Assume that you take Alison\'s advice. As part of your undergraduate research pr
ID: 14059 • Letter: A
Question
Assume that you take Alison's advice.
As part of your undergraduate research project, you are characterizing three polypeptides of approximately equal molecular weight from human plasma. Using various physical techniques, you have established that in their native states one of the polypeptides is a monomeric, cigar-shaped molecule, the second is monomeric and approximately spherical, and the third is the subunit of a teiramer of identical subunits. Another student (Lisa, who you arc working with on the project) in the research group has determined the amino acid compositions of the three proteins. However, when Lisa brings you the data, shown in the table below, you are greatly upset to discover that she failed to note which composition corresponds to which protein. Another student. Alison, working in the lab (on another project) tells you that you should simply deduce which is which from the amino acid compositions themselves. Lisa is sure that Alison is wrong. Should you take Alison's advice? If you decide to do so, which composition would you assign to which protein, and why?Explanation / Answer
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The residue properties is such that hydrophobic (nonpolar) residues are naturally buried in the protein where they are shielded from the polar solvent, whereas the polar residues are typically exposed to the compatible, polar aqueous solution.
(a) sphere has the minimum surface to volume ratio. Therefore, this protein should have the smallest polar to nonpolar residues ratio.
(b) cigar-shaped has large surface to volume ratio. Therefore, this protein should have a high polar to nonpolar residues ratio.
(c) It is reasonable to assume that both hydrophobic and polar residues may be involved in protein-protein interactions. So the subunit of a tetrameric protein is guessed to has a surface to volume ratio between the spherical and the cigar-shaped molecules.
Now, lets get the ratio data of the polar residues / nonpolar residues:
Protein 1: 131 / 71 = 1.85
Protein 2: 59 / 140 = 0.42
Protein 3: 99 / 100 = 0.99
Conclusion:
Protein 1 has the largest ratio polar to nonpolar residues. Therefore, it may be inferred that this is the cigar-shaped molecule (b) above.
Protein 2 has the smallest ratio polar to nonpolar residues. Therefore this is the spherical protein (a) above.
Protein 3 has the intermediate ratio polar to non-polar residues. Therefore it is the subunit of a tetrameric protein (c) above.
/update -- typo corrected