Microbial biotransformation of chiral drugs is a vital area in advanced pharmaceutical biotechnology, focusing on how microorganisms or their enzymes selectively convert racemic mixtures into single enantiomers or unique metabolites. This blog-style MCQ set is tailored for M.Pharm students to deepen understanding of enantioselective biocatalysis, mechanisms such as kinetic resolution, deracemization and stereoinversion, and practical considerations including whole-cell versus isolated enzyme systems, cofactor regeneration, immobilization and analytical evaluation of enantiomeric excess. Questions emphasize real microbial enzymes, strain examples, process variables, and calculation of enantiomeric purity to bridge theoretical stereochemistry with practical bioprocess design for chiral drug development.
Q1. What is the primary objective of microbial biotransformation when applied to chiral drugs?
- To completely mineralize the drug into carbon dioxide and water
- To convert pro-drugs into activated metabolites only
- To achieve enantioselective conversion producing a single pharmacologically active enantiomer
- To increase molecular weight through glycosylation
Correct Answer: To achieve enantioselective conversion producing a single pharmacologically active enantiomer
Q2. Which statement correctly contrasts kinetic resolution with deracemization in microbial processes?
- Kinetic resolution can, in theory, convert 100% of a racemate to a single enantiomer
- Deracemization involves selective removal of one enantiomer leaving the other unchanged
- Kinetic resolution is limited to a theoretical maximum yield of 50% for one enantiomer
- Deracemization always requires chemical reagents and cannot be enzymatic
Correct Answer: Kinetic resolution is limited to a theoretical maximum yield of 50% for one enantiomer
Q3. What is a distinct advantage of using whole microbial cells over isolated enzymes for chiral biotransformations?
- Whole cells eliminate the need for cofactor regeneration
- Whole cells provide intrinsic cofactor regeneration and multiple enzyme activities for cascade reactions
- Whole cells always give higher enantioselectivity than purified enzymes
- Whole cells are immune to substrate inhibition and toxicity
Correct Answer: Whole cells provide intrinsic cofactor regeneration and multiple enzyme activities for cascade reactions
Q4. Which class of microbial enzymes is most commonly used for stereoselective reduction of prochiral ketones to chiral alcohols?
- Lipases
- Alcohol dehydrogenases (ADHs)
- Epoxide hydrolases
- Nitrile hydratases
Correct Answer: Alcohol dehydrogenases (ADHs)
Q5. What is the correct formula for calculating enantiomeric excess (ee)?
- ee = (minor enantiomer/major enantiomer) × 100%
- ee = (|R − S| / (R + S)) × 100%
- ee = (R × S) / (R + S)
- ee = (R + S) / 2
Correct Answer: ee = (|R − S| / (R + S)) × 100%
Q6. Microbial resolution of racemic profens (e.g., ibuprofen) is frequently achieved by which enzymatic activity?
- Decarboxylase-mediated decarboxylation
- Esterase- or lipase-mediated enantioselective hydrolysis of prodrug esters
- Direct photochemical racemization
- Transaminase-catalyzed racemization
Correct Answer: Esterase- or lipase-mediated enantioselective hydrolysis of prodrug esters
Q7. Which enzyme system is commonly used for in situ NAD(P)H regeneration during reductive microbial biotransformations?
- Lactate dehydrogenase with pyruvate
- Formate dehydrogenase with formate
- Glucose dehydrogenase with glucose
- Alcohol dehydrogenase with ethanol
Correct Answer: Glucose dehydrogenase with glucose
Q8. The enantiomeric ratio (E-value) used to quantify enzyme selectivity is best defined as which of the following?
- The ratio of enantiomeric excess to conversion
- The ratio of rate constants for conversion of the two enantiomers (kfast/kslow)
- The ratio of substrate solubility in water to organic solvent
- The ratio of product yield to enzyme concentration
Correct Answer: The ratio of rate constants for conversion of the two enantiomers (kfast/kslow)
Q9. Which microbial genus is widely used for enantioselective transformations of diverse chiral substrates, including reductions and oxidations?
- Escherichia
- Rhodococcus
- Bacillus subtilis only for proteases
- Clostridium exclusively for anaerobic fermentations
Correct Answer: Rhodococcus
Q10. Stereoinversion of a chiral secondary alcohol to its opposite configuration by microbes typically proceeds by which mechanistic sequence?
- Direct SN2-like attack at the stereocenter without intermediates
- Oxidation to a prochiral ketone followed by stereoselective reduction to the opposite enantiomer
- Radical-mediated hydrogen abstraction and rebound
- Concerted pericyclic rearrangement
Correct Answer: Oxidation to a prochiral ketone followed by stereoselective reduction to the opposite enantiomer
Q11. What is a principal advantage of immobilizing whole cells or enzymes for chiral biotransformations?
- Guaranteed 100% enantioselectivity regardless of conditions
- Enhanced operational stability and the ability to reuse biocatalyst
- Complete elimination of substrate inhibition
- Always increases reaction rate tenfold
Correct Answer: Enhanced operational stability and the ability to reuse biocatalyst
Q12. Which analytical technique is most appropriate for direct quantitative measurement of enantiomeric excess in chiral drugs?
- Achiral reversed-phase HPLC
- Chiral HPLC using a chiral stationary phase
- UV–Vis spectroscopy without chiral discrimination
- Standard GC with non-chiral column
Correct Answer: Chiral HPLC using a chiral stationary phase
Q13. How does pH most directly influence enantioselectivity in microbial enzyme-catalyzed reactions?
- By changing the bulk solvent density only
- By altering ionization states of active-site residues and substrates, thus modifying enzyme selectivity
- By converting the enzyme to a different primary sequence
- By increasing the molecular weight of the substrate
Correct Answer: By altering ionization states of active-site residues and substrates, thus modifying enzyme selectivity
Q14. Which term best describes a biotransformation in which both enantiomers of a racemate are converted into the same single enantiomeric product?
- Kinetic resolution
- Enantioconvergent transformation
- Simple racemization
- Dynamic covalent chemistry
Correct Answer: Enantioconvergent transformation
Q15. What is a common practical limitation encountered when scaling up whole-cell biotransformations of hydrophobic chiral substrates?
- Complete absence of side reactions
- Excessive thermal stability of cells
- Substrate toxicity and poor mass transfer due to low aqueous solubility
- Unlimited cofactor supply inside cells
Correct Answer: Substrate toxicity and poor mass transfer due to low aqueous solubility
Q16. For enantioselective opening or resolution of epoxides, which microbial enzyme class is most directly involved?
- Alcohol dehydrogenases
- Epoxide hydrolases
- Nitrilases
- Proteases
Correct Answer: Epoxide hydrolases
Q17. Which term is synonymous with enantiomeric excess when describing chiral purity of a sample?
- Optical purity
- Mass purity
- Chemical purity
- Isotopic purity
Correct Answer: Optical purity
Q18. What is the principal role of adding a water-miscible co-solvent (e.g., DMSO, ethanol) to a whole-cell biotransformation of a poorly soluble chiral drug?
- To act as the primary substrate for the biotransformation
- To increase substrate solubility but possibly reduce cell viability or enzyme activity
- To permanently denature the biocatalyst for easier downstream processing
- To remove the need for temperature control
Correct Answer: To increase substrate solubility but possibly reduce cell viability or enzyme activity
Q19. A common microbial strategy to obtain S-ibuprofen from a racemate involves which of the following steps?
- Nonselective hydrogenation of the aryl ring
- Enantioselective hydrolysis of a racemic ester precursor to yield S-ibuprofen
- Oxidative decarboxylation to a benzyl derivative
- Photochemical racemization followed by crystallization
Correct Answer: Enantioselective hydrolysis of a racemic ester precursor to yield S-ibuprofen
Q20. If a biotransformation yields a product mixture composed of 70% S-enantiomer and 30% R-enantiomer, what is the enantiomeric excess (ee)?
- 30%
- 40%
- 70%
- 100%
Correct Answer: 40%
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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