Protein Secondary, Tertiary, Quaternary Structure (1A) Practice Test

A researcher studies a soluble enzyme that requires a tightly packed hydrophobic core for function. When the enzyme is produced in bacteria at low temperature, it is active; when produced at higher temperature, it is mostly inactive despite identical amino acid sequence. Analysis shows no change in oligomeric state, but the high-temperature preparation is more susceptible to protease digestion. Based on tertiary structure, what is the most likely explanation for the loss of activity in the high-temperature preparation?

A secreted signaling protein is synthesized in the ER and contains four cysteines that form two intramolecular disulfide bonds in the mature protein. In a cell-based assay, a Cyse2e1Ser mutation at one of these positions yields a protein that is still secreted but shows reduced receptor binding. Nonreducing SDS-PAGE suggests altered disulfide pairing, while CD spectroscopy indicates similar overall secondary structure content. Based on the protein’s tertiary structure, what is the most likely consequence of the Cyse2e1Ser change?