NEET-PG 2015 - Biochemistry Practice Questions

Q: What primarily forms the core of chylomicrons?

Triglycerides. ***Triglycerides*** - Chylomicrons are primarily responsible for transporting **dietary triglycerides** from the intestines to other tissues. - Their large core, composed mainly of **triglycerides**, allows efficient transport of these hydrophobic molecules. *Triglycerides and Cholesterol together* - While **cholesterol** is present in chylomicrons, it is less abundant than **triglycerides** and primarily exists as **cholesterol esters** in the core. - The core is not an equal mixture; **triglycerides** overwhelmingly dominate the volume. *Free fatty acids* - **Free fatty acids** are transported in the blood primarily bound to **albumin**, not within the core of chylomicrons. - Chylomicrons typically carry **esterified fatty acids** as part of triglycerides. *Triglyceride, Cholesterol and Phospholipids* - **Phospholipids** form the outer monolayer of the chylomicron, along with apoproteins, making them **amphipathic**. - They do not constitute a core component but rather the **surface interface** with the aqueous environment.

Q: What type of receptor is the insulin receptor?

Tyrosine kinase. ***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.

Q: Which protein does the domain of plasminogen resemble?

Apolipoprotein (a) (a lipoprotein). ***Apolipoprotein (a) (a lipoprotein)*** - **Plasminogen** and **apolipoprotein (a)** share structural homology, specifically due to the presence of **kringle domains**. - This structural similarity suggests a potential for apolipoprotein (a) to **interfere with plasminogen’s fibrinolytic activity**, contributing to **atherosclerosis**. *Fibrinogen (a clotting protein)* - While plasmin acts on fibrinogen (and its derivative fibrin), its domain structure does not **resemble fibrinogen**. - **Fibrinogen** is a large, multi-domain glycoprotein crucial for **clot formation**, distinct from plasminogen's primarily **kringle-rich structure**. *LDL receptor (a lipid metabolism protein)* - The **LDL receptor** is involved in **cholesterol uptake** by cells and has structural features like ligand-binding repeats and epidermal growth factor (EGF) repeats. - Its domain structure is **not similar to plasminogen**, which is characterized by **kringle domains** and a protease domain. *Prothrombin (a coagulation protein)* - **Prothrombin** is a precursor to thrombin, featuring **gla domains**, kringle-like domains (though structurally distinct from plasminogen's), and a serine protease domain. - While both are involved in coagulation/fibrinolysis, their **overall domain arrangements and specific kringle structures differ** significantly.

Q: Which of the following stimulates Acetyl CoA Carboxylase?

Citrate. ***Citrate*** - **Citrate** is an allosteric activator of **Acetyl-CoA Carboxylase (ACC)**, indicating abundant energy and precursor availability for fatty acid synthesis. - This activation promotes the conversion of **Acetyl-CoA** to **Malonyl-CoA**, the committed step in **fatty acid synthesis**. *Starvation* - **Starvation** leads to energy deficit, which generally **inhibits** anabolic processes like fatty acid synthesis. - In this state, enzymes involved in anabolic pathways are often downregulated or inhibited to conserve energy. *Glucagon* - **Glucagon** is a hormone that signals low blood glucose and promotes catabolic processes such as **glycogenolysis** and **gluconeogenesis**. - It **inhibits** fatty acid synthesis by phosphorylating and inactivating **Acetyl-CoA Carboxylase**, thus opposing citrate's activating effect. *None of the options* - **Citrate** is a known stimulator of Acetyl CoA Carboxylase. - This option is incorrect because there is a correct answer among the choices.

Q: Which of the following is an example of an exopeptidase?

Carboxypeptidases. ***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.

Q: Calcium absorption is hampered by

Phytates. ***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.

Q: HDL is called good cholesterol because -

Removes cholesterol from peripheral tissues. ***Removes cholesterol from peripheral tissues*** - **High-density lipoprotein (HDL)** is known as "good cholesterol" due to its role in **reverse cholesterol transport**, a process where it collects excess cholesterol from peripheral cells and tissues. - This action helps to prevent the accumulation of cholesterol in arteries, thereby reducing the risk of **atherosclerosis** and cardiovascular disease. - HDL then transports this cholesterol to the liver for excretion via bile, completing the protective cycle. *Increases cholesterol delivery to peripheral tissues* - This is actually the opposite of HDL's function and describes the role of **LDL (low-density lipoprotein)**, which is considered "bad cholesterol." - LDL delivers cholesterol to peripheral tissues, and excess LDL can lead to **atherosclerotic plaque formation**. *Stimulates cholesterol synthesis in the liver* - HDL does not directly stimulate cholesterol synthesis in the liver; rather, its role is primarily in **cholesterol efflux** from cells and transport. - The liver's cholesterol synthesis is regulated by various factors, including dietary intake and cellular cholesterol levels via the **SREBP pathway**, but HDL does not upregulate hepatic cholesterol synthesis. *Activates enzymes that break down triglycerides* - While HDL does activate **LCAT (lecithin-cholesterol acyltransferase)** for cholesterol esterification, its primary "good" function is not the breakdown of triglycerides. - **Lipoprotein lipase (LPL)** is the primary enzyme responsible for triglyceride breakdown in lipoproteins like VLDL and chylomicrons.

Q: Amide group is present in which part of protein?

Peptide bond. ***Peptide bond*** - A **peptide bond** is formed between the **carboxyl group** of one amino acid and the **amino group** of another, releasing a water molecule. This bond has an **amide structure**. - The repeated formation of these amide (peptide) bonds links amino acids into long chains, forming a **polypeptide** or protein. *Amino-terminal* - The **amino-terminal (N-terminal)** end of a protein contains a free **amino group (-NH2)**, which is not part of an amide linkage within the polypeptide backbone. - It marks the beginning of the polypeptide chain and is typically involved in various cellular interactions and modifications. *Disulfide bond* - A **disulfide bond** is a covalent bond formed between two **sulfhydryl groups (-SH)** of **cysteine residues**, leading to the formation of a **cystine** residue. - This bond is crucial for stabilizing the **tertiary and quaternary structures** of proteins, but it does not contain an amide group. *Carboxy-terminal* - The **carboxy-terminal (C-terminal)** end of a protein contains a free **carboxyl group (-COOH)**, which is not part of an amide linkage within the polypeptide backbone. - It marks the end of the polypeptide chain and plays roles in protein processing, targeting, and regulation.

Q: In phenylketonuria, which substance should be restricted in the diet?

Phenylalanine. ***Phenylalanine*** - **Phenylketonuria (PKU)** is a genetic disorder where the body cannot effectively metabolize **phenylalanine** due to a deficiency in the enzyme **phenylalanine hydroxylase**. - Restricting dietary phenylalanine is crucial to prevent the accumulation of toxic byproducts that can lead to severe neurological damage and developmental delays. *Tyrosine* - Tyrosine is normally synthesized from phenylalanine. In PKU, this conversion is impaired. - While phenylalanine must be restricted, tyrosine supplementation is often necessary for individuals with PKU, as it becomes a conditionally essential amino acid. *Maize* - Maize (corn) is a carbohydrate-rich food and does not contain high levels of phenylalanine that would necessitate its restriction in PKU. - Dietary management in PKU focuses on regulating protein intake, as phenylalanine is an amino acid found in proteins. *None of the options* - This option is incorrect because phenylalanine must be strictly restricted in the diet of individuals with phenylketonuria to manage the condition effectively. - Without dietary restriction, the accumulation of phenylalanine can lead to severe and irreversible neurological damage.

Q: Which of the following statements are true regarding the visual cycle cascade?

All of the options are true. ***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 1

What primarily forms the core of chylomicrons?

Accepted Answer

Triglycerides

Answer

Triglycerides and Cholesterol together

Answer

Free fatty acids

Answer

Triglyceride, Cholesterol and Phospholipids

Question 2

What type of receptor is the insulin receptor?

Accepted Answer

Tyrosine kinase

Answer

Guanylyl cyclase

Answer

Adenylyl cyclase

Answer

IP3-DAG

Question 3

Which protein does the domain of plasminogen resemble?

Accepted Answer

Apolipoprotein (a) (a lipoprotein)

Answer

Fibrinogen (a clotting protein)

Answer

LDL receptor (a lipid metabolism protein)

Answer

Prothrombin (a coagulation protein)

Question 4

Which of the following stimulates Acetyl CoA Carboxylase?

Accepted Answer

Citrate

Answer

Starvation

Answer

Glucagon

Answer

None of the options

Question 5

Which of the following is an example of an exopeptidase?

Accepted Answer

Carboxypeptidases

Answer

Trypsin

Answer

Chymotrypsin

Answer

Elastase

Question 6

Calcium absorption is hampered by

Accepted Answer

Phytates

Answer

Protein

Answer

Lactose

Answer

Acid

Question 7

HDL is called good cholesterol because -

Accepted Answer

Removes cholesterol from peripheral tissues

Answer

Increases cholesterol delivery to peripheral tissues

Answer

Stimulates cholesterol synthesis in the liver

Answer

Activates enzymes that break down triglycerides

Question 8

Amide group is present in which part of protein?

Accepted Answer

Peptide bond

Answer

Amino-terminal

Answer

Disulfide bond

Answer

Carboxy-terminal

Question 9

In phenylketonuria, which substance should be restricted in the diet?

Accepted Answer

Phenylalanine

Answer

Tyrosine

Answer

Maize

Answer

None of the options

Question 10

Which of the following statements are true regarding the visual cycle cascade?

Accepted Answer

All of the options are true

Answer

Light causes isomerization of 11-cis-retinal to all-trans-retinal

Answer

Retinal is involved in the visual cycle

Answer

Involves a conformational change in opsin

NEET-PG 2015 — Biochemistry