Essential Amino Acids and Proteins — MCQs
Protein metabolism after trauma is characterized by the following except:
Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?
Digestion of proteins is initiated by:
Which of the following protein sources has the lowest biological value?
Which enzyme deficiency causes Maple Syrup Urine Disease?
Which amino acid needs to be supplemented through diet in patient with cystathionine beta synthase deficiency
A 6-year-old presents with developmental delay, musty body odor, and fair skin. Lab tests show high phenylalanine levels. What is the most appropriate management?
Which food item among the following has the highest protein content?
What is the Net Protein Utilization (NPU) for eggs?
Essential Amino Acids and Proteins Indian Medical PG Practice Questions and MCQs
Question 1: Protein metabolism after trauma is characterized by the following except:
- A. Increased liver gluconeogenesis
- B. Increased urinary nitrogen loss
- C. Hepatic synthesis of acute phase reactants
- D. Inhibition of skeletal muscle breakdown by interleukin 1 and tumour necrosis factor (Correct Answer)
Explanation: ***Inhibition of skeletal muscle breakdown by interleukin 1 and tumour necrosis factor*** - After trauma, **interleukin 1 (IL-1)** and **tumor necrosis factor (TNF)** actually **promote** skeletal muscle breakdown (catabolism) to provide amino acids for gluconeogenesis and acute phase protein synthesis. - This statement is incorrect because these cytokines are **pro-catabolic**, not inhibitory, in their effect on muscle protein. *Increased liver gluconeogenesis* - Trauma leads to a significant increase in **liver gluconeogenesis**, primarily to maintain glucose supply for **immune cells** and wound healing, which rely heavily on glucose. - This process utilizes amino acids obtained from muscle breakdown as substrates. *Increased urinary nitrogen loss* - The breakdown of muscle protein releases amino acids, which are then deaminated. The nitrogen waste product, **urea**, is excreted in the urine, leading to **increased urinary nitrogen loss**. - This is a direct consequence of the catabolic state. *Hepatic synthesis of acute phase reactants* - The liver increases the synthesis of **acute phase reactants** (e.g., C-reactive protein, fibrinogen, haptoglobin) in response to inflammatory cytokines like IL-1, **IL-6**, and TNF. - These proteins play a crucial role in the inflammatory response and tissue repair.
Question 2: Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
- A. Trypsin
- B. Carboxypeptidase
- C. Phenylisothiocyanate (Correct Answer)
- D. Cyanogen bromide
Explanation: ***Phenylisothiocyanate*** - **Phenylisothiocyanate** (PITC), also known as Edman's reagent, is used in the **Edman degradation** method to identify the N-terminal amino acid. - It sequentially cleaves the **N-terminal amino acid** without hydrolyzing the rest of the peptide chain, allowing for identification by chromatography. *Trypsin* - **Trypsin** is a protease that cleaves peptide bonds at the carboxyl side of **lysine** and **arginine** residues. - It is used for peptide fragmentation, not for determining the N-terminal amino acid. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave amino acids from the **C-terminal end** of a polypeptide chain. - They are used to identify the C-terminal amino acid, not the N-terminal. *Cyanogen bromide* - **Cyanogen bromide (CNBr)** is a chemical reagent that specifically cleaves peptide bonds on the C-terminal side of **methionine** residues. - It is used for specific peptide fragmentation and not for N-terminal sequencing.
Question 3: The α-helix and β-pleated sheet in proteins are examples of which level of protein structure?
- A. Primary structure
- B. Secondary structure (Correct Answer)
- C. Tertiary structure
- D. Quaternary structure
Explanation: ***Secondary structure*** - The **α-helix** and **β-pleated sheet** are formed by **hydrogen bonding** between the backbone atoms of amino acids within a polypeptide chain. - This level of structure describes the regular, recurring arrangements of **local regions** of the polypeptide backbone. *Primary structure* - This refers to the **linear sequence of amino acids** in a polypeptide chain, determined by the genetic code. - It does not involve the folding patterns of the polypeptide backbone but rather the order of its constituent monomers. *Tertiary structure* - This describes the **overall three-dimensional shape** of a single polypeptide chain, including the folding of helices and sheets and the arrangement of side chains. - It is stabilized by various interactions, including **hydrophobic interactions**, ionic bonds, hydrogen bonds, and disulfide bridges. *Quaternary structure* - This applies to proteins composed of **multiple polypeptide subunits**, describing how these subunits associate and are arranged in space. - It is established through interactions between different polypeptide chains, such as in **hemoglobin**.
Question 4: Digestion of proteins is initiated by:
- A. Amylase
- B. Sucrase
- C. Chymotrypsin
- D. Pepsin (Correct Answer)
Explanation: ***Pepsin*** - **Pepsin** is the primary enzyme responsible for initiating **protein digestion** in the **stomach**. - It cleaves proteins into smaller polypeptides, functioning optimally in the highly **acidic environment** of the stomach. *Amylase* - **Amylase** is responsible for the digestion of **carbohydrates**, breaking down starch into simpler sugars. - It is found in both saliva (salivary amylase) and pancreatic secretions (pancreatic amylase), and does not act on proteins. *Sucrase* - **Sucrase** is an enzyme located in the **small intestine** and is responsible for breaking down the disaccharide **sucrose** into glucose and fructose. - It plays no role in protein digestion. *Chymotrypsin* - **Chymotrypsin** is a proteolytic enzyme secreted by the **pancreas** that acts in the **small intestine** to further digest polypeptides into smaller peptides. - While it digests proteins, it is not the *initiating* enzyme; protein digestion is already underway by the time chymotrypsin acts.
Question 5: Which of the following protein sources has the lowest biological value?
- A. Gelatin (Correct Answer)
- B. Albumin
- C. Cashewnut protein
- D. Lactalbumin
Explanation: ***Gelatin*** - Gelatin is derived from collagen and is notably deficient in several **essential amino acids**, particularly **tryptophan**. - Its incomplete amino acid profile gives it a very **low biological value**, meaning it's poorly utilized by the body for protein synthesis. *Lactalbumin* - Lactalbumin is a **whey protein** found in milk, known for its **high biological value**. - It contains a **complete profile of essential amino acids** and is easily digestible. *Albumin* - Albumin, typically referring to **egg albumin** (egg white protein) or **serum albumin**, has an **excellent biological value**. - It is often used as a reference protein due to its well-balanced and **complete amino acid composition**. *Cashewnut protein* - While nuts like cashews provide protein, they generally have a **moderate to high biological value**, though typically not as high as animal proteins. - Cashew protein contains a good range of amino acids but might be slightly limited in one or two essential amino acids compared to a reference like egg or whey.
Question 6: Which enzyme deficiency causes Maple Syrup Urine Disease?
- A. Glucose-6-phosphatase
- B. Branched-chain ketoacid dehydrogenase (Correct Answer)
- C. Phenylalanine hydroxylase
- D. Pyruvate dehydrogenase
Explanation: ***Branched-chain ketoacid dehydrogenase*** - **Maple Syrup Urine Disease (MSUD)** is caused by deficiency in the branched-chain α-keto acid dehydrogenase complex (BCKDC) - This enzyme is crucial for metabolism of the **branched-chain amino acids (BCAAs)**: leucine, isoleucine, and valine - Accumulation of BCAAs and their keto acids gives urine a characteristic **maple syrup odor** - Presents with **neurological symptoms, ketoacidosis, and intellectual disability** if untreated *Glucose-6-phosphatase* - Deficiency causes Glycogen Storage Disease Type I (von Gierke disease), not MSUD - This enzyme is essential for the final step of gluconeogenesis and glycogenolysis - Leads to hypoglycemia and lactic acidosis *Phenylalanine hydroxylase* - Deficiency causes phenylketonuria (PKU), not MSUD - This enzyme converts phenylalanine to tyrosine - Absence leads to toxic accumulation of phenylalanine causing intellectual disability and musty odor *Pyruvate dehydrogenase* - Deficiency affects conversion of pyruvate to acetyl-CoA, not branched-chain amino acid metabolism - Leads to buildup of pyruvate shunted to lactate - Causes lactic acidosis and neurological symptoms
Question 7: Which amino acid needs to be supplemented through diet in patient with cystathionine beta synthase deficiency
- A. Tryptophan
- B. Serine
- C. Methionine
- D. Cysteine (Correct Answer)
- E. Tyrosine
Explanation: ***Cysteine*** - In **cystathionine beta synthase (CBS) deficiency**, the conversion of **homocysteine** to **cystathionine** (and subsequently to cysteine) is impaired. - This makes **cysteine** an **essential amino acid** for these patients, requiring dietary supplementation. *Tryptophan* - **Tryptophan** is an **essential amino acid** and a precursor for **serotonin** and **niacin**, but its metabolism is not directly affected by CBS deficiency. - Its supplementation is not specifically indicated for this condition. *Serine* - **Serine** is a **non-essential amino acid** that provides the **carbon skeleton** for the synthesis of cysteine from homocysteine in the presence of CBS. - While important in the pathway, CBS deficiency specifically disrupts the downstream conversion of homocysteine, making **cysteine** the deficient product, not serine. *Methionine* - **Methionine** is an **essential amino acid** that is a precursor to **homocysteine**; in CBS deficiency, homocysteine levels are already elevated due to impaired conversion to cystathionine. - Restricting methionine intake is typically recommended in CBS deficiency to reduce homocysteine accumulation, not supplementing it. *Tyrosine* - **Tyrosine** is a **non-essential amino acid** derived from **phenylalanine** and serves as a precursor for catecholamines and thyroid hormones. - Its metabolism is not affected by CBS deficiency, and supplementation is not indicated for this condition.
Question 8: A 6-year-old presents with developmental delay, musty body odor, and fair skin. Lab tests show high phenylalanine levels. What is the most appropriate management?
- A. Low-phenylalanine diet (Correct Answer)
- B. Avoidance of ascorbic acid
- C. Vitamin D supplementation
- D. High-protein diet
- E. Tetrahydrobiopterin (BH4) supplementation
Explanation: ***Low-phenylalanine diet*** - The patient's symptoms (developmental delay, musty body odor, fair skin) and high **phenylalanine levels** are classic for **phenylketonuria (PKU)**. - Management primarily involves a strict **low-phenylalanine diet** to prevent further neurological damage. - This is the **cornerstone of PKU management** and must be initiated as early as possible. *Tetrahydrobiopterin (BH4) supplementation* - While **BH4 (sapropterin)** can be beneficial in some patients with **BH4-responsive PKU** (a subset of PKU cases), it is not first-line management. - BH4 testing is performed after diagnosis, but dietary restriction remains the primary treatment. - Not all PKU patients respond to BH4, and it's used as an adjunct, not a replacement for dietary management. *Avoidance of ascorbic acid* - **Ascorbic acid** (vitamin C) is generally not contraindicated in PKU and does not impact phenylalanine metabolism. - This intervention is not relevant to the management of PKU. *Vitamin D supplementation* - While vitamin D supplementation might be necessary for general health, especially in children with restricted diets, it is not the primary treatment for **phenylketonuria (PKU)**. - It does not directly address the elevated phenylalanine levels. *High-protein diet* - A **high-protein diet** would exacerbate the condition, as proteins are a major source of phenylalanine. - This would lead to even higher phenylalanine levels and worsen the symptoms of PKU.
Question 9: Which food item among the following has the highest protein content?
- A. Egg
- B. Soyabean (Correct Answer)
- C. Rice
- D. Wheat
Explanation: ***Soyabean*** - **Soybeans** are an exceptional source of **complete protein**, containing all nine essential amino acids - They provide approximately **36 grams of protein per 100 grams**, making them one of the most protein-rich plant foods - Due to their high protein content and versatility, soybeans and soy-based products are staple foods in many diets, especially for vegetarians and vegans *Egg* - Eggs are a complete protein source with excellent bioavailability and quality - A whole egg typically contains about **6 grams of protein** (approximately 13 grams per 100 grams) - While eggs are an excellent protein source, **soybeans contain significantly higher protein per 100 grams** *Rice* - **Rice**, particularly white rice, is primarily a source of **carbohydrates** with relatively low protein content - Contains approximately **2-3 grams of protein per 100 grams** - Not considered a significant source of protein compared to legumes or animal products *Wheat* - **Wheat** is a grain rich in carbohydrates with moderate protein content - Contains approximately **10-15 grams of protein per 100 grams**, particularly in whole wheat - While it's a good source of plant-based protein, it contains significantly less protein than **soybeans**
Question 10: What is the Net Protein Utilization (NPU) for eggs?
- A. 70
- B. 80
- C. 94 (Correct Answer)
- D. 100
Explanation: ***94*** - **Eggs** are considered a **high-quality protein source** with a Net Protein Utilization (NPU) of approximately **94**, indicating very efficient protein absorption and utilization by the body. - This high NPU reflects the excellent balance of **essential amino acids** in eggs, making them a benchmark for protein quality. *70* - An NPU of 70 is generally considered good but is lower than that of **eggs**, which are among the most efficiently utilized proteins. - This value might be typical for some **plant-based proteins** or mixtures of proteins with slightly less optimal essential amino acid profiles. *80* - An NPU of 80 indicates good protein quality but is still significantly lower than the **NPU of eggs**. - This value is often seen in high-quality **meat proteins** or well-balanced **dairy products**. *100* - An NPU of 100 would mean that all ingested protein is perfectly absorbed and utilized by the body without any loss, which is **theoretically impossible** for biological systems. - While some protein quality metrics might approach 100, **NPU is a measure of actual utilization** and never reaches 100 due to metabolic losses.
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