What is the classification of Carcinoembryonic Antigen (CEA)?
Which of the following statements are true regarding the visual cycle cascade?
Calcium absorption is hampered by
Chymotrypsinogen is activated into chymotrypsin by:
Which of the following is an example of an exopeptidase?
What is the role of colipase in fat digestion?
Which of the following statements about adiponectin is incorrect?
What type of receptor is the insulin receptor?
Which protein is responsible for conserving iron in the body?
Converging point of both pathways in coagulation is at:
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 81: What is the classification of Carcinoembryonic Antigen (CEA)?
- A. Glycoprotein (Correct Answer)
- B. Lipoprotein
- C. Phosphoprotein
- D. Nucleoprotein
Explanation: ***Glycoprotein*** - Carcinoembryonic Antigen (CEA) is classified as a **glycoprotein** due to its structure, which consists of both **carbohydrate** and **protein** components. - This glycosylation is crucial for its function as a cell adhesion molecule and its recognition in diagnostic assays. *Lipoprotein* - **Lipoproteins** are complexes of lipids and proteins that function primarily in **lipid transport** in the blood. - CEA's primary role and structure are not related to lipid transport or being predominantly lipid-based. *Phosphoprotein* - A **phosphoprotein** is a protein that has been **covalently modified by the addition of a phosphate group**, a process crucial for cell signaling. - While proteins can be phosphorylated, the defining characteristic and major classification of CEA is its extensive glycosylation rather than phosphorylation state. *Nucleoprotein* - **Nucleoproteins** are proteins that are **structurally associated with nucleic acids** (DNA or RNA), such as histones or ribosomal proteins. - CEA does not have a structural or functional association with nucleic acids.
Question 82: Which of the following statements are true regarding the visual cycle cascade?
- A. All of the options are true (Correct Answer)
- B. Light causes isomerization of 11-cis-retinal to all-trans-retinal
- C. Retinal is involved in the visual cycle
- D. Involves a conformational change in opsin
Explanation: ***All of the statements are true*** The visual cycle cascade involves multiple interconnected events in phototransduction: **Light causes isomerization of 11-cis-retinal to all-trans-retinal** - This is the **primary photochemical event** that initiates vision - Light absorption causes the **cis-trans isomerization** in less than a picosecond - This conformational change is the only light-dependent step in the entire cascade **Retinal is involved in the visual cycle** - **11-cis-retinal** serves as the chromophore bound to opsin forming rhodopsin - After isomerization to **all-trans-retinal**, it must be converted back to 11-cis-retinal - This regeneration occurs through the **retinoid cycle** involving RPE cells **Involves a conformational change in opsin** - The isomerization of retinal triggers **conformational changes in opsin** - This converts rhodopsin to **metarhodopsin II** (the active form) - Activated opsin then activates **transducin** (G-protein), amplifying the signal and leading to hyperpolarization of photoreceptor cells All three statements accurately describe essential components of the visual cycle cascade.
Question 83: Calcium absorption is hampered by
- A. Protein
- B. Lactose
- C. Acid
- D. Phytates (Correct Answer)
Explanation: ***Phytates*** - **Phytates** (phytic acid) found in whole grains, legumes, nuts, and seeds bind to calcium, forming an insoluble complex that significantly **reduces its absorption** in the intestines. - This binding prevents the free calcium ions from crossing the intestinal wall into the bloodstream. *Protein* - **Protein** generally *enhances* calcium absorption, especially when consumed in moderate amounts, as some amino acids can form soluble calcium complexes. - However, very high protein intake, particularly from animal sources, *may* slightly increase urinary calcium excretion in the long term, but it does not directly hamper intestinal absorption. *Lactose* - **Lactose**, a sugar found in milk, is known to *enhance* calcium absorption. - It does so by creating a more acidic environment in the small intestine and by forming soluble complexes with calcium, making it more bioavailable. *Acid* - **Stomach acid** (hydrochloric acid) is crucial for calcium absorption as it helps to solubilize calcium salts from food. - A *reduced* acidic environment, such as from antacid use or certain medical conditions, would hamper calcium absorption, but acid itself is beneficial.
Question 84: Chymotrypsinogen is activated into chymotrypsin by:
- A. Trypsin (Correct Answer)
- B. Pepsin
- C. Renin
- D. HCl
Explanation: ***Activation of Chymotrypsinogen by Trypsin*** - **Trypsin** is the primary enzyme responsible for the activation of **chymotrypsinogen** into its active form, **chymotrypsin**, by cleaving a specific peptide bond. - This activation is part of a cascade of proteolytic enzyme activations in the **pancreatic juice**, crucial for protein digestion in the small intestine. *Pepsin* - **Pepsin** is a protease active in the **stomach**, requiring an acidic environment for its activity, and is involved in the initial breakdown of proteins. - It does not play a role in the activation of pancreatic zymogens like chymotrypsinogen; its primary function is protein digestion in the gastric lumen. *Renin* - **Renin** is an enzyme primarily involved in the **renin-angiotensin-aldosterone system** (RAAS), which regulates blood pressure and fluid balance. - Its action involves cleaving **angiotensinogen** to form angiotensin I, and it has no role in the activation of digestive enzymes like chymotrypsinogen. *HCl* - **Hydrochloric acid (HCl)** is produced in the stomach and is essential for providing the acidic environment required for **pepsin's activity** and for denaturing proteins. - While HCl is crucial for digestion, it does not directly activate chymotrypsinogen; this activation is an enzymatic process carried out by another protease.
Question 85: Which of the following is an example of an exopeptidase?
- A. Trypsin
- B. Chymotrypsin
- C. Elastase
- D. Carboxypeptidases (Correct Answer)
Explanation: ***Carboxypeptidases*** - **Carboxypeptidases** are enzymes that cleave the **C-terminal** (carboxyl end) amino acid from a polypeptide chain, making them a type of exopeptidase. - They are crucial in protein digestion, releasing individual amino acids from the end of protein chains. *Trypsin* - **Trypsin** is an **endopeptidase** that cleaves peptide bonds within protein chains, specifically at the carboxyl side of **lysine** or **arginine** residues. - It does not cleave amino acids from the ends of polypeptide chains. *Chymotrypsin* - **Chymotrypsin** is an **endopeptidase** that cleaves peptide bonds within a polypeptide chain, primarily at the carboxyl side of **tyrosine**, **tryptophan**, or **phenylalanine**. - Its action is internal to the protein sequence, not at the termini. *Elastase* - **Elastase** is also an **endopeptidase** that cleaves peptide bonds internally, specifically targeting small, uncharged amino acid residues like **alanine**, **glycine**, and **valine**. - Its primary role is to break down elastin, an elastic protein in connective tissues, but it does so by internal cleavage.
Question 86: What is the role of colipase in fat digestion?
- A. Encoded by the gene CLPS
- B. Assists pancreatic lipase in fat digestion (Correct Answer)
- C. Is secreted in an inactive form
- D. Is secreted by pancreatic cells
Explanation: ***Assists pancreatic lipase in fat digestion*** - Colipase **binds to pancreatic lipase** and the **lipid-water interface** of the fat droplet, providing a conformational change that enables lipase to access and hydrolyze triglycerides. - It also prevents bile salts from inactivating pancreatic lipase, ensuring efficient **fat emulsification and digestion**. *Is secreted in an inactive form* - Colipase is secreted as **procolipase** by the pancreas, which is then activated by **trypsin** in the duodenum. - While correct, this option describes its activation rather than its primary role in fat digestion. *Encoded by the gene CLPS* - The gene **CLPS** indeed encodes for colipase, but this is a genetic detail rather than its functional role in the digestive process. - Knowledge of the encoding gene is not directly relevant to understanding its biochemical function in fat digestion. *Is secreted by pancreatic cells* - Colipase is indeed synthesized and secreted by the **pancreas** into the small intestine. - This statement is true but describes the **origin** of colipase, not its specific functional role in fat digestion.
Question 87: Which of the following statements about adiponectin is incorrect?
- A. Secreted by adipose tissue
- B. Increases FFA oxidation
- C. Lowers glucose
- D. Positive Correlation with BMI (Correct Answer)
Explanation: ***Positive Correlation with BMI (INCORRECT STATEMENT)*** - Adiponectin levels are **inversely correlated with BMI**, NOT positively correlated; as BMI increases, adiponectin levels generally decrease. - This inverse relationship is significant because lower adiponectin levels are associated with increased insulin resistance and **metabolic syndrome**. - This statement is **false**, making it the correct answer to this question. *Secreted by adipose tissue (Correct statement)* - Adiponectin is a **hormone primarily secreted by adipocytes** (fat cells). - It plays a crucial role in regulating glucose and lipid metabolism, and its secretion is altered in conditions like obesity. - This statement is **true**. *Lowers glucose (Correct statement)* - Adiponectin **enhances insulin sensitivity** in peripheral tissues like skeletal muscle and liver, leading to increased glucose uptake and utilization. - This action helps to **lower blood glucose levels** and improve glycemic control. - This statement is **true**. *Increases FFA oxidation (Correct statement)* - Adiponectin **promotes fatty acid oxidation** in muscle and liver, reducing intracellular lipid accumulation. - By increasing fatty acid burning, it helps to **decrease circulating free fatty acid (FFA) levels**, which can contribute to insulin resistance if elevated. - This statement is **true**.
Question 88: What type of receptor is the insulin receptor?
- A. Guanylyl cyclase
- B. Adenylyl cyclase
- C. IP3-DAG
- D. Tyrosine kinase (Correct Answer)
Explanation: ***Tyrosine kinase*** - The insulin receptor is a **receptor tyrosine kinase (RTK)**, meaning it has intrinsic tyrosine kinase activity that phosphorylates specific tyrosine residues on itself and other intracellular proteins upon insulin binding. - This phosphorylation initiates a **signaling cascade** involving molecules like IRS proteins, PI3K/Akt, and MAPK pathways, leading to glucose uptake and metabolic regulation. *Guanylyl cyclase* - Guanylyl cyclase receptors, such as the **atrial natriuretic peptide receptor**, catalyze the conversion of GTP to **cGMP**, which acts as a second messenger. - This mechanism is distinct from the insulin receptor's direct protein phosphorylation. *Adenylyl cyclase* - Adenylyl cyclase is typically activated by **G-protein coupled receptors (GPCRs)**, leading to the conversion of ATP to **cAMP**, another second messenger. - The insulin receptor does not couple to G proteins or directly activate adenylyl cyclase. *IP3-DAG* - The **inositol triphosphate (IP3)** and **diacylglycerol (DAG)** pathway is primarily activated by certain **GPCRs** and involves the hydrolysis of PIP2 by phospholipase C, leading to calcium release and protein kinase C activation. - This pathway is not the primary signaling mechanism initiated by the insulin receptor.
Question 89: Which protein is responsible for conserving iron in the body?
- A. Ferritin (Correct Answer)
- B. Hepcidin
- C. Hemopexin
- D. Transferrin
Explanation: ***Ferritin*** - **Ferritin** is the primary intracellular protein that **conserves iron** in the body by storing it in a safe, non-toxic, and bioavailable form - It is found mainly in the liver, spleen, and bone marrow, serving as the body's **iron reserve** - When iron is abundant, ferritin stores it; when iron is needed, ferritin releases it, thus **conserving iron for future use** - Serum ferritin levels directly reflect total body iron stores *Hepcidin* - **Hepcidin** is a regulatory peptide hormone that controls iron homeostasis by inhibiting **ferroportin**, the iron export channel - It reduces iron absorption from the intestine and iron release from macrophages during inflammation or iron overload - While it regulates iron distribution, it is a hormone, not a storage protein, and does not directly conserve iron within cells *Hemopexin* - **Hemopexin** binds free **heme** in plasma, preventing oxidative damage and delivering it to the liver for catabolism - It helps recover iron from heme but does not store or conserve iron in the body *Transferrin* - **Transferrin** is a plasma protein that **transports iron** from absorption sites (intestine) and storage sites (liver, spleen) to tissues that need it - Its role is iron delivery, not conservation or storage
Question 90: Converging point of both pathways in coagulation is at:
- A. Factor VIII
- B. Stuart factor X (Correct Answer)
- C. Factor IX
- D. Factor VII
Explanation: ***Stuart factor X*** [1][2] - It is the main **converging point** of the coagulation cascade, where both the intrinsic and extrinsic pathways meet to initiate the common pathway [1]. - Activated factor X leads to the conversion of **prothrombin to thrombin**, pivotal for clot formation [2]. *Factor VII* [2] - Primarily involved in the **extrinsic pathway** of coagulation, activating factor X, but does not serve as a converging point. - Its function is limited to starting the coagulation cascade, particularly upon tissue injury. *Factor IX* [2] - A key component of the **intrinsic pathway**, it leads to the activation of factor X but is not the point where both pathways converge. - It requires **factor VIII** for its activation, further illustrating its role within a specific pathway. *Factor VIII* - Also part of the **intrinsic pathway**, it acts as a cofactor for factor IX but does not integrate both pathways into a common point. - Its deficiency is associated with **Hemophilia A**, underscoring its specific pathway involvement. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Hemodynamic Disorders, Thromboembolic Disease, and Shock, pp. 128-130. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 582-583.