INI-CET 2022 — Biochemistry

Q: Match the following molecules with their carriers in plasma: MOLECULES: 1. Thyroxine 2. Fatty acid 3. Hemoglobin 4. Heme CARRIERS: A. Hemopexin B. Prealbumin C. Albumin D. Haptoglobin

1-B, 2-C, 3-D, 4-A. ***1-B, 2-C, 3-D, 4-A*** - **Thyroxine** is primarily transported by **prealbumin (transthyretin)**, which has high affinity for thyroid hormones and prevents their rapid clearance. - **Fatty acids** bind to **albumin** at multiple binding sites, allowing efficient transport of these hydrophobic molecules in plasma. - **Hemoglobin** released from lysed red blood cells is bound by **haptoglobin** to prevent renal loss and iron wastage. - **Heme** groups are specifically transported by **hemopexin**, which has the highest binding affinity for heme among plasma proteins. *1-B, 2-C, 3-A, 4-D* - Incorrectly pairs **hemoglobin** with hemopexin; hemoglobin binds to **haptoglobin** to form stable complexes. - Incorrectly pairs **heme** with haptoglobin; free heme has highest affinity for **hemopexin**, not haptoglobin. *1-A, 2-B, 3-C, 4-D* - Incorrectly pairs **thyroxine** with hemopexin; thyroxine is primarily carried by **prealbumin** and thyroxine-binding globulin. - Incorrectly pairs **fatty acids** with prealbumin; fatty acids bind predominantly to **albumin** due to its multiple hydrophobic binding sites. *1-A, 2-D, 3-B, 4-C* - Incorrectly pairs **heme** with albumin; while albumin can bind heme weakly, **hemopexin** is the specific high-affinity carrier. - Incorrectly pairs **hemoglobin** with prealbumin; free hemoglobin forms specific complexes with **haptoglobin** for recycling. [Practice more Biochemistry PYQs on OnCourse]

Q: Cyclooxygenase plays a role in which pathway?

Prostaglandin. ***Prostaglandin*** - **Cyclooxygenase (COX)** enzymes specifically catalyze the conversion of **arachidonic acid** into **prostaglandins**, **prostacyclins**, and **thromboxanes**. - This pathway is crucial for mediating **inflammation**, **fever**, and **pain** responses in the body. *Leukotriene* - **Leukotrienes** are synthesized via the **lipoxygenase** pathway, not the cyclooxygenase pathway. - They are primarily involved in **allergic reactions** and **asthma**, causing bronchoconstriction and increased vascular permeability. *Vitamin K* - **Vitamin K** is a fat-soluble vitamin essential for the synthesis of **blood clotting factors** and does not involve cyclooxygenase enzymes. - It acts as a cofactor for the enzyme **gamma-glutamyl carboxylase**. *Krebs cycle* - The **Krebs cycle (citric acid cycle)** is a central metabolic pathway for **cellular respiration**, producing ATP, NADH, and FADH2. - It takes place in the **mitochondria** and is involved in the breakdown of carbohydrates, fats, and proteins for energy, unrelated to cyclooxygenase. [Practice more Biochemistry PYQs on OnCourse]

Q: Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography

2,1,3,5,4. ***2,1,3,5,4*** - This sequence accurately reflects the typical order of operations in **recombinant protein isolation**: first, the gene is introduced into bacteria, then protein is expressed and cells lysed, followed by **SDS-PAGE as an intermediate quality check** to confirm protein expression before proceeding to purification steps (column chromatography and elution). - The process starts with gene incorporation, includes an analytical checkpoint after lysis, and ends with purified protein elution. *2,4,5,3,1* - This sequence is incorrect because **protein elution (4)** and **column chromatography (5)** are purification steps that occur *after* protein expression and cell lysis. - **Lysis (1)** cannot happen after elution, as cells must be lysed first to release the protein for purification. *1,2,4,3,5* - This sequence is incorrect because **expression and lysis (1)** must occur *after* the gene has been **incorporated into bacteria (2)** - the gene must be present before it can be expressed. - Additionally, **protein elution (4)** should follow **column chromatography (5)**, as elution is the step where protein is collected from the chromatography column. *1,5,2,4,3* - This sequence is incorrect because **incorporation of genes (2)** must be the first step - the gene needs to be in the bacteria before any expression, lysis, or purification can occur. - Starting with **expression and lysis (1)** before gene incorporation is impossible. [Practice more Biochemistry PYQs on OnCourse]

Q: In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?

Methionine. ***Methionine*** - **Maple syrup urine disease (MSUD)** is a disorder affecting the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. - Therefore, methionine, which is not a BCAA, typically does not need to be restricted and is, in fact, an **essential amino acid** crucial for protein synthesis. *Isoleucine* - **Isoleucine** is a branched-chain amino acid (BCAA) whose metabolism is impaired in MSUD due to a deficiency in **branched-chain alpha-keto acid dehydrogenase complex**. - Accumulation of isoleucine and its corresponding alpha-keto acid is toxic and must be **restricted in the diet**. *Leucine* - **Leucine** is another branched-chain amino acid (BCAA) that cannot be properly metabolized in MSUD. - High levels of leucine and its metabolites are particularly **neurotoxic** and contribute to the characteristic neurological symptoms, necessitating strict dietary restriction. *Valine* - **Valine** is the third branched-chain amino acid (BCAA) whose breakdown is defective in MSUD. - Dietary restriction of valine is essential to prevent its accumulation, which can lead to metabolic crises and **developmental delays**. [Practice more Biochemistry PYQs on OnCourse]

Q: Deamination of methylated cytosine forms which of the following?

Thymine. ***Thymine*** - Deamination of **5-methylcytosine** removes the amine group at the 4-position and replaces it with a keto group, forming **thymine**. - This reaction can lead to a common type of point mutation, as the DNA repair machinery might fail to distinguish this naturally occurring base from normal thymine. *Uracil* - **Uracil** is formed by the deamination of **unmethylated cytosine**, not methylated cytosine. - Uracil is a base found in RNA but not typically in DNA, so its presence in DNA signals a repair event. *Cytosine* - **Cytosine** is the original base before deamination occurs; deamination is a chemical modification that changes cytosine into another base. - If a base remains cytosine, it means deamination has not taken place. *Guanine* - **Guanine** is a purine base and is structurally unrelated to cytosine or its deamination products. - Deamination primarily affects pyrimidine bases like cytosine and uracil, not purines like guanine. [Practice more Biochemistry PYQs on OnCourse]

7 Previous Year Questions with Answers & Explanations

Questions

Match the following molecules with their carriers in plasma: MOLECULES: 1. Thyroxine 2. Fatty acid 3. Hemoglobin 4. Heme CARRIERS: A. Hemopexin B. Prealbumin C. Albumin D. Haptoglobin

Cyclooxygenase plays a role in which pathway?

Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography

In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?

Deamination of methylated cytosine forms which of the following?

Increased H+ ions in the intermembrane space of mitochondria are due to?

The signal sequence in a type 2 membrane protein with N-terminal facing cytoplasm is located in?

INI-CET 2022 - Biochemistry INI-CET Practice Questions and MCQs

Question 1: Match the following molecules with their carriers in plasma: MOLECULES: 1. Thyroxine 2. Fatty acid 3. Hemoglobin 4. Heme CARRIERS: A. Hemopexin B. Prealbumin C. Albumin D. Haptoglobin

A. 1-B, 2-C, 3-A, 4-D
B. 1-A, 2-B, 3-C, 4-D
C. 1-B, 2-C, 3-D, 4-A (Correct Answer)
D. 1-A, 2-D, 3-B, 4-C

Explanation: ***1-B, 2-C, 3-D, 4-A*** - **Thyroxine** is primarily transported by **prealbumin (transthyretin)**, which has high affinity for thyroid hormones and prevents their rapid clearance. - **Fatty acids** bind to **albumin** at multiple binding sites, allowing efficient transport of these hydrophobic molecules in plasma. - **Hemoglobin** released from lysed red blood cells is bound by **haptoglobin** to prevent renal loss and iron wastage. - **Heme** groups are specifically transported by **hemopexin**, which has the highest binding affinity for heme among plasma proteins. *1-B, 2-C, 3-A, 4-D* - Incorrectly pairs **hemoglobin** with hemopexin; hemoglobin binds to **haptoglobin** to form stable complexes. - Incorrectly pairs **heme** with haptoglobin; free heme has highest affinity for **hemopexin**, not haptoglobin. *1-A, 2-B, 3-C, 4-D* - Incorrectly pairs **thyroxine** with hemopexin; thyroxine is primarily carried by **prealbumin** and thyroxine-binding globulin. - Incorrectly pairs **fatty acids** with prealbumin; fatty acids bind predominantly to **albumin** due to its multiple hydrophobic binding sites. *1-A, 2-D, 3-B, 4-C* - Incorrectly pairs **heme** with albumin; while albumin can bind heme weakly, **hemopexin** is the specific high-affinity carrier. - Incorrectly pairs **hemoglobin** with prealbumin; free hemoglobin forms specific complexes with **haptoglobin** for recycling.

Question 2: Cyclooxygenase plays a role in which pathway?

A. Leukotriene
B. Vitamin K
C. Krebs cycle
D. Prostaglandin (Correct Answer)

Explanation: ***Prostaglandin*** - **Cyclooxygenase (COX)** enzymes specifically catalyze the conversion of **arachidonic acid** into **prostaglandins**, **prostacyclins**, and **thromboxanes**. - This pathway is crucial for mediating **inflammation**, **fever**, and **pain** responses in the body. *Leukotriene* - **Leukotrienes** are synthesized via the **lipoxygenase** pathway, not the cyclooxygenase pathway. - They are primarily involved in **allergic reactions** and **asthma**, causing bronchoconstriction and increased vascular permeability. *Vitamin K* - **Vitamin K** is a fat-soluble vitamin essential for the synthesis of **blood clotting factors** and does not involve cyclooxygenase enzymes. - It acts as a cofactor for the enzyme **gamma-glutamyl carboxylase**. *Krebs cycle* - The **Krebs cycle (citric acid cycle)** is a central metabolic pathway for **cellular respiration**, producing ATP, NADH, and FADH2. - It takes place in the **mitochondria** and is involved in the breakdown of carbohydrates, fats, and proteins for energy, unrelated to cyclooxygenase.

Question 3: Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography

A. 2,1,3,5,4 (Correct Answer)
B. 2,4,5,3,1
C. 1,2,4,3,5
D. 1,5,2,4,3

Explanation: ***2,1,3,5,4*** - This sequence accurately reflects the typical order of operations in **recombinant protein isolation**: first, the gene is introduced into bacteria, then protein is expressed and cells lysed, followed by **SDS-PAGE as an intermediate quality check** to confirm protein expression before proceeding to purification steps (column chromatography and elution). - The process starts with gene incorporation, includes an analytical checkpoint after lysis, and ends with purified protein elution. *2,4,5,3,1* - This sequence is incorrect because **protein elution (4)** and **column chromatography (5)** are purification steps that occur *after* protein expression and cell lysis. - **Lysis (1)** cannot happen after elution, as cells must be lysed first to release the protein for purification. *1,2,4,3,5* - This sequence is incorrect because **expression and lysis (1)** must occur *after* the gene has been **incorporated into bacteria (2)** - the gene must be present before it can be expressed. - Additionally, **protein elution (4)** should follow **column chromatography (5)**, as elution is the step where protein is collected from the chromatography column. *1,5,2,4,3* - This sequence is incorrect because **incorporation of genes (2)** must be the first step - the gene needs to be in the bacteria before any expression, lysis, or purification can occur. - Starting with **expression and lysis (1)** before gene incorporation is impossible.

Question 4: In a patient with maple syrup urine disease, all of the following amino acids should be restricted in diet except?

A. Methionine (Correct Answer)
B. Isoleucine
C. Leucine
D. Valine

Explanation: ***Methionine*** - **Maple syrup urine disease (MSUD)** is a disorder affecting the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. - Therefore, methionine, which is not a BCAA, typically does not need to be restricted and is, in fact, an **essential amino acid** crucial for protein synthesis. *Isoleucine* - **Isoleucine** is a branched-chain amino acid (BCAA) whose metabolism is impaired in MSUD due to a deficiency in **branched-chain alpha-keto acid dehydrogenase complex**. - Accumulation of isoleucine and its corresponding alpha-keto acid is toxic and must be **restricted in the diet**. *Leucine* - **Leucine** is another branched-chain amino acid (BCAA) that cannot be properly metabolized in MSUD. - High levels of leucine and its metabolites are particularly **neurotoxic** and contribute to the characteristic neurological symptoms, necessitating strict dietary restriction. *Valine* - **Valine** is the third branched-chain amino acid (BCAA) whose breakdown is defective in MSUD. - Dietary restriction of valine is essential to prevent its accumulation, which can lead to metabolic crises and **developmental delays**.

Question 5: Deamination of methylated cytosine forms which of the following?

A. Uracil
B. Thymine (Correct Answer)
C. Cytosine
D. Guanine

Explanation: ***Thymine*** - Deamination of **5-methylcytosine** removes the amine group at the 4-position and replaces it with a keto group, forming **thymine**. - This reaction can lead to a common type of point mutation, as the DNA repair machinery might fail to distinguish this naturally occurring base from normal thymine. *Uracil* - **Uracil** is formed by the deamination of **unmethylated cytosine**, not methylated cytosine. - Uracil is a base found in RNA but not typically in DNA, so its presence in DNA signals a repair event. *Cytosine* - **Cytosine** is the original base before deamination occurs; deamination is a chemical modification that changes cytosine into another base. - If a base remains cytosine, it means deamination has not taken place. *Guanine* - **Guanine** is a purine base and is structurally unrelated to cytosine or its deamination products. - Deamination primarily affects pyrimidine bases like cytosine and uracil, not purines like guanine.

Question 6: Increased H+ ions in the intermembrane space of mitochondria are due to?

A. Decreased ATP synthase activity
B. Reduced proton pumping
C. Impaired inner mitochondrial membrane integrity
D. Increased electron transport chain activity (Correct Answer)

Explanation: ***Increased electron transport chain activity*** - The **electron transport chain (ETC)** complexes (I, III, and IV) actively pump **protons (H+)** from the mitochondrial matrix into the intermembrane space during electron transfer. - **Increased ETC activity** directly causes more protons to be pumped, creating a higher H+ concentration in the intermembrane space. - This is the **primary mechanism** for establishing the proton-motive force used in ATP synthesis. *Decreased ATP synthase activity* - While decreased ATP synthase activity would cause **passive accumulation** of protons in the intermembrane space (since fewer H+ flow back through ATP synthase), it does **not actively increase** proton pumping. - The question asks what causes the **increase** in H+ ions, which requires active transport by the ETC, not passive accumulation. - This option confuses the consequence (accumulation) with the cause (active pumping). *Reduced proton pumping* - **Reduced proton pumping** by the ETC would lead to a **decrease** in H+ concentration in the intermembrane space, as fewer protons are being actively transported. - This produces the opposite effect of what the question describes. *Impaired inner mitochondrial membrane integrity* - **Impaired membrane integrity** would cause protons to **leak back** into the mitochondrial matrix, dissipating the proton gradient. - This would **decrease**, not increase, the H+ concentration in the intermembrane space. - This is seen in uncoupling conditions where the membrane becomes permeable to protons.

Question 7: The signal sequence in a type 2 membrane protein with N-terminal facing cytoplasm is located in?

A. C-terminal
B. Middle of protein (Correct Answer)
C. Both N and C terminal
D. N-terminal

Explanation: ***Middle of protein*** - For a type 2 membrane protein with the **N-terminal facing the cytoplasm**, the **start-transfer sequence** (which acts as a signal sequence) is found in the **middle of the protein**. - This **internal signal sequence** allows for membrane integration with the correct orientation, often functioning as a **transmembrane domain**. *C-terminal* - A C-terminal signal is typically seen in **Type I membrane proteins** where the **N-terminus is in the ER lumen** and the C-terminus is in the cytoplasm. - This orientation requires a **stop-transfer sequence** in the middle of the protein. *Both N and C terminal* - While some proteins may have multiple signal sequences, a single protein typically utilizes **one dominant signal** for its initial targeting and membrane insertion. - Having both an N-terminal and C-terminal primary signal for integration would lead to **conflicting targeting signals** for this specific type of protein. *N-terminal* - An N-terminal signal sequence is characteristic of **Type I membrane proteins** or **secreted proteins**, guiding them to the ER and often being cleaved. - In a **type 2 protein**, with the **N-terminus in the cytoplasm**, an N-terminal signal would result in the N-terminus entering the ER lumen, contradicting the given orientation.