I need to identify the three proteins from the information given in this chart.
ID: 484571 • Letter: I
Question
I need to identify the three proteins from the information given in this chart. I also know that one protein is monomeric and cigar shaped, another is monomeric and spherical, and the last is a tetramer of identical subunits. I have NO idea how I'm supposed to do this or what I should be getting from this chart of residues, please help?
"As part of an undergraduate biochemistry labproject, you are characterizing three polypeptides ofapproximately equal molecular weight from humanplasma. Using various physical techniques, youhave established that in their native states one of thepolypeptides is a monomeric, cigar-shapedmolecule, the second is monomeric andapproximately spherical, and the third is the subunitof a tetramer of identical sub-units. Your lab partnerhas determined the amino acid compositions of thethree proteins. However, when he brings you thedata, shown in Table 1, you are greatly upset todiscover that he failed to note which compositioncorresponds to which protein. A student at the nextbench tells you that you should simply deducewhich is which from the amino acid compositionsthemselves. Your partner is sure that the student istrying to trick you into making an error that willlower your grade.
Should you take his advice?
If you decide to do so, which composition wouldyou assign to which protein, and why?"
Explanation / 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.