and very important for a protein molecule. They play a significant role in the functioning of the respective proteins.

The peptide C=O bond of the n^th residue of the backbone of alpha-helix points along and forms a hydrogen bond with the peptide N—H group of the (n+4)^th residue.This arrangement results in the formation of a strong hydrogen bond, which has nearly optimum N—O distance (2.8 Armstrong). Moreover, in the core of the helix, the atoms are in van der Waals contact, because the core of the helix is tightly packed.

Turns or beta-bends often connect alpha-helices or the strands of beta-sheets. These structures mostly occur on the surface of proteins. These structures usually contain a few amino acid residues (about 4).

Keratin is the principal component of the outer layer of the skin (epidermis) and its related appendages, such as hair, horn, nails, and feathers. It is mechanically durable and does not easily react. It occurs in all higher vertebrates. There are more than 50 keratin genes that are present in humans.

The deficiency of vitamin C in diet results in Scurvy. Collagen is a triple helix and contains some hydroxylated residues. The enzyme (prolyl hydroxylase) that catalyzes this reaction requires vitamin C to maintain its activity.

There are some flanking residues outside the alpha-helix or beta-sheets. Helix capping is the phenomenon in which, the side chains of the flanking Asn and Gln residues fold back to form hydrogen bonds with one of the four-terminal residues of the helix.

Due to the resonance occurring between the double bond and the lone pair of the oxygen atom, the peptide bond has a partial double bond character. This partial double bond character makes the peptide planar.

Linus Pauling discovered the alpha-helix structure in a protein molecule. Lynn Margulis gave the theory of endosymbiosis. Francis Collins discovered the gene for Cystic Fibrosis. Louis Pasteur is also known as the father of immunology.

The pitch of alpha-helix is 5.4 Armstrong, and residues per turn are 3.6 residues. Generally, the alpha-helix of a protein contains on an average around 12 residues, and have a length of around 18 Armstrong. The alpha-helix is stable majorly due to its strong hydrogen bonding.