As part of an undergraduate biochemistry lab project, you are characterizing thr
ID: 484558 • Letter: A
Question
As part of an undergraduate biochemistry lab 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 numeric, cigar-shaped molecule, the second is and approximately spherical, and the third is the submit of a tetrameter of identical sub-units. Your lab partner has determined the ammo acid compositions of the three proteins However, when he brings you the data, shown in Table I. you are greatly upset to discover that he failed to note which composition corresponds to which protein. A student at the next bench tells you that you should simply deduce which is which from the amino acid compositions themselves. Your partner is sure that the student is trying to trick you into making an error that will lower your grade. Should you take his advice? If you decide to do so. which composition would you assign to which protein, and why? Amino Acid Compositions of Three Hypothetical Plasma ProteinsExplanation / Answer
Ans. Molecule A: Monomeric, cigar shaped = Protein 2
Protein 2 is most likely to be molecule A.
Molecule A is cigar shaped, i.e. the axial ratio (length / width ratio) of the protein is most likely to be greater than 10- that is, the molecule is around 10 times or more longer than its width. Hence, the protein is fibrous.
A fibrous protein is usually insoluble in water. It is evident by the presence of relative abundance of hydrophobic (non-polar residues) in the protein. Presence of very large number of proline residue is responsible for the bend (bending at one end as seen in cigar) in the molecule because proline produces bend/kink in the molecule. Accumulation of large number of proline residues at specified region may cause bend in the 3D folded protein.
Molecule B: Monomeric, approx. spherical = Protein 1
An approximately spherical protein (axial ratio of 3-10) is a globular protein that is soluble in water. Relative abundance of hydrophilic or polar residues (as shown in table) are present on the surface of protein. The hydrophilic residues on surface interact with polar aqueous environment and make the molecule solvated in it.
So, protein 1 is most likely to be molecule B.
Molecule C: Tetramer of identical subunits: Protein 3
Two or more polypeptides interact among each other to form quaternary structure. Being tetramer, 4 polypeptides must associate together. Interaction between two polypeptides includes ionic interaction (salt bridges between oppositely charged residues), H-bonds, and hydrophobic interactions. Or, both the polar and non-polar interactions may weigh equal. Therefore, it shall have relative abundance of both polar and non-polar residues.
Therefore, protein 3 is the most appropriate candidate to fulfil the criteria of being a tetramer.
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