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)