Question

11.Which of the following sequences of amino acids is the most likely to be located on the exterior of a water soluble globular protein?

	a.	Arg-Ser-Gln-Pro-His
	b.	Met-Phe-Ile-Leu-Ala
	c.	Val-Leu-Ser-Ala-Val
	d.	Met-Val-Cys-Leu-Gln

QUESTION 12

1.     Which pair of amino acid side chains might form a London dispersion force type of van der Waals interaction in a globular protein?

	a.	His and Ser
	b.	Ala and Gln
	c.	His and Met
	d.	Val and Met

QUESTION 13

1.     The β-hairpin consists of antiparallel β-sheets.

  True

  False

QUESTION 14

1.     The α/β barrel topology found in the protein triose phosphate isomerase consists of

	a.	the Greek Key motif combined with the β-hairpin motif rolled into a continuous barrel structure.
	b.	a series of β-α-β motifs rolled into a double barrel structure.
	c.	the immunoglobulin fold combined with an α-α motif.
	d.	several β-hairpin motifs rolled into a barrel structure.

QUESTION 15

1.     Which treatment is least likely to cause a protein to denature?

	a.	Adding sodium chloride to 0.10 M.
	b.	SDS (sodium dodecyl sulfate)
	c.	Addition of NaOH to pH13.
	d.	8 M urea

Answer 1

11. Answer is a (Arg-Ser-Gln-Pro-His).

To be water soluble, a molecule has to make polar contacts with the water molecules. In proteins, six amino acids have side chains that are polar but not charged. These are serine (Ser), threonine (Thr), cysteine (Cys), asparagine (Asn), glutamine (Gln), and tyrosine (Tyr). These amino acids are usually found at the surface of proteins. Other amino acids are generally buried in the hydrophobic core in the presence of polar solvents. Among the given sequences, b-d have at least three hydrophobic amino acids. Therefore their tendency to occur at the surface is very less. The correct answer is a.

12. The correct answer is d. Van der waals interactions are mostly seen among the hydrphobic amino acids. Among the given pairs, Val and Met will form the most hydrophobic interactions.

13. This is true. A beta hairpin is made by a beta strand-loop-beta strand. When oriented in space the beta strand after the loop will be running anti-parallel to the one before the loop.

14. The correct answer is b.

The α/β barrel topology found in the protein triose phosphate isomerase (TIM) consists of a series of β-α-β motifs rolled into a double barrel structure. It is also known as (αβ)₈ - or (βα)₈- barrel. It has a central cylinder or barrel of β-sheet formed from eight parallel β-strands; the barrel is closed by hydrogen bonding between the first and the last strands in the sequence. Adjacent β-strands are connected by an α-helix, and the eight α-helices form an external coat for the central β-barrel.

15. The answer is a (0.10 M NaCl).

This condition does not denature protein. In fact, a high conc. upto 500 mM (or 0.5 M) of salts are used to stabilise proteins in solvents by increasing the ionic strength and osmolarity. Whereas the other reagents such as SDS, NaOH (at high pH) or Urea are harsh to proteins and these can denature the protein structure substantially.