NEET-PG 2013 - Biochemistry Practice Questions

Q: Carbonic anhydrase activity is found in all of the following except?

Plasma. ***Plasma*** - **Carbonic anhydrase** is an intracellular enzyme that catalyzes the rapid interconversion of carbon dioxide and water to carbonic acid, **bicarbonate**, and protons. - It is notably **absent in plasma** in healthy individuals, as it is primarily found within cells where its function is crucial for pH regulation and CO2 transport. *Brain* - Carbonic anhydrase is found in various brain cells, including **neurons**, **oligodendrocytes**, and **astrocytes**. - It plays a vital role in pH regulation, fluid balance, and the production of cerebrospinal fluid (CSF) within the **central nervous system**. *Kidney* - The kidney is rich in carbonic anhydrase, particularly in the **proximal tubules** and collecting ducts. - It is critical for **bicarbonate reabsorption** and proton excretion, essential processes for maintaining acid-base balance. *RBC* - **Red blood cells (RBCs)** contain a high concentration of carbonic anhydrase (specifically CA-I and CA-II isoforms). - This enzyme facilitates the rapid conversion of CO2 to bicarbonate for transport to the lungs and the reverse reaction for **CO2 exhalation**.

Q: What is the approximate half-life of albumin in the human body?

20 days. ***20 days*** - The **half-life of albumin** in the human body is approximately **20 days**, reflecting the time it takes for half of the circulating albumin to be catabolized or excreted. - This relatively long half-life means that changes in albumin levels, such as those due to malnutrition or liver disease, may take several weeks to become evident. *3 days* - A half-life of 3 days is too short for albumin, which is a major, long-lasting plasma protein. - Proteins with such a short half-life typically include more rapidly turnover proteins or small peptides. *7 days* - A half-life of 7 days is also too short for albumin, which plays a critical role in maintaining plasma oncotic pressure and transporting various substances. - While some proteins have a 7-day half-life, albumin's is considerably longer. *30 days* - A half-life of 30 days is longer than the typical half-life of albumin. - While some proteins may have half-lives in this range, 20 days is the more commonly accepted value for albumin.

Q: What is the half-life of Prealbumin?

2 days. ***2 days*** - Prealbumin, also known as transthyretin, has a **short half-life** of approximately 2-3 days, making it a sensitive indicator of recent changes in **nutritional status**. - Its rapid turnover allows for prompt reflection of improvement or deterioration in protein synthesis. *10 days* - A half-life of 10 days would make prealbumin less responsive to acute changes in nutrition compared to its actual turnover rate. - This duration is longer than the typical half-life of proteins used to monitor **short-term nutritional status**. *20 days* - A 20-day half-life would indicate a protein with a much slower turnover, unsuitable for monitoring **acute nutritional interventions**. - Proteins with such long half-lives, like **albumin**, reflect more chronic states rather than rapid changes. *40 days* - A half-life of 40 days is characteristic of proteins like **albumin**, which are influenced by longer-term nutritional and inflammatory processes. - Such a long half-life would not be useful for assessing immediate responses to **nutritional support** or acute disease states.

Q: Which of the following is activated by calmodulin?

Calcium/calmodulin-dependent protein kinase. ***Calcium/calmodulin-dependent protein kinase*** - **Calmodulin** is a **calcium-binding messenger protein** that, when bound to calcium, undergoes a conformational change allowing it to activate various enzymes, including **calcium/calmodulin-dependent protein kinases** (CaMKs). - CaMKs play crucial roles in many cellular processes, including **metabolism**, **gene expression**, and **neurotransmission**, by phosphorylating target proteins. *Muscle phosphorylase* - **Muscle phosphorylase** (glycogen phosphorylase) is primarily activated by **epinephrine**, **AMP**, and **nerve stimulation** (via calcium), but not directly by calmodulin. - Its activation leads to the breakdown of **glycogen** into glucose-1-phosphate. *Phospholipase C* - **Phospholipase C (PLC)** is typically activated by **G protein-coupled receptors** and **tyrosine kinase receptors**, leading to the production of **inositol trisphosphate (IP3)** and **diacylglycerol (DAG)**. - While it plays a role in calcium signaling upstream (releasing calcium from stores), it is not directly activated by calmodulin. *Adenylyl cyclase* - **Adenylyl cyclase (AC)** is a key enzyme in generating **cyclic AMP (cAMP)**, and is commonly regulated by **G proteins** (specifically Gs and Gi subunits). - While certain isoforms (AC1, AC3, AC8) can be directly activated by calcium/calmodulin, **CaMK** remains the most classical and direct example of calmodulin activation.

Q: What is the normal range of ferritin levels in adult males?

30-300 ng/ml. ***30-300 ng/ml*** - The normal range for **ferritin levels** in adult males is typically **30-300 ng/ml** (some laboratories report 30-400 ng/ml). - Ferritin is an **iron storage protein**, and its levels reflect the body's iron stores. - Values below 30 ng/ml suggest **iron deficiency**, while values above 300 ng/ml may indicate iron overload or inflammatory conditions. *10-20 ng/ml* - These levels are **significantly low** and indicate **iron deficiency**. - This range is well below the normal threshold and would warrant investigation and likely iron supplementation. - Levels below 15 ng/ml are diagnostic of **iron deficiency** even in the absence of anemia. *300-500 ng/ml* - Levels in this range are considered **elevated** and can indicate iron overload, chronic inflammation, liver disease, or malignancy. - While some laboratories extend the upper limit to 400 ng/ml, persistent elevation above 300 ng/ml warrants further investigation. - Common causes include **hemochromatosis**, **chronic liver disease**, or **inflammatory conditions**. *500-700 ng/ml* - These levels are **significantly elevated** and strongly suggest **iron overload conditions** such as **hemochromatosis**, severe inflammatory states, or hepatocellular injury. - High ferritin levels can be associated with organ damage, leading to conditions like **cirrhosis** or **cardiomyopathy**. - Requires urgent investigation to identify the underlying cause.

Q: What is the major site of protein glycosylation?

ER and Golgi body. ***ER and Golgi body*** - The **endoplasmic reticulum (ER)** is the primary site for **N-linked glycosylation**, where carbohydrates are added to the asparagine residues of nascent proteins. - The **Golgi apparatus** is crucial for further modification and processing of these N-linked glycans, as well as the site for **O-linked glycosylation**, where sugars are added to serine or threonine residues. *Ribosome and Golgi body* - **Ribosomes** are responsible for **protein synthesis (translation)** but do not directly perform glycosylation, which is a post-translational modification. - While the **Golgi body** is a site of glycosylation, the ribosome's inclusion makes this option incorrect as the ribosome's role precedes glycosylation. *ER and Ribosome* - The **ER** is a major site of protein glycosylation, especially N-linked glycosylation. - However, **ribosomes** are involved in protein synthesis and lack the enzymatic machinery for adding sugar moieties to proteins. *Ribosome and Cytoplasm* - **Ribosomes** synthesize proteins, but glycosylation does not occur there. - The **cytoplasm** is the site for many metabolic pathways, but major protein glycosylation events mostly occur within the ER and Golgi.

Q: In type IA Maple Syrup Urine Disease, which gene mutation is responsible?

BCKDHA. ***BCKDHA*** - **Maple Syrup Urine Disease (MSUD)** type IA is caused by a mutation in the **BCKDHA gene**, which codes for the E1α subunit of the **branched-chain α-keto acid dehydrogenase (BCKD) complex**. - This **enzyme complex** is crucial for the metabolism of **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine. *BCKDHB* - The **BCKDHB gene** codes for the E1β subunit of the **BCKD complex**. - Mutations in **BCKDHB** are associated with **type IB MSUD**, not type IA. *DBT* - The **DBT gene** codes for the E2 subunit (dihydrolipoyl transacylase) of the **BCKD complex**. - Mutations in **DBT** are responsible for **type II MSUD**. *DLD* - The **DLD gene** codes for the E3 subunit (dihydrolipoyl dehydrogenase), which is a component shared by several **α-keto acid dehydrogenase complexes**. - Mutations in the **DLD gene** lead to **type III MSUD** and other pyruvate dehydrogenase complex deficiencies, rather than type IA.

Q: Which element is required by phosphofructokinase?

Magnesium. **Magnesium** - **Phosphofructokinase** (PFK) is an enzyme in **glycolysis** that catalyzes the phosphorylation of fructose-6-phosphate. - This reaction requires **ATP**, and like many enzymes that utilize ATP, PFK requires **magnesium ions (Mg²⁺)** as a cofactor, typically forming a complex with ATP (MgATP²⁻). *Inorganic phosphate* - **Inorganic phosphate** is a substrate for some kinase reactions, but not a direct cofactor requirement for the *activation* of phosphofructokinase itself. - While phosphate is incorporated into molecules during phosphorylation, it does not act as a metal ion cofactor to facilitate the enzyme's activity. *Manganese* - While **manganese (Mn²⁺)** can sometimes substitute for magnesium in certain enzyme reactions, it is not the primary or required cofactor for phosphofructokinase under normal physiological conditions. - Many enzymes have a preference for specific metal ions based on their active site structure and coordination chemistry. *Copper* - **Copper (Cu²⁺)** is a cofactor for a variety of enzymes, particularly those involved in **redox reactions** (e.g., cytochrome c oxidase, superoxide dismutase). - However, copper is not a required metallic cofactor for the activity of **phosphofructokinase** in glycolysis.

Q: Kcat/Km is a measure of which of the following?

Enzyme efficiency. **Correct: Enzyme efficiency** - The ratio **kcat/Km** is the definitive measure of an enzyme's **catalytic efficiency** or **specificity constant** - It reflects how effectively an enzyme converts substrate to product at low substrate concentrations - A higher **kcat/Km** value indicates greater efficiency, combining high catalytic rate (kcat) with strong substrate affinity (low Km) - This is the most important parameter for comparing different enzymes or different substrates for the same enzyme *Incorrect: Speed of enzymatic reaction* - **kcat** (turnover number) alone measures the maximum speed when enzyme is saturated with substrate - **kcat/Km** is a more comprehensive measure that includes substrate binding affinity, not just reaction speed - Speed also depends on enzyme and substrate concentrations, which kcat/Km doesn't directly represent *Incorrect: Concentration of substrate* - **Km** (Michaelis constant) represents the substrate concentration at which reaction velocity is half of Vmax - **kcat/Km** is a ratio that describes enzyme performance across substrate concentrations, not the concentration itself - It's particularly useful for predicting enzyme behavior at physiological (low) substrate concentrations *Incorrect: Enzyme turnover* - **kcat** specifically measures enzyme turnover: the number of substrate molecules converted per enzyme molecule per unit time at saturation - **kcat/Km** incorporates both kcat and Km, providing overall efficiency rather than just turnover rate - Turnover is only one component of the efficiency measure

Question 1

Carbonic anhydrase activity is found in all of the following except?

Accepted Answer

Plasma

Answer

Brain

Answer

Kidney

Answer

RBC

Question 2

What is the approximate half-life of albumin in the human body?

Accepted Answer

20 days

Answer

30 days

Answer

3 days

Answer

7 days

Question 3

What is the half-life of Prealbumin?

Accepted Answer

2 days

Answer

10 days

Answer

20 days

Answer

40 days

Question 4

Which of the following is activated by calmodulin?

Accepted Answer

Calcium/calmodulin-dependent protein kinase

Answer

Muscle phosphorylase

Answer

Phospholipase C

Answer

Adenylyl cyclase

Question 5

Which of the following statements about G protein-coupled receptors (GPCRs) is true?

Accepted Answer

G proteins can act as either inhibitory or excitatory based on the type of alpha subunit.

Answer

The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.

Answer

G proteins bind directly to hormones to become activated.

Answer

In the resting state, G proteins are bound to GTP.

Question 6

What is the normal range of ferritin levels in adult males?

Accepted Answer

30-300 ng/ml

Answer

300-500 ng/ml

Answer

10-20 ng/ml

Answer

500-700 ng/ml

Question 7

What is the major site of protein glycosylation?

Accepted Answer

ER and Golgi body

Answer

Ribosome and Golgi body

Answer

ER and Ribosome

Answer

Ribosome and Cytoplasm

Question 8

In type IA Maple Syrup Urine Disease, which gene mutation is responsible?

Accepted Answer

BCKDHA

Answer

BCKDHB

Answer

DBT

Answer

DLD

Question 9

Which element is required by phosphofructokinase?

Accepted Answer

Magnesium

Answer

Inorganic phosphate

Answer

Manganese

Answer

Copper

Question 10

Kcat/Km is a measure of which of the following?

Accepted Answer

Enzyme efficiency

Answer

Speed of enzymatic reaction

Answer

Concentration of substrate

Answer

Enzyme turnover

NEET-PG 2013 — Biochemistry