MCAT Practice Problems: Carbohydrates

May 28, 2024

Lecture Notes on MCAT Practice Problems for Carbohydrates

Introduction

  • Presenter: Iman
  • Topic: MCAT practice problems focusing on carbohydrates
  • Previous session: Covered carbohydrate structure and function

Problem 1: Properties of Glucose in Straight-Chain Form

  • Glucose: An aldohexose (1 aldehyde group, 6 carbons)
  • Key Points:
    • Aldohexose, not Aldoketose or Pentose
    • 4 chiral centers
    • Number of stereoisomers: 2^n (where n = number of chiral centers)
    • Glucose has 16 stereoisomers
  • Correct Answer: D (16 stereoisomers)

Problem 2: Properties of Epimers

  • Epimers: Monosaccharide diastereomers that differ at only one carbon
  • Key Points:
    • Epimers are diastereomers, not enantiomers
    • Different chemical & physical properties
    • Optical activity has no specific relation
  • Correct Answer: D (They have equal but opposite optical activities—this is false for epimers)

Problem 3: Aldonic Acids

  • Aldonic Acids:
    • Formed when the aldehyde group of a reducing sugar reduces another compound and gets oxidized
    • Act as reducing agents
  • Correct Answer: C (Compounds that have been oxidized and act as reducing agents)

Problem 4: Formation of Alpha and Beta D-Glucopyranose

  • Mutarotation:
    • Interconversion between anomers (alpha and beta forms)
    • Alpha: Hydroxyl group trans to CH2OH
    • Beta: Hydroxyl group cis to CH2OH
    • Anomeric carbons: New chiral centers formed in cyclization
  • Correct Answer: B (Mutarotation)

Problem 5: Reducing Ability of Keto Sugars

  • Keto Sugars:
    • Undergo tautomerization (rearrangement to form aldose)
    • Aldose can act as reducing sugars
  • Correct Answer: A (Undergo tautomerization)

Problem 6: Reaction of Glucose with Methanol

  • Reaction:
    • Glucose reacts with methanol under acid catalysis
    • Hemiacetal converted to acetal, forming glycoside
    • Anomeric hydroxyl group replaced with methoxy group (CH3O)
  • Correct Answer: B

Problem 7: Enzyme Cleaving Polysaccharide Chains

  • Beta-Amylase:
    • Cleaves amylose at non-reducing end, yields maltose exclusively
  • Alpha-Amylase: Cleaves amylose to yield shorter polysaccharides, maltose, and glucose
  • Debranching Enzyme: Removes oligosaccharides
  • Glycogen Phosphorylase: Yields glucose-1-phosphate
  • Correct Answer: B (Beta-Amylase)