Shikimic, acetate and amino acid pathways and radio-tracing techniques MCQs With Answer

Introduction

This quiz set focuses on the shikimic, acetate and amino acid biosynthetic pathways and practical radio-tracing techniques — core areas in phytochemistry for M.Pharm students. Questions emphasize biochemical steps, key enzymes, branch points, and how plant secondary metabolites (alkaloids, phenylpropanoids, polyphenols, polyketides) arise from these pathways. The radio-tracing portion explores isotopic labeling strategies (14C, 3H, 32P), pulse–chase designs, autoradiography and liquid scintillation methods used to trace precursor–product relationships and carbon flow. These MCQs are designed to test conceptual understanding and experimental interpretation required for research and advanced coursework in natural product biosynthesis and analytical tracing methods.

Q1. Which intermediate is the direct product of the shikimate pathway that serves as the universal precursor to aromatic amino acids?

• Prephenate
• Chorismate
• Shikimate-3-phosphate
• Aromatic amino acid aminotransferase

Correct Answer: Chorismate

Q2. Glyphosate, a broad-spectrum herbicide, exerts its effect by inhibiting which enzyme in the shikimate pathway?

• Chorismate mutase
• DAHP synthase
• 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase)
• Shikimate kinase

Correct Answer: 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase)

Q3. In plants, the shikimate pathway is localized predominantly in which subcellular compartment?

• Cytosol
• Chloroplast (plastid)
• Mitochondrion
• Endoplasmic reticulum

Correct Answer: Chloroplast (plastid)

Q4. Which enzyme catalyzes the non-oxidative decarboxylative condensation of p-coumaroyl-CoA with three malonyl-CoA units to form naringenin chalcone in flavonoid biosynthesis?

• Chalcone synthase (CHS)
• Chalcone isomerase (CHI)
• Flavanone 3-hydroxylase (F3H)
• Polyketide synthase type I

Correct Answer: Chalcone synthase (CHS)

Q5. The acetate (polyketide) pathway commonly uses which two-carbon extender unit for successive condensations in polyketide and fatty acid biosynthesis?

• Acetyl-CoA
• Malonyl-CoA
• Succinyl-CoA
• Methylmalonyl-CoA

Correct Answer: Malonyl-CoA

Q6. During malonyl-CoA–driven Claisen condensations, one carbon is lost as CO2. This decarboxylation provides what principal advantage to the condensation reaction?

• It supplies NADPH for reduction steps
• It activates the acyl acceptor by forming an enolate for nucleophilic attack
• It increases ATP production
• It generates radical intermediates for cyclization

Correct Answer: It activates the acyl acceptor by forming an enolate for nucleophilic attack

Q7. Chorismate mutase directs chorismate into which biosynthetic branch leading to phenylalanine and tyrosine?

• Anthranilate pathway
• Prephenate pathway
• Isochorismate pathway
• Shikimate pathway backward flux

Correct Answer: Prephenate pathway

Q8. Phenylalanine ammonia-lyase (PAL) catalyzes the conversion of L-phenylalanine to which initial phenylpropanoid intermediate?

• p-Coumaric acid
• Cinnamic acid
• Benzoic acid
• Caffeic acid

Correct Answer: Cinnamic acid

Q9. In radio-tracing experiments, which isotope is most commonly used for long-term carbon tracing because of its suitable half-life and beta emission?

• 3H (Tritium)
• 32P
• 14C
• 35S

Correct Answer: 14C

Q10. A pulse–chase experiment using [U-14C]-phenylalanine followed by unlabeled phenylalanine is primarily designed to reveal which information?

• The total pool size of phenylalanine in tissue
• The rate of conversion of phenylalanine into downstream phenylpropanoids over time
• Subcellular localization of phenylalanine biosynthesis enzymes
• The degradation pathway of labeled phenylalanine to CO2

Correct Answer: The rate of conversion of phenylalanine into downstream phenylpropanoids over time

Q11. Which of the following is the correct precursor–product relationship for tryptophan biosynthesis starting from chorismate?

• Chorismate → Prephenate → Anthranilate → Tryptophan
• Chorismate → Anthranilate → 3-indole glycerol phosphate → Tryptophan
• Chorismate → Shikimate → Indole-3-acetic acid → Tryptophan
• Chorismate → Isochorismate → Salicylate → Tryptophan

Correct Answer: Chorismate → Anthranilate → 3-indole glycerol phosphate → Tryptophan

Q12. When interpreting autoradiographs of sectioned plant tissue fed with position-specifically labeled acetate, unequal grain density across cell types most likely indicates what?

• Differences in autoradiographic emulsion thickness only
• Differential metabolic activity and incorporation of label among cell types
• Uniform transport of acetate but unequal drying artifacts
• Contamination by external 14CO2

Correct Answer: Differential metabolic activity and incorporation of label among cell types

Q13. In polyketide biosynthesis, iterative type II polyketide synthases are predominantly associated with which organisms and products?

• Plants producing terpenes
• Bacteria producing aromatic polyketides such as actinorhodin
• Mammals producing fatty acids
• Fungi producing nonribosomal peptides

Correct Answer: Bacteria producing aromatic polyketides such as actinorhodin

Q14. Specific activity in a radio-labeling experiment is defined as:

• Total radioactivity present in the scintillation vial
• Radioactivity per unit mass (or mole) of compound
• The half-life of the radioisotope used
• The background counts per minute of the detector

Correct Answer: Radioactivity per unit mass (or mole) of compound

Q15. Which technique provides the highest spatial resolution for detection of radioactive label within tissue sections?

• Liquid scintillation counting of homogenates
• Whole-body autoradiography with phosphorimager
• Emulsion autoradiography (microscopic silver grain autoradiography)
• Geiger counter surface scanning

Correct Answer: Emulsion autoradiography (microscopic silver grain autoradiography)

Q16. When using 14C-labeled acetate to trace polyketide assembly, incorporation at every other carbon position of the product typically indicates what biosynthetic feature?

• Use of propionyl-CoA starter units exclusively
• Successive additions of malonyl-CoA with decarboxylative condensation
• Direct condensation of intact acetate dimers without decarboxylation
• Incorporation via shikimate pathway intermediates

Correct Answer: Successive additions of malonyl-CoA with decarboxylative condensation

Q17. Isochorismate synthase converts chorismate to isochorismate, a precursor for which plant compound commonly derived from the shikimate pathway?

• Auxin (IAA)
• Salicylic acid (SA)
• Jasmonic acid (JA)
• Squalene

Correct Answer: Salicylic acid (SA)

Q18. In designing a radio-tracing experiment to prove that a secondary metabolite is derived from phenylalanine, which control strengthens evidence for precursor–product relationship?

• Feeding unlabeled phenylalanine and measuring growth only
• Feeding [U-14C]-phenylalanine and demonstrating co-chromatography of radioactivity with the isolated metabolite plus competitive inhibition by excess unlabeled phenylalanine
• Measuring ATP levels after labeled feeding
• Feeding labeled acetate instead and observing no incorporation

Correct Answer: Feeding [U-14C]-phenylalanine and demonstrating co-chromatography of radioactivity with the isolated metabolite plus competitive inhibition by excess unlabeled phenylalanine

Q19. Which enzyme provides the entry point of primary metabolism into phenylpropanoid metabolism by deamination of phenylalanine?

• Phenylalanine hydroxylase
• Phenylalanine ammonia-lyase (PAL)
• Tyrosine aminotransferase
• Cinnamate 4-hydroxylase (C4H)

Correct Answer: Phenylalanine ammonia-lyase (PAL)

Q20. When interpreting labeling patterns, dilution of specific activity of a precursor pool can cause underestimation of incorporation into products. Which experimental approach reduces precursor pool dilution?

• Using very long chase periods with unlabeled precursor
• Feeding high specific activity tracer and minimizing the duration between feed and sampling (short pulse)
• Adding excess unlabeled precursor during pulse
• Using isotopes with very short half-lives only

Correct Answer: Feeding high specific activity tracer and minimizing the duration between feed and sampling (short pulse)

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