NEET-PG 2015 - Biochemistry Practice Questions

Q: Which of the following is a positive acute phase protein that enhances the acute phase response?

Fibrinogen. ***Fibrinogen*** - **Fibrinogen** is a key **positive acute phase protein** whose concentration increases significantly during inflammation - Its elevation contributes to the acute phase response by promoting **blood clotting** and influencing **erythrocyte sedimentation rate (ESR)** - Along with C-reactive protein (CRP), haptoglobin, and serum amyloid A, fibrinogen is among the major positive acute phase reactants *Transferrin* - **Transferrin** is a **negative acute phase protein**, meaning its concentration decreases during inflammation - This reduction is part of the body's iron-sequestration strategy to limit iron availability for invading pathogens - The decrease in transferrin helps restrict bacterial growth by reducing available iron *Albumin* - **Albumin** is a prominent **negative acute phase protein**, with its concentration decreasing during acute inflammation due to redistribution and reduced synthesis - It plays a vital role in maintaining **oncotic pressure** and transporting various substances - Its decline reflects the severity of inflammation and is used as a marker of the acute phase response *Prealbumin* - **Prealbumin** (also known as transthyretin) is a **negative acute phase protein** and a sensitive marker of nutritional status - Its rapid decline during inflammation makes it a useful indicator, as its synthesis is quickly reduced - It has a short half-life (2-3 days), making it more sensitive to acute changes than albumin

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: Glucagon activates which enzyme ?

Adenylyl cyclase. ***Adenylyl cyclase*** - **Glucagon** binds to specific G protein-coupled receptors on target cells, activating the **Gαs subunit**. - The activated **Gαs subunit** then stimulates **adenylyl cyclase**, leading to the production of **cyclic AMP (cAMP)**, which mediates glucagon's metabolic effects. *Pepsin* - **Pepsin** is a protease produced in the stomach, involved in protein digestion, and its activity is regulated by **gastrin** and **acid secretion**, not glucagon. - It is synthesized as **pepsinogen** and activated by hydrochloric acid (HCl) at low pH. *Trypsin* - **Trypsin** is a digestive enzyme produced in the pancreas and secreted into the small intestine, primarily involved in protein digestion. - Its activation is dependent on **enteropeptidase**, which cleaves **trypsinogen**, and its activity is not directly regulated by glucagon. *None of the options* - This option is incorrect because **adenylyl cyclase** is indeed activated by glucagon as part of its signaling pathway.

Q: What is the classification of Carcinoembryonic Antigen (CEA)?

Glycoprotein. ***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.

Q: What is the respiratory quotient of carbohydrates?

1. ***Option: 1 (Correct Answer)*** - The **respiratory quotient (RQ)** is the ratio of **carbon dioxide produced to oxygen consumed** during metabolism. - For carbohydrates, complete oxidation yields equal moles of CO2 and O2, resulting in an **RQ of 1.0**. - Example: C6H12O6 + 6O2 → 6CO2 + 6H2O, giving RQ = 6CO2/6O2 = 1.0 - This value reflects that carbohydrates are highly oxygenated molecules, requiring less external oxygen for their oxidation relative to the CO2 produced. *Option: 0.5* - An RQ of 0.5 is not observed for any major macronutrient during complete oxidation. - This value would imply significantly lower CO2 production relative to O2 consumption, which doesn't match any physiological substrate metabolism. *Option: 0.8* - An RQ of approximately 0.8 is characteristic of a **mixed diet** or the average value sometimes cited for **protein metabolism**. - Protein RQ typically ranges from 0.8-0.85, as proteins require more oxygen for their oxidation compared to the CO2 produced. - The exact RQ can vary depending on the specific amino acids being metabolized. *Option: 0.75* - An RQ around 0.7-0.75 may represent **fat-predominant metabolism** or a mixed diet with fats and carbohydrates. - Pure **fat metabolism** has an RQ of approximately **0.7**, as fats require substantial oxygen for oxidation due to their lower oxygen content relative to carbon and hydrogen. - Fats contain many C-H bonds and few C-O bonds, necessitating more oxygen for complete combustion.

Q: Which of the following compounds is not formed with the involvement of glycine?

Thyroxine. ***Thyroxine*** - **Thyroxine** (and other thyroid hormones) are derived from the amino acid **tyrosine**. - Their synthesis involves iodination and coupling reactions of tyrosine residues within the protein **thyroglobulin**. *Heme* - **Glycine** is a direct precursor for the initial step in **heme synthesis**. - It condenses with **succinyl CoA** to form α-amino-β-ketoadipate, which then decarboxylates to form δ-aminolevulinate (ALA). *Purines* - **Glycine** contributes part of its structure to the **purine ring**. - Specifically, the **nitrogen at position 7** and the **carbons at positions 4 and 5** of the purine ring are derived from glycine. *Glutathione* - **Glutathione** is a tripeptide composed of three amino acids: **glutamate**, **cysteine**, and **glycine**. - **Glycine** is the C-terminal amino acid of glutathione and is essential for its structure and function as an antioxidant.

Q: Boiled cabbage or rancid butter smelling urine is seen in

Isovaleric Acidaemia. ***Isovaleric Acidaemia*** - **Boiled cabbage or rancid butter odor** in urine is a classic feature of isovaleric acidemia, caused by the accumulation of isovaleric acid. - This **inborn error of metabolism** affects **leucine metabolism** due to deficiency of isovaleryl-CoA dehydrogenase. *Tyrosinemia* - Does NOT present with boiled cabbage or rancid butter odor. The characteristic features are **liver dysfunction** and **renal tubular defects**. - Tyrosinemia Type I is caused by deficiency of **fumarylacetoacetate hydrolase**, leading to accumulation of tyrosine metabolites. *Phenylketonuria* - Characterized by a **mousy or musty odor** in urine, resulting from the accumulation of phenylacetic acid. - The defect is in the enzyme **phenylalanine hydroxylase**, not associated with boiled cabbage odor. *Multiple carboxylase deficiency* - Typically presents with a **"cat urine" smell** due to the accumulation of various organic acids. - The deficiency impairs the function of several **biotin-dependent carboxylases**, not specifically linked to the boiled cabbage odor.

Q: How many isoenzymes does lactate dehydrogenase (LDH) have?

5, based on H and M polypeptide subunits. **5, based on H and M polypeptide subunits** - **Lactate dehydrogenase (LDH)** is a tetrameric enzyme, meaning it is composed of four polypeptide subunits. - These subunits can be either **H (heart)** type or **M (muscle)** type, leading to five distinct isoenzymes (**LDH-1, LDH-2, LDH-3, LDH-4, LDH-5**) based on their combinations (HHHH, HHHM, HHMM, HMMM, MMMM). *7, based on H and M polypeptide subunits* - While LDH is composed of two types of subunits, H and M, the possible combinations of these four subunits result in **five distinct isoenzymes**, not seven. - Seven isoenzymes are not a recognized number for LDH. *9, based on H and M polypeptide subunits* - The combination of two types of subunits in a tetrameric structure cannot yield nine unique isoenzymes. - This number is incorrect and not supported by the biochemistry of LDH. *3, based on H and M polypeptide subunits* - Three isoenzymes would imply either fewer than four subunits or a more restricted combination, which is not the case for LDH's tetrameric structure with H and M subunits. - This number is insufficient to account for all possible combinations.

Q: If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?

35%. ***35%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If A = 15%, then T must also be 15%. This means A + T = 30%. Since the total percentage of all bases is 100%, G + C must be 100% - 30% = 70%. As G = C, then G = 70% / 2 = 35%. *15%* - This would only be correct if guanine paired with adenine, which it does not; guanine pairs with **cytosine**. - This answer incorrectly assumes that all four bases are present in equal proportions, or that G equals A, which violates **Chargaff's rules**. *85%* - This percentage would imply an incorrect base pairing or an imbalanced ratio of purines and pyrimidines, violating the fundamental structure of DNA. - An 85% guanine content would mean that G + C far exceeds 100% or that T is extremely low, which is biologically impossible. *70%* - This represents the combined percentage of **guanine and cytosine**, not guanine alone. - While it correctly acknowledges the remaining proportion of bases, it fails to divide this sum between the two equal components, **G and C**.

Q: GlcNAc-P-P-oligosaccharide is -

Glycoprotein. ***Glycoprotein*** - **GlcNAc-P-P-oligosaccharide** refers to the **N-linked oligosaccharide precursor** that is synthesized on a **dolichol pyrophosphate** carrier (`-P-P`). This complex is characteristic of the initial stages of **N-linked glycosylation**, a process that forms glycoproteins. - **N-acetylglucosamine (GlcNAc)** is a crucial sugar residue found at the reducing end of this precursor, linking it to the dolichol carrier. *Proteoglycan* - Proteoglycans consist of a **core protein** covalently attached to long, unbranched **glycosaminoglycan (GAG)** chains, such as chondroitin sulfate or heparin. - While they contain sugar units, their structure and synthesis pathway are distinct from the GlcNAc-P-P-oligosaccharide described, which is specific to N-linked glycoprotein synthesis. *Collagen* - **Collagen** is a fibrous protein, primarily composed of a triple helix of polypeptide chains rich in **glycine, proline, and hydroxyproline**. - Although collagen undergoes some post-translational modifications like **glycosylation**, it does not involve the GlcNAc-P-P-oligosaccharide precursor in its typical synthesis. *Phospholipid* - **Phospholipids** are a major component of cell membranes, composed of a **hydrophilic head** (containing a phosphate group) and two **hydrophobic fatty acid tails**. - They are lipids and do not contain carbohydrate structures like GlcNAc-P-P-oligosaccharide.

Question 1

Which of the following is a positive acute phase protein that enhances the acute phase response?

Accepted Answer

Fibrinogen

Answer

Transferrin

Answer

Albumin

Answer

Prealbumin

Question 2

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

Accepted Answer

Phenylalanine

Answer

Tyrosine

Answer

Maize

Answer

None of the options

Question 3

Glucagon activates which enzyme ?

Accepted Answer

Adenylyl cyclase

Answer

Pepsin

Answer

Trypsin

Answer

None of the options

Question 4

What is the classification of Carcinoembryonic Antigen (CEA)?

Accepted Answer

Glycoprotein

Answer

Lipoprotein

Answer

Phosphoprotein

Answer

Nucleoprotein

Question 5

What is the respiratory quotient of carbohydrates?

Accepted Answer

1

Answer

0.5

Answer

0.8

Answer

0.75

Question 6

Which of the following compounds is not formed with the involvement of glycine?

Accepted Answer

Thyroxine

Answer

Purines

Answer

Glutathione

Answer

Heme

Question 7

Boiled cabbage or rancid butter smelling urine is seen in

Accepted Answer

Isovaleric Acidaemia

Answer

Tyrosinemia

Answer

Phenylketonuria

Answer

Multiple carboxylase deficiency

Question 8

How many isoenzymes does lactate dehydrogenase (LDH) have?

Accepted Answer

5, based on H and M polypeptide subunits

Answer

7, based on H and M polypeptide subunits

Answer

9, based on H and M polypeptide subunits

Answer

3, based on H and M polypeptide subunits

Question 9

If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?

Accepted Answer

35%

Answer

15%

Answer

85%

Answer

70%

Question 10

GlcNAc-P-P-oligosaccharide is -

Accepted Answer

Glycoprotein

Answer

Proteoglycan

Answer

Collagen

Answer

Phospholipid

NEET-PG 2015 — Biochemistry