Biochemistry Protein Structure and Function 2 — Questions and Answers
Question 1: What is the isoelectric point (pI) of a protein?
- The pH at which the protein has maximum solubility
- The pH at which the protein carries no net charge (Correct answer)
- The pH at which the protein denatures
- The pH at which the protein has maximum enzymatic activity
Correct answer: The pH at which the protein carries no net charge
The isoelectric point is the pH at which the net charge of the protein is zero, meaning positive and negative charges are balanced.
Question 2: Chaperone proteins assist in which process?
- Protein degradation
- mRNA translation initiation
- Correct protein folding (Correct answer)
- Post-translational glycosylation
Correct answer: Correct protein folding
Molecular chaperones bind to unfolded or misfolded proteins and provide a protected environment that facilitates correct folding, preventing aggregation.
Question 3: Which prosthetic group is found in myoglobin and hemoglobin, enabling oxygen binding?
- Flavin adenine dinucleotide (FAD)
- Pyridoxal phosphate (PLP)
- Heme group (Correct answer)
- Biotin
Correct answer: Heme group
The heme group contains an iron atom (Fe²⁺) coordinated within a porphyrin ring, which reversibly binds molecular oxygen.
Question 4: The cooperative binding of oxygen to hemoglobin is best described by which model?
- Lock-and-key model
- Induced fit model
- Concerted (MWC) model (Correct answer)
- Michaelis-Menten model
Correct answer: Concerted (MWC) model
The concerted (Monod-Wyman-Changeux) model proposes that all subunits switch simultaneously between T (tense, low affinity) and R (relaxed, high affinity) states.
Question 5: 2,3-Bisphosphoglycerate (2,3-BPG) affects hemoglobin oxygen binding by:
- Increasing oxygen affinity by stabilizing the R state
- Decreasing oxygen affinity by stabilizing the T state (Correct answer)
- Blocking the heme iron from binding oxygen
- Promoting dissociation of hemoglobin subunits
Correct answer: Decreasing oxygen affinity by stabilizing the T state
2,3-BPG binds in the central cavity of deoxyhemoglobin and stabilizes the low-affinity T state, promoting oxygen release to tissues.
Question 6: Which technique separates proteins based on their isoelectric points?
- SDS-PAGE
- Size-exclusion chromatography
- Isoelectric focusing (Correct answer)
- Affinity chromatography
Correct answer: Isoelectric focusing
Isoelectric focusing separates proteins in a pH gradient; each protein migrates until it reaches the pH equal to its pI, where it has no net charge and stops.
Biochemistry Protein Structure and Function 2
What is the isoelectric point (pI) of a protein?