Mass Spectrometry in Biochemistry — MCQs
Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
Which test is used for detecting gunshot residue?
Which technique is used for protein separation based on molecular size?
Which of the following techniques is used for the detection of variations in DNA sequence and gene expression?
Which of the following hormones is an example of a peptide hormone?
Which technique is used for the separation of proteins based on their mass?
Equilibrium potential for an ion is calculated by:
The technique shown in the image is:
A patient reports a change in colour of urine on air exposure. All are true about the condition shown below except:
Which of the following methods of protein separation is not dependent on molecular size?
Mass Spectrometry in Biochemistry Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
- A. Trypsin
- B. Carboxypeptidase
- C. Phenylisothiocyanate (Correct Answer)
- D. Cyanogen bromide
Explanation: ***Phenylisothiocyanate*** - **Phenylisothiocyanate** (PITC), also known as Edman's reagent, is used in the **Edman degradation** method to identify the N-terminal amino acid. - It sequentially cleaves the **N-terminal amino acid** without hydrolyzing the rest of the peptide chain, allowing for identification by chromatography. *Trypsin* - **Trypsin** is a protease that cleaves peptide bonds at the carboxyl side of **lysine** and **arginine** residues. - It is used for peptide fragmentation, not for determining the N-terminal amino acid. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave amino acids from the **C-terminal end** of a polypeptide chain. - They are used to identify the C-terminal amino acid, not the N-terminal. *Cyanogen bromide* - **Cyanogen bromide (CNBr)** is a chemical reagent that specifically cleaves peptide bonds on the C-terminal side of **methionine** residues. - It is used for specific peptide fragmentation and not for N-terminal sequencing.
Question 2: Which test is used for detecting gunshot residue?
- A. Lie test for Firearm injury
- B. Neutron activation analysis for firearm use (Correct Answer)
- C. Toluidine blue test
- D. Benzidine test for blood stain
Explanation: ***Neutron activation analysis for firearm use*** - **Neutron activation analysis (NAA)** is a highly sensitive and reliable method for detecting specific elements characteristic of **gunshot residue (GSR)**, such as **barium**, **antimony**, and **lead**. - This technique works by irradiating samples with neutrons, causing them to emit gamma rays that are unique to each element, allowing for precise identification and quantification of GSR particles. *Lie test for Firearm injury* - A "lie test" typically refers to a **polygraph test**, which assesses physiological responses to detect deception, not physical evidence like gunshot residue. - Polygraph tests are not used for identifying **firearm injury** or the presence of actual physical traces. *Toluidine blue test* - The **Toluidine blue test** is primarily used in dentistry to detect and delineate **dysplastic or malignant lesions** in the oral mucosa. - It has no application in the forensic analysis of gunshot residue or firearm use. *Benzidine test for blood stain* - The **Benzidine test** was historically used as a preliminary test for the presence of **blood stains**, as it reacts with the heme component of hemoglobin. - It is not used for detecting **gunshot residue** and has largely been replaced by safer and more specific tests due to its carcinogenic properties.
Question 3: Which technique is used for protein separation based on molecular size?
- A. Affinity chromatography
- B. Gel filtration chromatography (Correct Answer)
- C. HPLC
- D. Salting out
Explanation: ***Gel filtration chromatography*** - Also known as **size-exclusion chromatography**, this method separates proteins by passing them through a porous matrix. **Larger proteins** elute first as they cannot enter the pores, while smaller proteins get trapped and elute later. - This technique effectively separates proteins based solely on their **hydrodynamic radius**, which is closely related to their molecular size. *Affinity chromatography* - This method separates proteins based on their **specific binding affinity** to a ligand immobilized on a stationary phase, not molecular size. - It is used for purifying proteins that bind to a specific molecule, such as an antibody or substrate. *HPLC* - **High-performance liquid chromatography** is a general technique that can use various separation mechanisms (e.g., reverse-phase, ion-exchange, size-exclusion) under high pressure. - While it *can* be used for size-exclusion, HPLC itself describes the *method* of chromatographic performance rather than a specific separation principle based on molecular size alone. *Salting out* - This technique separates proteins based on their **solubility** in high salt concentrations. - As salt concentration increases, the proteins lose their hydration shells and precipitate out of solution, with different proteins precipitating at different salt concentrations.
Question 4: Which of the following techniques is used for the detection of variations in DNA sequence and gene expression?
- A. Southern blot
- B. Western blot
- C. Microarray (Correct Answer)
- D. Northern blot
Explanation: ***Microarray*** - **Microarrays** are designed to detect thousands of DNA or RNA sequences simultaneously, making them ideal for analyzing **gene expression profiles** and identifying **sequence variations** like SNPs. - They involve hybridizing labeled sample DNA/RNA to probes fixed on a solid surface, with the intensity of hybridization indicating the presence or abundance of specific sequences. *Northern blot* - The **Northern blot** technique is primarily used to study **gene expression** by detecting specific **RNA sequences** in a sample. - It does not directly analyze DNA sequence variations. *Southern blot* - The **Southern blot** is a molecular biology method used to detect specific **DNA sequences** in DNA samples. - While it can identify large-scale DNA rearrangements or deletions, it is not optimized for simultaneous detection of multiple gene expression levels or subtle sequence variations. *Western blot* - The **Western blot** is used to detect specific **proteins** in a sample. - It analyzes protein expression levels and modifications and is not designed for the detection of DNA sequence variations or gene expression at the RNA level.
Question 5: Which of the following hormones is an example of a peptide hormone?
- A. Dopamine
- B. Thyroxine
- C. Parathormone (PTH) (Correct Answer)
- D. Cortisol
Explanation: ***Parathormone (PTH)*** - **Parathormone (PTH)** is a **peptide hormone** composed of 84 amino acids, synthesized in the parathyroid glands. - Peptide hormones are typically **water-soluble** and bind to receptors on the cell surface to exert their effects. *Dopamine* - **Dopamine** is a **catecholamine** and a neurotransmitter, derived from the amino acid **tyrosine**. - It functions as a **neurotransmitter** and hormone, regulating various bodily functions but is not a peptide. *Thyroxine* - **Thyroxine** (T4) is an **amine hormone** derived from the amino acid **tyrosine**, produced by the thyroid gland. - It is an **iodinated amino acid derivative**, not a peptide. *Cortisol* - **Cortisol** is a **steroid hormone**, part of the glucocorticoid class, derived from **cholesterol**. - It is **lipid-soluble** and functions by binding to intracellular receptors, distinctly different from peptide hormones.
Question 6: Which technique is used for the separation of proteins based on their mass?
- A. Electrophoresis
- B. Salting out
- C. SDS-PAGE (Correct Answer)
- D. Ion exchange chromatography
Explanation: ***Correct Option: SDS-PAGE*** - **SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis)** separates **denatured proteins** almost exclusively by their **molecular mass**. - **SDS** binds to proteins, imparting a uniform negative charge-to-mass ratio, ensuring that separation is primarily based on their size as they migrate through a **polyacrylamide gel**. - This is the gold standard technique for analyzing proteins by molecular weight. *Incorrect Option: Electrophoresis* - This is a general technique that uses an **electric field** to separate molecules based on their **charge** and **size**. - While it can separate proteins, it doesn't exclusively rely on **mass** without additional modifications (like SDS). - Native electrophoresis separates by charge-to-mass ratio, not mass alone. *Incorrect Option: Salting out* - This technique separates proteins based on their **solubility** in high salt concentrations. - Proteins "salt out" or precipitate at different salt concentrations, which is not directly related to their **mass**. - Based on protein surface properties and hydrophobicity. *Incorrect Option: Ion exchange chromatography* - This method separates proteins based on their **net charge** at a particular pH. - Proteins bind to a charged resin and are eluted by changing the **ionic strength** or **pH** of the buffer. - Two types: cation exchange (negative resin) and anion exchange (positive resin).
Question 7: Equilibrium potential for an ion is calculated by:
- A. Gibbs Donnan Equilibrium
- B. Nernst Equation (Correct Answer)
- C. Goldman Equation
- D. None of the options
Explanation: ***Nernst Equation*** - The **Nernst equation** is used to calculate the **equilibrium potential** for a **single ion** across a semi-permeable membrane. - It considers the **charge of the ion**, the **temperature**, and the **concentration gradient** of the ion across the membrane. *Gibbs Donnan Equilibrium* - The **Gibbs-Donnan equilibrium** describes the distribution of **permeable ions** when there is a **non-permeable charged molecule** on one side of a membrane. - It focuses on the **overall distribution of ions** and water, rather than the equilibrium potential of a *single* ion. *Goldman Equation* - The **Goldman-Hodgkin-Katz (GHK) equation**, often referred to as the **Goldman equation**, calculates the **resting membrane potential** of a cell. - It accounts for the **permeability and concentration gradients** of *multiple* ions (e.g., Na+, K+, Cl-) that contribute to the membrane potential. *None of the options* - This option is incorrect because the **Nernst Equation** is specifically designed for calculating the equilibrium potential of a single ion.
Question 8: The technique shown in the image is:
- A. High performance liquid chromatography (Correct Answer)
- B. Haemoglobin electrophoresis
- C. Gel electrophoresis
- D. Tandem mass spectrometry
Explanation: ***High performance liquid chromatography*** - The image displays a **chromatogram** with distinct peaks labeled HbA1c, HbF, HbA0, and HbA2, separated based on their chemical properties. - This separation and detection method is characteristic of **High Performance Liquid Chromatography (HPLC)**, a technique used for quantifying different hemoglobin fractions. *Tandem mass spectrometry* - **Tandem mass spectrometry (MS/MS)** identifies compounds based on their mass-to-charge ratio and fragmentation patterns, which would look like mass spectra, not peaks on a time-based chromatogram. - While MS/MS is highly sensitive and specific, it doesn't produce the type of **elution profile** seen in the image. *Haemoglobin electrophoresis* - **Hemoglobin electrophoresis** separates hemoglobins based on their electrical charge, resulting in bands on a gel or a densitometric scan, not the **distinct chromatogram peaks** shown here. - While used for hemoglobin analysis, the visual representation is typically different, often displaying bands that reflect migration distance. *Gel electrophoresis* - **Gel electrophoresis** separates molecules, such as proteins or nucleic acids, by size and charge through a gel matrix, producing distinct **bands** that can be visualized. - This method would not produce the continuous **elution peaks over time** as observed in the provided graph, which indicates a liquid chromatography technique.
Question 9: A patient reports a change in colour of urine on air exposure. All are true about the condition shown below except:
- A. Blackening of urine is accelerated on exposure to sunlight
- B. Alkaptone bodies are deposited in intervertebral disc
- C. Urine Benedict's test is negative (Correct Answer)
- D. The condition is caused by deficiency of tyrosine aminotransferase
Explanation: ***Urine Benedict's test is negative*** - This is FALSE - Benedict's test is actually **POSITIVE** in alkaptonuria because **homogentisic acid** is a reducing agent. - Homogentisic acid readily **reduces Benedict's reagent**, giving a positive test result in alkaptonuria patients. - This is the **correct answer** to this EXCEPT question. *Blackening of urine is accelerated on exposure to sunlight* - This is TRUE - **UV light** and sunlight accelerate the **oxidation of homogentisic acid** in urine. - The characteristic **dark discoloration** occurs more rapidly when exposed to light and air. *Alkaptone bodies are deposited in intervertebral disc* - This is TRUE - **Homogentisic acid (alkaptone bodies)** polymerizes to form **ochronotic pigment** deposits. - These deposits accumulate in **cartilage** including intervertebral discs, causing degenerative changes and spondylosis. *The condition is caused by deficiency of tyrosine aminotransferase* - This is FALSE - Alkaptonuria is caused by deficiency of **homogentisate 1,2-dioxygenase**, not tyrosine aminotransferase. - **Tyrosine aminotransferase** deficiency causes Tyrosinemia Type II (Richner-Hanhart syndrome), a different condition. - However, Option C (Benedict's test) is the **more clearly incorrect** statement and the intended answer.
Question 10: Which of the following methods of protein separation is not dependent on molecular size?
- A. Gel filtration chromatography
- B. SDS-PAGE
- C. Ultracentrifugation
- D. Ion-exchange chromatography (Correct Answer)
Explanation: ***Ion-exchange chromatography*** - This method separates proteins based on their **net charge** at a specific pH. - Proteins bind to a charged resin based on their charge, independent of their size. *Gel filtration chromatography* - Separates proteins based on their **molecular size** and shape as they pass through a porous matrix. - **Larger molecules** elute first as they cannot enter the pores, while smaller molecules are retained. *SDS-PAGE* - **Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis** separates proteins primarily based on their **molecular weight**. - SDS denatures proteins and confers a uniform negative charge, allowing migration through a gel matrix based on size. *Ultracentrifugation* - Separates macromolecules and particles based on their **sedimentation rate**, which is influenced by **molecular mass**, density, and shape. - While molecular size is a factor, density and shape also play significant roles in the separation process.
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