Suppose that a protein consists of 380 amino acids in its primary sequence, and the least abundant amino acid is cysteine, of which there is only one in the sequence. Which kind of intramolecular bond or attraction can NOT be used to stabilize this protein after folding?
a.) Hydrogen bonds
b.) Hydrophobic interactions
c.) Disulfide bonds
d.) Salt bridges
e.) All of the above can be used
The correct answer and explanation is:
The correct answer is c.) Disulfide bonds.
Explanation:
A protein’s stability after folding is determined by a variety of intermolecular forces. These forces help maintain the protein’s three-dimensional structure and function. Among the different types of bonding interactions, hydrogen bonds, hydrophobic interactions, and salt bridges all contribute to the stabilization of a protein’s tertiary structure. However, disulfide bonds are unique in their formation and have specific requirements.
Hydrogen bonds are interactions between a hydrogen atom and an electronegative atom like oxygen or nitrogen. They play a crucial role in stabilizing protein structures by forming between the peptide backbone and side chains of the amino acids. Since hydrogen bonds are not dependent on the presence of cysteine residues, they can certainly form in a protein regardless of the presence of cysteine.
Hydrophobic interactions occur when nonpolar side chains of amino acids cluster together in the interior of the protein, away from the aqueous environment. These interactions stabilize the protein by reducing the exposure of hydrophobic residues to the surrounding solvent. These interactions are independent of cysteine, so they are not affected by the absence of cysteine in the protein.
Salt bridges, or ionic bonds, are formed between positively and negatively charged side chains of amino acids. These interactions stabilize the protein structure by creating strong electrostatic forces. Again, the presence or absence of cysteine does not impact the formation of salt bridges.
Disulfide bonds, on the other hand, are covalent bonds formed between the sulfur atoms of two cysteine residues. They play a crucial role in stabilizing the folded structure of many proteins, especially extracellular proteins that need to withstand harsh environments. However, in this case, since there is only one cysteine residue in the protein’s sequence, disulfide bonds cannot form. Disulfide bonds require at least two cysteine residues, making them irrelevant for stabilizing a protein with only a single cysteine. Thus, disulfide bonds cannot be used to stabilize the protein.