1.7 Proteins
A biochemist analyzes a newly discovered protein and finds that most of its nonpolar amino acids are located in the interior of the folded molecule, while most of its polar and charged amino acids are on the surface. Which of the following best explains this arrangement?
Which of the following correctly describes the formation of a peptide bond?
A researcher observes that a protein loses its biological activity when heated to 80°C, but analysis shows that all peptide bonds in the molecule remain intact. Which of the following best explains this observation?
A patient with sickle cell disease has a single amino acid substitution in which a hydrophilic glutamic acid is replaced by a hydrophobic valine on the surface of the hemoglobin beta subunit. Which levels of protein structure are most likely disrupted by this mutation?
An enzyme functions optimally at pH 7. When the pH is lowered to 2, the enzyme loses all catalytic activity. Which of the following best explains why the acidic environment destroys enzyme function?
Which of the following correctly distinguishes secondary structure from tertiary structure in a protein?
A food scientist heats egg white (rich in the protein albumin) and observes that it changes from transparent and liquid to opaque and solid. The scientist then cools the egg white but it does not return to its original state. Which of the following best explains why the change is irreversible?
A polypeptide contains 150 amino acids. How many water molecules are released during the complete synthesis of this polypeptide from free amino acids?
A cell biologist discovers a protein that functions as a single folded polypeptide chain with no partner subunits. Which level of protein structure does this protein lack?
During an experiment, a student adds a heavy metal ion solution to a protein sample and observes that the protein loses its function. The student hypothesizes that the heavy metal ions disrupted the protein's structure. Which of the following interactions are most likely directly disrupted by heavy metal ions binding to charged R groups?
A marine biologist studies a deep-sea organism whose proteins function normally at 4°C and extremely high pressure. When the same proteins are brought to the surface at 25°C and atmospheric pressure, they denature. A colleague suggests that this is because the proteins rely on different proportions of stabilizing interactions than typical human proteins. Which of the following changes in R-group composition would most plausibly help a protein maintain its tertiary structure under cold, high-pressure conditions but make it vulnerable to denaturation at warmer, lower-pressure conditions?
The protein hemoglobin consists of four polypeptide subunits — two alpha chains and two beta chains — that must assemble correctly to transport oxygen. Which of the following correctly identifies the structural level at which the four subunits associate, and the types of interactions responsible?
A student claims that if a protein is denatured, its primary structure is destroyed. Which of the following statements best corrects this misconception?
An alpha-helix and a beta-pleated sheet are both examples of secondary structure. What do these two structures have in common?
A genetic mutation replaces a cysteine residue with an alanine residue at a position in a protein where a disulfide bridge normally forms. Which of the following is the most likely structural consequence?