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Chapter 6: Problem 8

Two polypeptides, \(A\) and \(B\), have similar tertiary structures, but \(A\) normally exists as a monomer, whereas \(\mathrm{B}\) exists as a tetramer, \(\mathrm{B}_{4}\) What differences might be expected in the amino acid composition of A versus B?

Short Answer

Expert verified
The major difference in the amino acid composition of polypeptides A and B would be that A, which exists as a monomer, would largely consist of hydrophobic amino acids. In contrast, B, which exists as a tetramer, would have more polar or charged amino acids to support intermolecular interactions leading to the tetramer structure.

Step by step solution

01

- Understand the Basics

A polypeptide is a single linear chain of amino acids. The exact sequence of amino acids in a polypeptide chain is the primary structure of the protein. Now, a monomer is a molecule that forms the basic unit for polymers, meaning it exists independently. A tetramer, on the other hand, is a polymer comprising four monomer units.
02

- Amino Acid Composition in Monomers and Tetramers

Polypeptide A is a monomer, implies that it is a single entity, which exists independently. Hence, the amino acid composition might largely consist of hydrophobic amino acids to ensure stability. On the other hand, polypeptide B exists as a tetramer. Therefore, it is logical to assume that it includes amino acids that favor interaction with other chains or units. It would rely on intermolecular interactions such as hydrogen bonds, electrostatic interactions, or disulfide bonds. This means it may have more polar or charged amino acids to support these interactions, leading to the formation of a tetramer structure from individual monomer units.
03

- Conclusion

In conclusion, the main difference between polypeptides A and B would be in their amino acid composition. Polypeptide A, being a monomer, would largely consist of hydrophobic amino acids, while polypeptide B, which forms a tetramer, would have a higher proportion of polar or charged amino acids. These differences in amino acid composition drive their ability to exist as a monomer and a tetramer respectively.

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Most popular questions from this chapter

A new protein of unknown structure has been purified. Gel filtration chromatography reveals that the native protein has a molecular weight of 240,000 , Chromatography in the presence of \(6 M\) guanidine hydrochloride yields only a peak for a protein of \(M\) \(60,000 .\) Chromatography in the presence of \(6 \mathrm{M}\) guanidine hydrochloride and \(10 \mathrm{m}\) M \(\beta\) -mercaptoethanol yields peaks for proteins of \(M, 34,000\) and \(26,000 .\) Explain what can be determined about the structure of this protein from these data.

It is often observed that Gly residues are conserved in proteins to a greater degree than other amino acids. From what you have learned in this chapter, suggest a reason for this observation.

Poly-L-glutamate adopts an a-helical structure at low pH but becomes a random coil above pH \(5 .\) Explain this behavior.

Discuss the potential contributions to hydrophobic and van der Waals interactions and ionic and hydrogen bonds for the side chains of Asp, Leu, Tyr, and His in a protein.

Choose any three regions in the Ramachandran plot and discuss the likelihood of observing that combination of \(\phi\) and \(\psi\) in a peptide or protein. Defend your answer using suitable molecular models of a peptide.
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