FMGE 2019 — Biochemistry

Q: Keshan's disease is due to deficiency of:

Selenium. **Selenium (Correct Answer)** - **Keshan disease** is an endemic cardiomyopathy characterized by multifocal myocyte necrosis, which has been directly linked to a severe dietary deficiency of **selenium**. - Selenium is an essential component of **glutathione peroxidase**, an enzyme critical for protecting cells against oxidative damage. - The disease is named after Keshan County in China where it was first identified in selenium-deficient regions. *Iron (Incorrect)* - **Iron deficiency** primarily leads to **anemia**, characterized by fatigue, pallor, and weakness due to insufficient hemoglobin production. - It does not directly cause the specific cardiomyopathy seen in Keshan disease. *Zinc (Incorrect)* - **Zinc deficiency** can cause a range of symptoms including impaired immune function, skin lesions (acrodermatitis enteropathica), hair loss, and growth retardation. - It is not associated with the cardiac manifestations of Keshan disease. *Copper (Incorrect)* - **Copper deficiency** can result in anemia, neutropenia, impaired immune function, and neurological dysfunction (Menkes disease). - While copper is important for various metalloenzymes, its deficiency does not cause Keshan disease. [Practice more Biochemistry PYQs on OnCourse]

Q: Protein content in cow's milk in comparison to human milk is?

Triple. ***Triple*** - The protein content in **cow's milk** is approximately **three times higher** than that found in **human milk**. - **Cow's milk** contains approximately **3.3 g/100 mL** of protein, while **human milk** contains about **1.0-1.3 g/100 mL**. - This higher protein load in cow's milk can be challenging for an infant's immature kidneys and digestion. - The predominant protein in cow's milk is **casein (80%)**, while human milk has more **whey proteins (60%)**. *Quadruple* - While cow's milk has significantly more protein than human milk, it is not **four times** the amount. - The exact ratio is closer to **three times (3:1)**, making quadruple an overestimation. *Double* - The protein content of cow's milk is **more than double** that of human milk. - Therefore, stating it is merely double **underestimates** the difference in protein concentration. *Same* - The protein content of **cow's milk and human milk are not the same**. - There are significant differences in both the **quantity** (3:1 ratio) and **type of proteins** (casein vs whey predominance). - These differences make each milk suited for different species' developmental needs. [Practice more Biochemistry PYQs on OnCourse]

Q: Zinc is cofactor of which enzyme?

Carbonic anhydrase. ***Carbonic anhydrase*** - **Zinc** is an essential cofactor for **carbonic anhydrase**, crucial for its enzymatic activity in catalyzing the reversible hydration of carbon dioxide. - This enzyme plays a vital role in processes like **pH regulation**, **carbon dioxide transport**, and **bicarbonate production** in various tissues. *Carboxylase* - Carboxylases typically require **biotin** as a cofactor for their activity, which involves the addition of a carboxyl group to a substrate. - Examples include **pyruvate carboxylase** and **acetyl-CoA carboxylase**, which are fundamental in metabolic pathways. *Kinase* - Kinases are enzymes that catalyze the transfer of a **phosphate group** from a high-energy donor molecule (like ATP) to a substrate. - Their activity often depends on cofactors like **magnesium (Mg2+)** or **manganese (Mn2+)**, not zinc. *Lysyl oxidase* - **Lysyl oxidase** is an enzyme that requires **copper** as a cofactor for its activity. - It plays a critical role in the **cross-linking of collagen and elastin**, essential for the integrity of connective tissues. [Practice more Biochemistry PYQs on OnCourse]

Q: All are cofactors for Dehydrogenase except:

SAM. ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂. [Practice more Biochemistry PYQs on OnCourse]

Q: Which structure of protein is not denatured after heating up to 100 degrees Celsius?

Primary. ***Primary*** - The **primary structure** refers to the specific linear sequence of **amino acids** forming the polypeptide chain, linked by **covalent peptide bonds**. - These strong **peptide bonds** are generally resistant to heat denaturation at 100°C, meaning the amino acid sequence remains intact. *Quaternary* - The **quaternary structure** involves the arrangement of multiple polypeptide subunits and is maintained by weaker interactions like **hydrophobic interactions**, hydrogen bonds, and salt bridges. - These interactions are highly susceptible to disruption by heat, causing the subunits to dissociate and the quaternary structure to be lost. *Tertiary* - The **tertiary structure** describes the three-dimensional folding of a single polypeptide chain, stabilized by various non-covalent interactions (e.g., hydrogen bonds, ionic bonds, hydrophobic interactions) and **disulfide bonds**. - Heat disrupts these weaker non-covalent interactions and can even break disulfide bonds, leading to the unfolding and loss of the specific 3D shape. *Secondary* - The **secondary structure** (e.g., **alpha-helices** and **beta-pleated sheets**) arises from hydrogen bonds between the backbone atoms of the polypeptide chain. - While peptide bonds remain intact, these vital **hydrogen bonds** are easily broken by heat, causing the unraveling of helices and sheets. [Practice more Biochemistry PYQs on OnCourse]

18 Previous Year Questions with Answers & Explanations

Questions

Keshan's disease is due to deficiency of:

Protein content in cow's milk in comparison to human milk is?

Zinc is cofactor of which enzyme?

All are cofactors for Dehydrogenase except:

Which structure of protein is not denatured after heating up to 100 degrees Celsius?

Most abundant amino acid found in collagen?

Which is a product of purine metabolism?

Melanin is derived from which amino acid?

Which bases are present in human DNA?

Q10

Type IV complex of ETC is inhibited by

FMGE 2019 - Biochemistry FMGE Practice Questions and MCQs

Question 1: Keshan's disease is due to deficiency of:

A. Iron
B. Zinc
C. Copper
D. Selenium (Correct Answer)

Explanation: **Selenium (Correct Answer)** - **Keshan disease** is an endemic cardiomyopathy characterized by multifocal myocyte necrosis, which has been directly linked to a severe dietary deficiency of **selenium**. - Selenium is an essential component of **glutathione peroxidase**, an enzyme critical for protecting cells against oxidative damage. - The disease is named after Keshan County in China where it was first identified in selenium-deficient regions. *Iron (Incorrect)* - **Iron deficiency** primarily leads to **anemia**, characterized by fatigue, pallor, and weakness due to insufficient hemoglobin production. - It does not directly cause the specific cardiomyopathy seen in Keshan disease. *Zinc (Incorrect)* - **Zinc deficiency** can cause a range of symptoms including impaired immune function, skin lesions (acrodermatitis enteropathica), hair loss, and growth retardation. - It is not associated with the cardiac manifestations of Keshan disease. *Copper (Incorrect)* - **Copper deficiency** can result in anemia, neutropenia, impaired immune function, and neurological dysfunction (Menkes disease). - While copper is important for various metalloenzymes, its deficiency does not cause Keshan disease.

Question 2: Protein content in cow's milk in comparison to human milk is?

A. Quadruple
B. Triple (Correct Answer)
C. Double
D. Same

Explanation: ***Triple*** - The protein content in **cow's milk** is approximately **three times higher** than that found in **human milk**. - **Cow's milk** contains approximately **3.3 g/100 mL** of protein, while **human milk** contains about **1.0-1.3 g/100 mL**. - This higher protein load in cow's milk can be challenging for an infant's immature kidneys and digestion. - The predominant protein in cow's milk is **casein (80%)**, while human milk has more **whey proteins (60%)**. *Quadruple* - While cow's milk has significantly more protein than human milk, it is not **four times** the amount. - The exact ratio is closer to **three times (3:1)**, making quadruple an overestimation. *Double* - The protein content of cow's milk is **more than double** that of human milk. - Therefore, stating it is merely double **underestimates** the difference in protein concentration. *Same* - The protein content of **cow's milk and human milk are not the same**. - There are significant differences in both the **quantity** (3:1 ratio) and **type of proteins** (casein vs whey predominance). - These differences make each milk suited for different species' developmental needs.

Question 3: Zinc is cofactor of which enzyme?

A. Carboxylase
B. Carbonic anhydrase (Correct Answer)
C. Kinase
D. Lysyl oxidase

Explanation: ***Carbonic anhydrase*** - **Zinc** is an essential cofactor for **carbonic anhydrase**, crucial for its enzymatic activity in catalyzing the reversible hydration of carbon dioxide. - This enzyme plays a vital role in processes like **pH regulation**, **carbon dioxide transport**, and **bicarbonate production** in various tissues. *Carboxylase* - Carboxylases typically require **biotin** as a cofactor for their activity, which involves the addition of a carboxyl group to a substrate. - Examples include **pyruvate carboxylase** and **acetyl-CoA carboxylase**, which are fundamental in metabolic pathways. *Kinase* - Kinases are enzymes that catalyze the transfer of a **phosphate group** from a high-energy donor molecule (like ATP) to a substrate. - Their activity often depends on cofactors like **magnesium (Mg2+)** or **manganese (Mn2+)**, not zinc. *Lysyl oxidase* - **Lysyl oxidase** is an enzyme that requires **copper** as a cofactor for its activity. - It plays a critical role in the **cross-linking of collagen and elastin**, essential for the integrity of connective tissues.

Question 4: All are cofactors for Dehydrogenase except:

A. SAM (Correct Answer)
B. NADP
C. NAD
D. FAD

Explanation: ***SAM*** - **S-adenosylmethionine (SAM)** is a cofactor involved in **methyl group transfer reactions**, carried out by enzymes known as methyltransferases. - Dehydrogenase enzymes catalyze **redox reactions**, typically involving the transfer of hydride ions, and thus do not utilize SAM as a cofactor. *NADP* - **Nicotinamide adenine dinucleotide phosphate (NADP)** is a crucial coenzyme for many **dehydrogenase reactions**, particularly in **anabolic pathways** like fatty acid synthesis and the pentose phosphate pathway. - It acts as an **electron carrier**, accepting or donating hydride ions. *NAD* - **Nicotinamide adenine dinucleotide (NAD)** is a highly common coenzyme for numerous **dehydrogenase enzymes**, especially in **catabolic pathways** such as glycolysis, the Krebs cycle, and oxidative phosphorylation. - It functions as an **electron acceptor** or donor in redox reactions. *FAD* - **Flavin adenine dinucleotide (FAD)** is a coenzyme derived from **riboflavin (Vitamin B2)** and is associated with various dehydrogenase enzymes, particularly those involved in **electron transport** and fatty acid oxidation. - FAD can accept two hydrogen atoms (one hydride and one proton) to become FADH₂.

Question 5: Which structure of protein is not denatured after heating up to 100 degrees Celsius?

A. Primary (Correct Answer)
B. Quaternary
C. Tertiary
D. Secondary

Explanation: ***Primary*** - The **primary structure** refers to the specific linear sequence of **amino acids** forming the polypeptide chain, linked by **covalent peptide bonds**. - These strong **peptide bonds** are generally resistant to heat denaturation at 100°C, meaning the amino acid sequence remains intact. *Quaternary* - The **quaternary structure** involves the arrangement of multiple polypeptide subunits and is maintained by weaker interactions like **hydrophobic interactions**, hydrogen bonds, and salt bridges. - These interactions are highly susceptible to disruption by heat, causing the subunits to dissociate and the quaternary structure to be lost. *Tertiary* - The **tertiary structure** describes the three-dimensional folding of a single polypeptide chain, stabilized by various non-covalent interactions (e.g., hydrogen bonds, ionic bonds, hydrophobic interactions) and **disulfide bonds**. - Heat disrupts these weaker non-covalent interactions and can even break disulfide bonds, leading to the unfolding and loss of the specific 3D shape. *Secondary* - The **secondary structure** (e.g., **alpha-helices** and **beta-pleated sheets**) arises from hydrogen bonds between the backbone atoms of the polypeptide chain. - While peptide bonds remain intact, these vital **hydrogen bonds** are easily broken by heat, causing the unraveling of helices and sheets.

Question 6: Most abundant amino acid found in collagen?

A. Hydroxyproline
B. Proline
C. Glycine (Correct Answer)
D. Lysine

Explanation: ***Glycine*** - **Glycine** is the most abundant amino acid in collagen, making up approximately one-third of its total amino acid content. - Its small size (due to a single hydrogen atom as its side chain) is crucial for the formation of the **triple helix structure** of collagen, allowing the tight packing of the three alpha chains. *Hydroxyproline* - **Hydroxyproline** is a modified amino acid derived from proline, and while abundant in collagen, it accounts for about 13% of the total amino acids, less than glycine. - It plays a vital role in stabilizing the collagen triple helix through **hydrogen bonding**. *Proline* - **Proline** is a structurally important amino acid in collagen, contributing to the kinks and turns necessary for the formation of the triple helix. - However, its abundance is less than that of glycine, making up around 10-17% of collagen's amino acids. *Lysine* - **Lysine** is a less abundant but essential amino acid in collagen, making up about 2-5% of its composition. - It is critical for **cross-linking** collagen fibers, which provides tensile strength, and can be hydroxylated to form **hydroxylysine**.

Question 7: Which is a product of purine metabolism?

A. Xanthine
B. Uric acid (Correct Answer)
C. Beta alanine
D. Urea

Explanation: ***Uric acid*** - **Uric acid** is the final breakdown product of **purine metabolism** in humans. - It's formed from the oxidation of **xanthine** by the enzyme **xanthine oxidase**. *Xanthine* - **Xanthine** is an intermediate compound in the purine metabolic pathway, but not the final product in humans. - It is converted to **uric acid** via the enzyme **xanthine oxidase**. *Beta alanine* - **Beta-alanine** is a **non-proteogenic amino acid** and is a breakdown product of **pyrimidine metabolism**, not purine. - It is involved in the synthesis of **carnosine**, a buffer in muscle tissue. *Urea* - **Urea** is the primary end product of **protein (amino acid) metabolism** in mammals, representing the main way the body excretes **nitrogen**. - It is produced in the **urea cycle** in the liver and is unrelated to purine breakdown.

Question 8: Melanin is derived from which amino acid?

A. Tyrosine (Correct Answer)
B. Phenylalanine
C. Alanine
D. Tryptophan

Explanation: ***Tyrosine*** - **Melanin synthesis** (melanogenesis) begins with the hydroxylation of **tyrosine** to DOPA (dihydroxyphenylalanine), catalyzed by the enzyme **tyrosinase**. - Subsequent steps involve the oxidation of DOPA and its derivatives to form various types of melanin, such as **eumelanin** (black/brown) and **pheomelanin** (red/yellow). *Phenylalanine* - **Phenylalanine** is an essential amino acid that can be converted to tyrosine in the body by the enzyme **phenylalanine hydroxylase**. - While it's a precursor to tyrosine, it's not directly converted to melanin; rather, it must first be hydroxylated to tyrosine. *Alanine* - **Alanine** is a non-essential amino acid involved in glucose metabolism and protein synthesis. - It is not a precursor for melanin synthesis. *Tryptophan* - **Tryptophan** is an essential amino acid that is a precursor for **serotonin**, **melatonin**, and **niacin** (Vitamin B3). - It is not involved in the biosynthesis of melanin.

Question 9: Which bases are present in human DNA?

A. Adenine-guanine-cytosine-thymine (Correct Answer)
B. None of the above
C. Adenine-guanine-cytosine-uracil
D. Adenine-guanine-thiamine-uracil

Explanation: ***Adenine- guanine-cytosine-thymine*** - DNA (deoxyribonucleic acid) in humans, and most other organisms, is composed of four nitrogenous bases: **adenine (A), guanine (G), cytosine (C), and thymine (T)**. - These bases are paired specifically: **adenine with thymine (A-T)** and **guanine with cytosine (G-C)**, forming the rungs of the DNA double helix. *None of the above* - This option is incorrect because there is a definitive set of bases that make up human DNA, as described in the correct option. - The fundamental building blocks of human DNA are well-established and universally recognized in biology. *Adenine-guanine- cytosine-uracil* - This list of bases is characteristic of **RNA (ribonucleic acid)**, not DNA. - In RNA, **uracil (U)** replaces thymine (T), meaning RNA contains adenine (A), guanine (G), cytosine (C), and uracil (U). *Adenine-guanine-thiamine- uracil* - This option incorrectly includes **thiamine**, which is a **vitamin (B1)** and not a nitrogenous base found in nucleic acids. - It also incorrectly includes **uracil**, which is found in RNA, not DNA.

Question 10: Type IV complex of ETC is inhibited by

A. Antimycin
B. Oligomycin
C. CO2
D. Cyanide (Correct Answer)

Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to its ferric iron center and preventing the reduction of oxygen to water. - This inhibition effectively blocks electron flow, leading to a rapid cessation of ATP production and cellular respiration. *Antimycin* - **Antimycin A** specifically inhibits **Complex III (cytochrome bc1 complex)** of the electron transport chain. - It binds to the Qn site of Complex III, preventing the transfer of electrons from reduced ubiquinone to cytochrome c. *Oligomycin* - **Oligomycin** is an inhibitor of **ATP synthase (Complex V)**, not Complex IV. - It blocks the flow of protons through the Fo subunit of ATP synthase, thereby inhibiting ATP synthesis, but it does not directly affect electron transport itself. *CO2* - **CO2** is a waste product of cellular respiration and is not an inhibitor of any complex within the electron transport chain. - While high levels of CO2 can affect pH and cellular function, it does not directly interfere with the catalytic activity of ETC complexes.