NEET-PG 2013 - Biochemistry Practice Questions

Q: Trypsinogen is converted to trypsin by?

Removal of specific amino acids from trypsinogen. ***Removal of specific amino acids from trypsinogen*** - Trypsinogen is an **inactive zymogen** that is activated by the enzymatic cleavage of a **short N-terminal peptide**. - This cleavage event, primarily catalyzed by **enteropeptidase** (or trypsin itself), transforms trypsinogen into active **trypsin**, a process known as **proteolytic activation**. *Combination of 2 molecules of trypsinogen* - The activation of trypsinogen to trypsin is a **unimolecular conformational change** followed by proteolytic cleavage, not a combination reaction between two zymogen molecules. - While trypsin can activate other trypsinogen molecules, the initial activation does not involve the physical combination of two zymogen molecules. *Phosphorylation* - **Phosphorylation** is a common regulatory mechanism in proteins but is not the primary method for activating inactive trypsinogen. - Trypsinogen activation relies on a **proteolytic cleavage event**, rather than the addition of a phosphate group. *Addition of alkyl group* - The addition of an **alkyl group** is not a known mechanism for the physiological activation of trypsinogen. - Enzymatic activation typically involves **hydrolysis of peptide bonds** or other specific post-translational modifications.

Q: Carboxypeptidase contains which mineral?

Zinc. ***Zinc*** - **Carboxypeptidase** is a **metalloenzyme**, meaning it requires a metal ion for its catalytic activity. - **Zinc** acts as a crucial cofactor in the active site of carboxypeptidase, enabling its proteolytic function. *Copper* - **Copper** is a component of enzymes like **cytochrome c oxidase** and **superoxide dismutase**, but not carboxypeptidase. - Its presence is essential for processes like **electron transport** and **antioxidant defense**. *Iron* - **Iron** is a central component of **hemoglobin** and **myoglobin** for oxygen transport, and in enzymes like **catalase** and **peroxidase**. - It is not involved in the catalytic mechanism of carboxypeptidase. *None of the options* - This option is incorrect because **Zinc** is a known and essential mineral for the function of carboxypeptidase. - Carboxypeptidase is a metalloenzyme, and a metal cofactor is required for its activity.

Q: Which of the following is a neutral amino acid?

Glycine. ***Glycine*** - **Glycine** has a hydrogen atom as its side chain, making it the **simplest amino acid** and electrically neutral at physiological pH. - Its **nonpolar side chain** contributes to its neutral charge and allows it to fit into various protein structures. *Aspartate* - **Aspartate** is an **acidic amino acid** with a carboxyl group in its side chain. - This **carboxyl group** can lose a proton, giving aspartate a net negative charge at physiological pH. *Arginine* - **Arginine** is a **basic amino acid** characterized by a guanidinium group in its side chain. - The **guanidinium group** contains multiple nitrogen atoms that can accept protons, making arginine positively charged at physiological pH. *Histidine* - **Histidine** is classified as a **basic amino acid** due to the imidazole ring in its side chain. - The **imidazole ring** has a pKa close to physiological pH, allowing it to be protonated and positively charged, but it is not neutral.

Q: Which organelle produces and destroys H2O2?

Peroxisome. ***Peroxisome*** - **Peroxisomes** are organelles that both produce and break down **hydrogen peroxide (H2O2)** during metabolic processes. - They contain **oxidases** (such as D-amino acid oxidase and urate oxidase) that produce H2O2 as a byproduct during oxidation reactions. - They also contain the enzyme **catalase** that converts H2O2 into water and oxygen, protecting the cell from oxidative damage. - This dual function makes peroxisomes unique in H2O2 metabolism. *Lysosome* - **Lysosomes** are responsible for breaking down waste materials and cellular debris through **hydrolytic enzymes**. - They are primarily involved in **cellular digestion** and waste removal, not H2O2 metabolism. *Golgi body* - The **Golgi apparatus** modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles. - It is crucial for **protein trafficking** and glycosylation, but does not produce or destroy H2O2. *Ribosome* - **Ribosomes** are responsible for **protein synthesis** (translation) based on genetic information carried by mRNA. - They are involved in the assembly of amino acids into proteins, not the metabolism of hydrogen peroxide.

Q: Which of the following lipoproteins is most strongly associated with an increased risk of cardiovascular diseases and is commonly referred to as "bad cholesterol"?

LDL. ***LDL*** - **Low-density lipoprotein (LDL)** is commonly referred to as "bad" cholesterol because elevated levels are the **primary driver** of atherosclerotic plaque buildup in arterial walls. - LDL particles transport cholesterol from the liver to peripheral tissues; when present in excess, they infiltrate the arterial intima and undergo oxidative modification, triggering inflammatory responses that lead to atherosclerosis. - **Clinical significance:** LDL cholesterol is the primary target of lipid-lowering therapy in cardiovascular disease prevention. *VLDL* - **Very low-density lipoprotein (VLDL)** primarily transports endogenously synthesized **triglycerides** from the liver to peripheral tissues. - While elevated VLDL levels do contribute to cardiovascular risk (particularly through conversion to small, dense LDL particles), it is not the primary lipoprotein targeted in cardiovascular risk assessment. *Chylomicron* - **Chylomicrons** transport **dietary lipids** (triglycerides and cholesterol) from the intestines to tissues after meals. - They are rapidly cleared from circulation (half-life of 5-10 minutes) and are typically not present during fasting, making their contribution to chronic atherosclerotic plaque formation minimal. *Lp(a)* - **Lipoprotein(a) [Lp(a)]** is structurally similar to LDL but contains an additional apolipoprotein(a) molecule, which has homology to plasminogen and may interfere with fibrinolysis. - While Lp(a) is an independent cardiovascular risk factor, it is less commonly measured in routine clinical practice, and **LDL remains the cornerstone lipoprotein** for cardiovascular risk stratification and management.

Q: What is the blood form of folic acid?

Methyltetrahydrofolate. ***Methyltetrahydrofolate*** - **5-methyltetrahydrofolate (5-MTHF)** is the **primary circulating form** of folate in the blood plasma and the most metabolically active form of folate. - It plays a crucial role in various metabolic pathways, especially in **one-carbon metabolism** for DNA synthesis and repair. *Folinic acid* - **Folinic acid** (leucovorin) is a **reduced form of folic acid** that does not require reduction by dihydrofolate reductase for activity. - It is often used as a therapeutic agent, particularly to **counteract the effects of methotrexate** toxicity, but it is not the main physiological circulating form. *Pteroglutamate* - **Pteroglutamate** is a generic term referring to compounds structurally related to folic acid, which is itself chemically known as pteroylglutamic acid. - While it describes the **general structure**, it is not the specific blood form of folic acid. *None of the options* - This option is incorrect because **methyltetrahydrofolate** is indeed the correct answer.

Q: What is the unit for a prolactin level of 20 in blood?

ng/ml. ***ng/ml*** - Prolactin levels in blood are typically measured in **nanograms per milliliter (ng/mL)**, reflecting the very small concentrations of hormones. - A value of 20 ng/mL falls within the typical reference range for prolactin. *mg/mL* - **Milligrams per milliliter (mg/mL)** is a unit used for much higher concentrations, more common for drugs or larger molecules, not hormones like prolactin. - If prolactin were measured in mg/mL, a value of 20 mg/mL would be an astronomically high and physiologically impossible level. *mg/L* - **Milligrams per liter (mg/L)** is also a unit for higher concentrations than those typically seen for hormones in blood. - 20 mg/L is equivalent to 20 µg/mL or 20,000 ng/mL, which would indicate severe hyperprolactinemia. *ng/L* - **Nanograms per liter (ng/L)** is a unit for extremely low concentrations. - A reading of 20 ng/L would be too low for normal physiological prolactin levels, as 1 ng/mL equals 1000 ng/L.

Q: Which of the following molecular interactions are found in the structure of DNA?

All of the options. ***All of the options*** - All three types of molecular interactions listed are present in DNA structure, making this the correct answer. - **Hydrogen bonds** hold together the two strands of the DNA double helix, forming between complementary base pairs (A-T with 2 hydrogen bonds, G-C with 3 hydrogen bonds). - **Glycosidic bonds** (N-glycosidic bonds) link the nitrogenous bases to the C1' carbon of the deoxyribose sugar in each nucleotide. - **Covalent interactions** (phosphodiester bonds) form the strong, stable sugar-phosphate backbone by linking the 3' hydroxyl group of one sugar to the 5' phosphate group of the next. *Hydrogen bond* - This is a **true statement** - hydrogen bonds are essential structural components of DNA. - However, this option alone is **incomplete** as DNA structure also contains glycosidic bonds and covalent phosphodiester bonds. - If only hydrogen bonds were present, there would be no nucleotides or backbone structure. *Glycosidic bond* - This is a **true statement** - glycosidic bonds are present in every nucleotide of DNA. - However, this option alone is **incomplete** as DNA also requires hydrogen bonds for base pairing and phosphodiester bonds for the backbone. - Without other bonds, individual nucleotides could not form a functional double helix. *Covalent interactions* - This is a **true statement** - covalent phosphodiester bonds form the DNA backbone within each strand. - However, this option alone is **incomplete** as it doesn't account for glycosidic bonds (nucleotide formation) or hydrogen bonds (strand pairing). - While the strongest bonds in DNA, they alone cannot create the complete double helix structure.

Q: Which of the following organs does not primarily utilize fatty acids for energy?

Brain. ***Brain*** - The **brain primarily uses glucose** as its main energy source because fatty acids cannot efficiently cross the **blood-brain barrier**. - During prolonged starvation, the brain can adapt to use **ketone bodies**, which are derived from fatty acid breakdown in the liver. *Muscle* - **Skeletal muscle** can utilize both **glucose and fatty acids** for energy, with fatty acids becoming a more prominent fuel source during prolonged exercise and at rest. - **Cardiac muscle** (heart) heavily relies on **fatty acid oxidation** as its primary energy substrate, especially during basal conditions. *Liver* - The **liver is highly metabolically flexible** and readily oxidizes fatty acids for its own energy needs, particularly during fasting states. - It also plays a key role in **fatty acid metabolism**, including synthesis, breakdown, and conversion into ketone bodies. *Kidney* - The **renal cortex** is rich in mitochondria and has a high metabolic rate, primarily utilizing **fatty acid oxidation** to meet its significant energy demands for filtration and reabsorption. - While the renal medulla can use glucose, the cortex's reliance on fatty acids makes it a significant consumer.

Question 1

Trypsinogen is converted to trypsin by?

Accepted Answer

Removal of specific amino acids from trypsinogen

Answer

Combination of 2 molecules of trypsinogen

Answer

Phosphorylation

Answer

Addition of alkyl group

Question 2

Which of the following statements about hemoglobin is true?

Accepted Answer

Hemoglobin consists of two alpha and two beta subunits, each capable of binding one O2 molecule.

Answer

Each hemoglobin molecule can bind up to six O2 molecules.

Answer

Each hemoglobin subunit contains two heme groups, which bind oxygen.

Answer

Each hemoglobin molecule is made of 6 polypeptide chains.

Question 3

Carboxypeptidase contains which mineral?

Accepted Answer

Zinc

Answer

Copper

Answer

Iron

Answer

None of the options

Question 4

Which of the following is a neutral amino acid?

Accepted Answer

Glycine

Answer

Aspartate

Answer

Arginine

Answer

Histidine

Question 5

Which organelle produces and destroys H2O2?

Accepted Answer

Peroxisome

Answer

Lysosome

Answer

Golgi body

Answer

Ribosome

Question 6

Which of the following lipoproteins is most strongly associated with an increased risk of cardiovascular diseases and is commonly referred to as "bad cholesterol"?

Accepted Answer

LDL

Answer

VLDL

Answer

Chylomicron

Answer

Lp (a)

Question 7

What is the blood form of folic acid?

Accepted Answer

Methyltetrahydrofolate

Answer

Folinic acid

Answer

Pteroglutamate

Answer

None of the options

Question 8

What is the unit for a prolactin level of 20 in blood?

Accepted Answer

ng/ml

Answer

mg/ml

Answer

mg/l

Answer

ng/l

Question 9

Which of the following molecular interactions are found in the structure of DNA?

Accepted Answer

All of the options

Answer

Hydrogen bond

Answer

Glycosidic bond

Answer

Covalent interactions

Question 10

Which of the following organs does not primarily utilize fatty acids for energy?

Accepted Answer

Brain

Answer

Muscle

Answer

Liver

Answer

Kidney

NEET-PG 2013 — Biochemistry