Question1 Pictured below are the structures of several proteins. Answer the foll

Question

Question1 Pictured below are the structures of several proteins. Answer the following questions about secondary and tertiary levels of organization. (a) What type of secondary structure dominates in the protein on the left? What molecular forces dominate in this type of secondary structure? (be specific) (b) Same questions as (a) for the protein on the right -note that the top picture is a side view and the bottom is the same protein looking from the top (c) How are the forces different in protein (a) vs. protein (b)? (d) For this one, describe the various secondary structures in the protein at the right, being as specific as possible (e) Where is the "hydrophobic core" of the molecule on the right compared to the one of the top left (part -a-)? (1) How many chains is insulin made from? Are they the same or different? Denote the quaternary structure of insulin, using correct nomenclature (g) In solution, two molecules of insulin form a hydrogen bonded pair. What is the new quaternary structure? NOTE: this is an active, blood-soluble form of the molecule

Explanation / Answer

1. [a] Alpha-helical structure of protein dominates the protein molecule on the left side.

In an alpha-helix, the polypeptide backbone forms a repeating helical structure that s stabilized by hydrogen bonds between a carbonyl oxygen and an amine hydrogen. These hydrogen-oxygen covalent bonds dominate the structure.

[b] Beta- sheet structure dominates the protein on the left.

[c] The beta strands are arranged adjacent to each other and form an extensive hydrogen bond network with their neighbors in which the N-H groups in the backbone of one strand establishes hydrogen bonds with the C=O groups in the backbone of the adjacent strands. The 'pleated' structure of beta-sheet arises from the tetrahedral chemical bonding at the C a^ atom.

In alpha-helices, the bond between the carbonyl oxygen and an amine hydrogen which occur at regular intervals of one hydrogen bond every fourth amino acid and this causes the polypeptide bond to form a helix.

[d] The third structure of the protein shows both the alpha-helical and beta-sheet structure.

The beta-sheet molecule is in the center surrounded by alpha-helical proteins from all sides.

[e] In alpha helices, the hydrophobic core of the amino acids is oriented towards the interior of the protein molecule.

In beta sheets too, the hydrophobic residues are oriented into the interior of the barrel to form a hydrophilic core.

In both the secondary structures, the hydrophilic or polar residues are oriented towards the outside face of the protein molecules.

[f] A normal insulin molecule exists as a single molecule or is monomeric. It has two dissimilar amino acid chains- i] chain A with 21 amino acids, and

ii] chain B with 30 amino acids  

[g] Insulin can form into granules consisting of hexamers [ 6 insulin molecules grouped around 2 zinc ions] due to interactions between hydrophobic surfaces. Insulin is stored in this form in the beta cells of the 'islets of Langerhans' n the pancreas and then released into the blood. In the blood, it changes into dimers [ two insulin molecules held together by hydrogen bonds ].

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