Insulin was the first protein to be sequenced. Its quaternary structure was dedu

Question

Insulin was the first protein to be sequenced. Its quaternary structure was deduced by gel filtration.  The chromatograms are described below.  

two peaks of MW 3500 and 2500

(a) What is the total number of N-termini in the protein?

(b) How many types of subunits are there? Name them (alpha, beta, gamma…).

(c) What are the molecular weights of the subunits?

(d) What kind of bonds connect one subunit to the next in the intact protein?  Identify subunits by name (your answer in (b)) and list the interactions each has with the others.

(e) The crystal structure of insulin shows an all alpha-helical structure. Describe the hydrogen-bonding pattern in an alpha-helix, indicating which atoms are involved.
(f) The protein is 0.373% Zn by mass (Zn mass = 65.37 AMU).  How many Zn2+ ions are bound to the protein?
(g) You decide to confirm the gel filtration result using native polyacrylamide gel electrophoresis.  You use the same conditions as above and obtain single bands that migrate 1.2 cm, 2.6 cm and 4.5 cm, respectively.  Give a reason why you are missing a band in the third experiment

Gel Filtration Condition Result Buffer only one peak of MW 36,000 Buffer with 8 M urea one peak of MW 6000 Buffer with 8 M urea and 10 mM ?-mercaptoethanol

two peaks of MW 3500 and 2500

Explanation / Answer

A. Total number of n-terminus in a protein is always one.

B. Total number of subunits are four i.e 2 alpha and 2 beta subunits.

C. Molecular weight of alpha subunit is approx. 150,000 and of beta subunit is 102,000

D. The subunits are linked to each other by dilsulfide bonds.

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