Which of the following is not an androgen?
Carbonic anhydrase activity is found in all of the following except?
What is the approximate half-life of albumin in the human body?
What is the half-life of Prealbumin?
Which of the following is activated by calmodulin?
Which of the following statements about G protein-coupled receptors (GPCRs) is true?
What is the normal range of ferritin levels in adult males?
What is the major site of protein glycosylation?
In type IA Maple Syrup Urine Disease, which gene mutation is responsible?
Which of the following plant components is not fermented by gastrointestinal microorganisms?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 21: Which of the following is not an androgen?
- A. 17α-hydroxyprogesterone (Correct Answer)
- B. Testosterone
- C. Dihydrotestosterone
- D. Androstenedione
Explanation: ***17α-hydroxyprogesterone*** - This is a **progesterone derivative** and an intermediate in the synthesis of androgens and corticosteroids, but it does **not possess significant androgenic activity** itself. - Its primary role is as a precursor, rather than a direct androgen. *Testosterone* - **Testosterone** is the **primary male sex hormone** and a potent androgen, responsible for the development of male secondary sexual characteristics. - It plays crucial roles in muscle mass, bone density, libido, and erythropoiesis. *Dihydrotestosterone* - **Dihydrotestosterone (DHT)** is a potent androgen, formed from testosterone by the enzyme 5α-reductase. - DHT is responsible for the development of external male genitalia during fetal development and contributes to prostate growth and male pattern baldness in adults. *Androstenedione* - **Androstenedione** is a **weak androgen** and an important **precursor hormone** in the biosynthesis of testosterone and estrogens. - It is produced in the adrenal glands and gonads, serving as an intermediate step in steroidogenesis.
Question 22: Carbonic anhydrase activity is found in all of the following except?
- A. Brain
- B. Kidney
- C. RBC
- D. Plasma (Correct Answer)
Explanation: ***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**.
Question 23: What is the approximate half-life of albumin in the human body?
- A. 30 days
- B. 20 days (Correct Answer)
- C. 3 days
- D. 7 days
Explanation: ***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.
Question 24: What is the half-life of Prealbumin?
- A. 2 days (Correct Answer)
- B. 10 days
- C. 20 days
- D. 40 days
Explanation: ***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.
Question 25: Which of the following is activated by calmodulin?
- A. Muscle phosphorylase
- B. Calcium/calmodulin-dependent protein kinase (Correct Answer)
- C. Phospholipase C
- D. Adenylyl cyclase
Explanation: ***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.
Question 26: Which of the following statements about G protein-coupled receptors (GPCRs) is true?
- A. The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.
- B. G proteins can act as either inhibitory or excitatory based on the type of alpha subunit. (Correct Answer)
- C. G proteins bind directly to hormones to become activated.
- D. In the resting state, G proteins are bound to GTP.
Explanation: ***G proteins can act as either inhibitory or excitatory based on the type of alpha subunit.*** - Different classes of Gα subunits (e.g., **Gαs**, **Gαi**, **Gαq**) couple to diverse downstream effectors, leading to either **stimulation** (excitatory) or **inhibition** of cellular processes. - For example, **Gαs** activates adenylyl cyclase, while **Gαi** inhibits it, demonstrating their opposing roles. *The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.* - Upon activation, the **Gα subunit dissociates** from the **Gβγ dimer**, and both free units can then independently modulate effector molecules. - For the G protein to function in signal transduction, the α subunit often separates from the βγ dimer to interact with its target enzyme or ion channel. *G proteins bind directly to hormones to become activated.* - **GPCRs** (the receptors themselves) bind to hormones or other ligands on the **extracellular side** of the membrane. - The binding of the ligand to the GPCR induces a conformational change in the receptor, which then activates the associated G protein on the intracellular side. *In the resting state, G proteins are bound to GTP.* - In the **resting (inactive) state**, the Gα subunit of the trimeric G protein is bound to **GDP**. - Activation occurs when the GPCR facilitates the exchange of **GDP for GTP** on the Gα subunit.
Question 27: What is the normal range of ferritin levels in adult males?
- A. 30-300 ng/ml (Correct Answer)
- B. 300-500 ng/ml
- C. 10-20 ng/ml
- D. 500-700 ng/ml
Explanation: ***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.
Question 28: What is the major site of protein glycosylation?
- A. Ribosome and Golgi body
- B. ER and Ribosome
- C. Ribosome and Cytoplasm
- D. ER and Golgi body (Correct Answer)
Explanation: ***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.
Question 29: In type IA Maple Syrup Urine Disease, which gene mutation is responsible?
- A. BCKDHB
- B. DBT
- C. DLD
- D. BCKDHA (Correct Answer)
Explanation: ***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.
Question 30: Which of the following plant components is not fermented by gastrointestinal microorganisms?
- A. Lignin (Correct Answer)
- B. Cellulose
- C. Hemicellulose
- D. Pectin
Explanation: ***Lignin*** - **Lignin** is a complex polymer found in plant cell walls that is highly resistant to degradation by digestive enzymes and microbial fermentation in the gastrointestinal tract. - Its complex, cross-linked structure makes it **non-fermentable** by the microorganisms typically present in the human gut. *Cellulose* - **Cellulose** is a major component of plant cell walls and is a type of dietary fiber that can be fermented by certain gut bacteria. - While humans lack the enzymes to digest cellulose, colonic microorganisms possess cellulases that break it down into **short-chain fatty acids (SCFAs)**. *Hemicellulose* - **Hemicellulose** is a diverse group of plant polysaccharides that are a significant source of fermentable fiber for gut microbiota. - It is readily broken down by gastrointestinal bacteria into **SCFAs** and gases, contributing to colonic health. *Pectin* - **Pectin** is a soluble dietary fiber found in fruits and vegetables, known for its gel-forming properties. - It is highly fermentable by gut microorganisms, leading to the production of **SCFAs** like butyrate, propionate, and acetate.