MCAT Practice Problems Lecture

Glucose in Straight Chain Form

• Type: Aldohexose
  • 1 aldehyde group
  • 6 carbons
• Eliminated Options: A & B
  • Not an Aldoketose
  • Not a Pentose
• Chiral Centers: 4 (each non-terminal carbon is chiral)
• Stereoisomers Formula: 2^n (where n = number of chiral centers)
  • For glucose: 2^4 = 16 stereoisomers
• Answer: D

Epimers Characteristics

• Definition: Monosaccharide diastereomers differing at one carbon
• Chemical & Physical Properties: Slightly different
• Optical Activities: No relation (different from enantiomers which have equal but opposite activity)
• False Statement (Answer): They have equal but opposite Optical activities

Aldonic Acids

• Formation: Oxidation of the aldehyde group on a reducing sugar
  • Act as reducing agents
• Answer: C

Anomerization of Glucopyranose

• Term: Mutarotation
  • Definition: Interconversion between anomers (alpha and beta forms)
• Alpha Form: OH group trans to CH2OH
• Beta Form: OH group cis to CH2OH
• Interconversion: Mutarotation
• Incorrect Options: Racemization, Enantiomerization, Glycosidation
• Answer: B

Reducing Property of Ketose Sugars

• Process: Tautomerization (rearrangement of bonds to keto-enol shift)
  • Formation: Aldose, then acting as a reducing sugar
• Ketones: Cannot be oxidized directly
• Answer: A

Reaction of Glucose with Methanol

• Under Acid Catalysis: Converts hemiacetal to acetal
  • Replacement: Anomeric hydroxyl group replaced by alkoxy group
  • Product: Glycoside
• Reaction: Glucose + Methanol → Glucose with OCH3 replacing OH
• Answer: B

Enzyme Cleaving Polysaccharides to Yield Maltose

• Enzyme: Beta Amylase
  • Action: Cleaves amylose at non-reducing end, yields maltose exclusively
• Other Enzymes:
  • Alpha Amylase: Cleaves anywhere along the chain (yields maltose, glucose, shorter polysaccharides)
  • Debranching Enzyme: Removes oligosaccharides from a branch in glycogen/starch
  • Glycogen Phosphorylase: Yields glucose-1-phosphate
• Answer: B