MCAT Biochemistry Review: Non-Enzymatic Protein Functions and Protein Analysis

Practice Problems Overview

• Topic: Non-enzymatic protein functions and protein analysis
• Focus: Key concepts and problem-solving related to MCAT preparation

Problem 1: Electrophoresis and pH

• Given: Proteins A, B, C with pIs 4.5, 6, and 9.5 respectively
• Isoelectric Point (pI): pH at which a protein is electrically neutral
• Objective: Isolate Protein A
  • pH for best separation: 5.5 (Protein A becomes negative; Proteins B and C remain neutral or positive)
  • Incorrect choice: pH 4.5 (Protein A would be neutral and not migrate)
• Answer: D (5.5)

Problem 2: Sodium Dodecyl Sulfate (SDS) in SDS-PAGE

• SDS Function: Detergent that digests proteins to form micelles with uniform negative charges
• Purpose: Separates proteins based on size
  • Denatures proteins: Masks native charge, size-based separation
  • Note: Functional protein can't be recaptured
• Correct Answer: B

Problem 3: Cell Migration

• Choices: Dynein, Flagella, Actin, Centrioles
• Non-participant in cell Migration: Centrioles (involved in mitosis, composed of microtubules)
• Answer: D

Problem 4: Extracellular Protein

• Choices: Tubulin, Myosin, Collagen, Actin
• Extracellular Proteins: Keratin, Elastin, Collagen
  • Answer: Collagen (
• Correct Answer: C

Problem 5: Hormone Receptors

• Characteristics: Low concentration, strong effect
• Types: Ligand-gated ion channels, Enzyme-linked receptors, G-protein coupled receptors
  • Mechanism: Second messenger cascade (amplification of signal)
• Correct Answer: C (enzyme-linked and G-protein coupled receptors)

Problem 6: Ions Bound to Proteins

• Choices: Sodium, Potassium, Chloride, Calcium
• Binding Propensity: Calcium (sequestered in both bloodstream and intracellular spaces)
  • Sodium and Potassium: Must exist in free state for action potentials
  • Chloride: Readily excreted by kidneys
• Correct Answer: D (Calcium)

Problem 7: Antibodies Characteristics

• Function: Specific to a single antigen
• Characteristics:
  • Label antigens, cause agglutination, have two heavy and light chains
  • False Statement: Antibodies bind to more than one antigen
• Correct Answer: A

Problem 8: Resting Membrane Potential

• Channels: Ungated, Voltage-gated, Ligand-gated
• Maintaining Resting Potential: Ungated channels (permit limited ion flow)
  • Voltage-gated and ligand-gated: Involved in cell signaling
• Correct Answer: A

Problem 9: Trimeric G Proteins

• Components: Alpha, Beta, Gamma subunits
• Not a component: G of I (GS, GI, and GQ are subtypes of G Alpha)
• Correct Answer: D

Problem 10: Protein Separation Methods

• Given: Proteins with close pIs and different masses
• Methods:
  • Not Suitable: Ion-exchange chromatography, isoelectric focusing, native PAGE
  • Suitable: Size exclusion chromatography (best for large quantities)
• Correct Answer: D

Problem 11: Amino Acids Influencing pI

• Choices: Lysine, Glycine, Arginine
• Influence on pI: Basic amino acids (Arginine, Lysine, Histidine) and acidic amino acids (Aspartic and Glutamic Acid)
• Relevant amino acids from choices: Lysine, Arginine
• Correct Answer: C

Conclusion

• Key Points: pH and protein separation, SDS-PAGE, cell migration, antibody specificity, resting membrane potential, protein-bound ions, G proteins, and separation methods
• Encouragement: Happy studying and best of luck!

Additional Notes

• Review basic concepts: Particularly those related to second messenger systems and electrophoresis
• Practice: More problems on protein isolation and analysis