Question 1

Which principle allows isoelectric focusing to separate proteins during resolution electrophoresis?

Accepted Answer

Proteins migrate until they reach a pH where their net charge is zero, known as the isoelectric point ($pI$).

Question 2

In SDS-PAGE, what is the role of $eta$-mercaptoethanol (BME)?

Accepted Answer

It reduces disulfide bonds, denaturing proteins and allowing separation by size.

Question 3

During size exclusion chromatography, which proteins elute first from the column?

Accepted Answer

Larger proteins, because they cannot enter the pores of the gel beads.

Question 4

Two-dimensional gel electrophoresis separates proteins based on which two properties?

Accepted Answer

Isoelectric point ($pI$) and molecular weight.

Question 5

Which enzyme is commonly used to cleave proteins at the carboxyl side of lysine and arginine residues?

Accepted Answer

Trypsin

Question 6

What is the main chemical reaction in Edman degradation for sequencing proteins?

Accepted Answer

Phenylisothiocyanate reacts with the N-terminal amino acid to form a thiazolinone derivative.

Question 7

In mass spectrometry-based protein identification, what does the $m/z$ ratio represent?

Accepted Answer

The mass-to-charge ratio of ionized protein fragments.

Question 8

Which disease is most commonly associated with a deletion of phenylalanine in the CFTR protein, leading to misfolding and degradation?

Accepted Answer

Cystic fibrosis

Question 9

Polyglutamine (poly(Q)) expansion is implicated in which type of disease mechanism?

Accepted Answer

Protein aggregation due to unstable, expanded trinucleotide repeats.

Question 10

Which statement best describes the evolutionary relationship between myoglobin and hemoglobin?

Accepted Answer

They are structurally related proteins with approximately 17% sequence identity.

Question 11

What is the thermodynamic driving force for protein folding?

Accepted Answer

Minimization of free energy ($\Delta G$) through hydrophobic interactions and entropy changes.

Question 12

Which technique would you use to determine the primary structure of a protein with high accuracy and speed?

Accepted Answer

Mass spectrometry

Question 13

Why do proteins migrate differently in SDS-PAGE compared to native PAGE?

Accepted Answer

SDS-PAGE separates proteins by size because SDS imparts a uniform negative charge, eliminating charge differences.

Question 14

A patient presents with symptoms of a neurodegenerative disorder. Genetic analysis reveals more than 35 CAG repeats in a gene encoding a poly(Q) tract. What is the most likely molecular consequence?

Accepted Answer

Increased protein aggregation leading to neuronal dysfunction.

Question 15

Which of the following is a function of molecular chaperones in protein folding?

Accepted Answer

They provide a protected environment for proper folding and prevent aggregation.

Question 16

Which method would best distinguish between healthy and diseased states in a complex protein mixture?

Accepted Answer

Two-dimensional gel electrophoresis (2D-GE)

Question 17

What is the effect of a point mutation in the $eta$-globin gene that changes glutamic acid to valine at position 6?

Accepted Answer

It causes sickle cell disease by altering hemoglobin structure and function.

Question 18

During Edman degradation, which group does phenylisothiocyanate (PITC) specifically react with?

Accepted Answer

The N-terminal amino group of the polypeptide.

Question 19

Which of the following best explains why proteins with similar functions can have low sequence identity?

Accepted Answer

Protein families can retain structural and functional similarities despite sequence divergence due to evolutionary pressures.

Protein Separation, Identification, and Folding in Biochemistry

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