FMGE 2018 — Biochemistry

Q: Muscles cannot release free glucose from glycogen stores because of a deficiency of:

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - **Glucose-6-phosphatase** is the enzyme that dephosphorylates glucose-6-phosphate to free glucose, allowing its release into the bloodstream. - This enzyme is **physiologically absent in muscle tissue** (present only in liver and kidneys), meaning muscles can break down glycogen for their own energy needs but cannot release free glucose into circulation. - This ensures that muscle glycogen stores are reserved exclusively for muscle's own metabolic needs during contraction. *Glycogen phosphorylase* - **Glycogen phosphorylase** is present in muscle and catalyzes the breakdown of glycogen by cleaving α-1,4 glycosidic bonds to release glucose-1-phosphate. - Muscles have this enzyme and can normally break down glycogen for energy; deficiency causes **McArdle disease** (glycogen storage disease type V) with exercise intolerance. *Hexokinase* - **Hexokinase** is abundant in muscle tissue and phosphorylates free glucose to glucose-6-phosphate for entry into glycolysis. - This enzyme is necessary for utilizing both blood glucose and glycogen-derived glucose-6-phosphate. *Phosphoglucomutase* - **Phosphoglucomutase** is present in muscle and converts glucose-1-phosphate (from glycogen breakdown) to glucose-6-phosphate. - This enzyme is essential for channeling glycogen-derived glucose into glycolysis. [Practice more Biochemistry PYQs on OnCourse]

Q: Km increases, but Vmax remains same. This is which type of inhibition?

Competitive. ***Competitive*** - In **competitive inhibition**, the inhibitor **reversibly binds** to the **active site** of the enzyme, competing with the substrate. - This competition means that a higher substrate concentration is required to achieve half-maximal velocity, thus **increasing the Km**, while the maximum velocity (**Vmax**) remains unchanged if sufficient substrate is present. *Uncompetitive* - **Uncompetitive inhibition** involves the inhibitor binding only to the **enzyme-substrate complex**. - This type of inhibition typically leads to a **decrease in both Km and Vmax**. *Non-competitive* - In **non-competitive inhibition**, the inhibitor binds to a site other than the active site (allosteric site) on either the free enzyme or the enzyme-substrate complex. - This binding usually **decreases the Vmax** (due to reduced enzyme efficiency) but does not affect the Km (as substrate binding is not directly hindered). *Irreversible* - **Irreversible inhibition** involves the formation of a strong, often covalent, bond between the inhibitor and the enzyme, permanently inactivating it. - This type of inhibition effectively **reduces the concentration of active enzyme**, leading to a **decrease in Vmax** (as fewer enzyme molecules are available to catalyze the reaction) with varying effects on Km depending on the mechanism. [Practice more Biochemistry PYQs on OnCourse]

Q: Apoprotein for chylomicron remnants:

Apo E. ***Apo E*** - **Apolipoprotein E** (**Apo E**) is crucial for the uptake of **chylomicron remnants** and **VLDL remnants** (IDL) by the liver via the **LDL receptor-related protein 1 (LRP1)**. - It acts as a **ligand** for this receptor, facilitating the clearance of dietary fats from the circulation. *Apo A2* - **Apo A2** is a primary apolipoprotein of **high-density lipoprotein (HDL)**, not chylomicron remnants. - Its exact function is not fully understood, but it may modulate the activity of **hepatic lipase**. *Apo C1* - **Apo C1** is found on **chylomicrons**, **VLDL**, and **HDL**, but it is not the primary apoprotein responsible for the uptake of chylomicron remnants by the liver. - It is known to **activate lecithin-cholesterol acyltransferase (LCAT)** and may inhibit **cholesterol ester transfer protein (CETP)**. *Apo A1* - **Apo A1** is the most abundant apolipoprotein in **HDL** and is essential for its structure and function. - It is a potent **activator of LCAT**, an enzyme that esterifies cholesterol in HDL, which is key for **reverse cholesterol transport**. [Practice more Biochemistry PYQs on OnCourse]

Q: Low glycemic index food is:

Has slower absorption. ***Has slower absorption*** - **Low glycemic index (GI)** foods are digested and absorbed more slowly, leading to a gradual rise in blood glucose and insulin levels. - This characteristic is beneficial for managing **blood sugar** and providing sustained energy. *Easily digestible* - **Easily digestible** foods often have a **high glycemic index** because their carbohydrates are rapidly broken down and absorbed. - Low GI foods, by contrast, contain more complex carbohydrates and fiber, making them slower to digest. *Increase glycogen deposits* - While all carbohydrates are eventually converted to **glucose** and can contribute to **glycogen synthesis**, low GI foods do not uniquely or preferentially increase glycogen deposits compared to high GI foods. - Glycogen synthesis is primarily influenced by insulin levels and the total amount of carbohydrates consumed, irrespective of GI. *Increases plasma glucose* - All carbohydrate-containing foods will eventually increase **plasma glucose**, but low GI foods cause a **slower and smaller rise** in blood glucose compared to high GI foods. - They prevent the sharp spikes in blood sugar that are associated with high GI foods. [Practice more Biochemistry PYQs on OnCourse]

Q: Which is branching enzyme?

Amylo-1, 4-1, 6-transglycolase. ***Amylo-1, 4-1, 6-transglycolase*** - This enzyme is also known as **glycogen branching enzyme**. - It catalyzes the formation of **α-1,6-glycosidic bonds** by transferring a segment of four to six glucosyl residues from the non-reducing end of a growing glycogen chain to another chain. *Glycogen synthase* - This enzyme is responsible for the **elongation of glycogen chains** by forming **α-1,4-glycosidic bonds**. - It adds glucose units to the non-reducing end of a pre-existing glycogen primer. *Glycogen Phosphorylase* - This enzyme is involved in **glycogen degradation**. - It catalyzes the **phosphorolytic cleavage** of α-1,4-glycosidic bonds, releasing glucose-1-phosphate. *Glucose-6 phosphatase* - This enzyme is primarily found in the **liver** and kidneys and is crucial for **gluconeogenesis** and **glycogenolysis**. - It dephosphorylates glucose-6-phosphate to **free glucose**, allowing its release into the bloodstream. [Practice more Biochemistry PYQs on OnCourse]

15 Previous Year Questions with Answers & Explanations

Questions

Muscles cannot release free glucose from glycogen stores because of a deficiency of:

Km increases, but Vmax remains same. This is which type of inhibition?

Apoprotein for chylomicron remnants:

Low glycemic index food is:

Which is branching enzyme?

Preferred biochemical marker(s) in patients presenting with myocardial infarction:

Most Common enzyme deficient in galactosemics:

Chaperones are:

Which organelle contains its own DNA apart from the nucleus?

Q10

Western blot is done for:

FMGE 2018 - Biochemistry FMGE Practice Questions and MCQs

Question 1: Muscles cannot release free glucose from glycogen stores because of a deficiency of:

A. Glucose-6-phosphatase (Correct Answer)
B. Hexokinase
C. Phosphoglucomutase
D. Glycogen phosphorylase

Explanation: ***Glucose-6-phosphatase*** - **Glucose-6-phosphatase** is the enzyme that dephosphorylates glucose-6-phosphate to free glucose, allowing its release into the bloodstream. - This enzyme is **physiologically absent in muscle tissue** (present only in liver and kidneys), meaning muscles can break down glycogen for their own energy needs but cannot release free glucose into circulation. - This ensures that muscle glycogen stores are reserved exclusively for muscle's own metabolic needs during contraction. *Glycogen phosphorylase* - **Glycogen phosphorylase** is present in muscle and catalyzes the breakdown of glycogen by cleaving α-1,4 glycosidic bonds to release glucose-1-phosphate. - Muscles have this enzyme and can normally break down glycogen for energy; deficiency causes **McArdle disease** (glycogen storage disease type V) with exercise intolerance. *Hexokinase* - **Hexokinase** is abundant in muscle tissue and phosphorylates free glucose to glucose-6-phosphate for entry into glycolysis. - This enzyme is necessary for utilizing both blood glucose and glycogen-derived glucose-6-phosphate. *Phosphoglucomutase* - **Phosphoglucomutase** is present in muscle and converts glucose-1-phosphate (from glycogen breakdown) to glucose-6-phosphate. - This enzyme is essential for channeling glycogen-derived glucose into glycolysis.

Question 2: Km increases, but Vmax remains same. This is which type of inhibition?

A. Uncompetitive
B. Non-competitive
C. Competitive (Correct Answer)
D. Irreversible

Explanation: ***Competitive*** - In **competitive inhibition**, the inhibitor **reversibly binds** to the **active site** of the enzyme, competing with the substrate. - This competition means that a higher substrate concentration is required to achieve half-maximal velocity, thus **increasing the Km**, while the maximum velocity (**Vmax**) remains unchanged if sufficient substrate is present. *Uncompetitive* - **Uncompetitive inhibition** involves the inhibitor binding only to the **enzyme-substrate complex**. - This type of inhibition typically leads to a **decrease in both Km and Vmax**. *Non-competitive* - In **non-competitive inhibition**, the inhibitor binds to a site other than the active site (allosteric site) on either the free enzyme or the enzyme-substrate complex. - This binding usually **decreases the Vmax** (due to reduced enzyme efficiency) but does not affect the Km (as substrate binding is not directly hindered). *Irreversible* - **Irreversible inhibition** involves the formation of a strong, often covalent, bond between the inhibitor and the enzyme, permanently inactivating it. - This type of inhibition effectively **reduces the concentration of active enzyme**, leading to a **decrease in Vmax** (as fewer enzyme molecules are available to catalyze the reaction) with varying effects on Km depending on the mechanism.

Question 3: Apoprotein for chylomicron remnants:

A. Apo A2
B. Apo E (Correct Answer)
C. Apo C1
D. Apo A1

Explanation: ***Apo E*** - **Apolipoprotein E** (**Apo E**) is crucial for the uptake of **chylomicron remnants** and **VLDL remnants** (IDL) by the liver via the **LDL receptor-related protein 1 (LRP1)**. - It acts as a **ligand** for this receptor, facilitating the clearance of dietary fats from the circulation. *Apo A2* - **Apo A2** is a primary apolipoprotein of **high-density lipoprotein (HDL)**, not chylomicron remnants. - Its exact function is not fully understood, but it may modulate the activity of **hepatic lipase**. *Apo C1* - **Apo C1** is found on **chylomicrons**, **VLDL**, and **HDL**, but it is not the primary apoprotein responsible for the uptake of chylomicron remnants by the liver. - It is known to **activate lecithin-cholesterol acyltransferase (LCAT)** and may inhibit **cholesterol ester transfer protein (CETP)**. *Apo A1* - **Apo A1** is the most abundant apolipoprotein in **HDL** and is essential for its structure and function. - It is a potent **activator of LCAT**, an enzyme that esterifies cholesterol in HDL, which is key for **reverse cholesterol transport**.

Question 4: Low glycemic index food is:

A. Easily digestible
B. Increase glycogen deposits
C. Has slower absorption (Correct Answer)
D. Increases plasma glucose

Explanation: ***Has slower absorption*** - **Low glycemic index (GI)** foods are digested and absorbed more slowly, leading to a gradual rise in blood glucose and insulin levels. - This characteristic is beneficial for managing **blood sugar** and providing sustained energy. *Easily digestible* - **Easily digestible** foods often have a **high glycemic index** because their carbohydrates are rapidly broken down and absorbed. - Low GI foods, by contrast, contain more complex carbohydrates and fiber, making them slower to digest. *Increase glycogen deposits* - While all carbohydrates are eventually converted to **glucose** and can contribute to **glycogen synthesis**, low GI foods do not uniquely or preferentially increase glycogen deposits compared to high GI foods. - Glycogen synthesis is primarily influenced by insulin levels and the total amount of carbohydrates consumed, irrespective of GI. *Increases plasma glucose* - All carbohydrate-containing foods will eventually increase **plasma glucose**, but low GI foods cause a **slower and smaller rise** in blood glucose compared to high GI foods. - They prevent the sharp spikes in blood sugar that are associated with high GI foods.

Question 5: Which is branching enzyme?

A. Glycogen synthase
B. Amylo-1, 4-1, 6-transglycolase (Correct Answer)
C. Glycogen Phosphorylase
D. Glucose-6 phosphatase

Explanation: ***Amylo-1, 4-1, 6-transglycolase*** - This enzyme is also known as **glycogen branching enzyme**. - It catalyzes the formation of **α-1,6-glycosidic bonds** by transferring a segment of four to six glucosyl residues from the non-reducing end of a growing glycogen chain to another chain. *Glycogen synthase* - This enzyme is responsible for the **elongation of glycogen chains** by forming **α-1,4-glycosidic bonds**. - It adds glucose units to the non-reducing end of a pre-existing glycogen primer. *Glycogen Phosphorylase* - This enzyme is involved in **glycogen degradation**. - It catalyzes the **phosphorolytic cleavage** of α-1,4-glycosidic bonds, releasing glucose-1-phosphate. *Glucose-6 phosphatase* - This enzyme is primarily found in the **liver** and kidneys and is crucial for **gluconeogenesis** and **glycogenolysis**. - It dephosphorylates glucose-6-phosphate to **free glucose**, allowing its release into the bloodstream.

Question 6: Preferred biochemical marker(s) in patients presenting with myocardial infarction:

A. Cardiac specific Troponins (Correct Answer)
B. All of the options
C. Myoglobin
D. CK-MB

Explanation: ***Cardiac specific Troponins*** - **Cardiac troponins** (cTnI and cTnT) are the preferred and most sensitive and specific biomarkers for diagnosing **myocardial infarction (MI)**. - They are proteins released into the bloodstream when myocardial cells are damaged, and their levels rise within hours of MI onset and remain elevated for several days. *All of the options* - While other markers like **CK-MB** and **Myoglobin** were historically used, **cardiac troponins** have superior specificity and sensitivity for MI. - The latest guidelines from major cardiology societies recommend troponins as the primary diagnostic markers. *Myoglobin* - **Myoglobin** is an early marker, but it is not specific to cardiac muscle and can be elevated in various conditions involving skeletal muscle damage. - Its short half-life means it can return to normal quickly, making it less reliable for diagnosing MI, especially if there's a delay in presentation. *CK-MB* - **Creatine Kinase-MB (CK-MB)** is a more specific marker than total CK for cardiac muscle damage but is still less specific than cardiac troponins. - It can be elevated in conditions like **myocarditis** or **pericarditis**, and its levels typically peak and decline earlier than troponins, limiting its diagnostic window.

Question 7: Most Common enzyme deficient in galactosemics:

A. Galactosidase
B. UDP galactose epimerase
C. Galactokinase
D. Galactose-1-phosphate uridyl transferase/GALT (Correct Answer)

Explanation: ***Galactose-1-phosphate uridyl transferase/GALT*** - **GALT deficiency** is the most common cause of **classic galactosemia** (Type I), a severe inherited metabolic disorder. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the main pathway of galactose metabolism. - Accounts for approximately **95%** of all galactosemia cases. *Galactosidase* - **Galactosidase** enzymes are involved in the hydrolysis of galactose-containing oligosaccharides or glycoconjugates but are not the primary enzymes deficient in classic galactosemia. - This enzyme is not part of the Leloir pathway of galactose metabolism, which is the pathway affected in galactosemia. *UDP galactose epimerase* - Deficiency of **UDP galactose epimerase** (GALE) causes a milder form of galactosemia (Type III), but it is much less common than GALT deficiency. - GALE is involved in the interconversion of UDP-galactose and UDP-glucose. - This is the rarest form of galactosemia. *Galactokinase* - **Galactokinase deficiency** (GALK) causes a different, milder form of galactosemia (Type II), characterized by **cataracts** as the primary symptom. - It prevents the initial phosphorylation of galactose to galactose-1-phosphate. - This accounts for less than 5% of galactosemia cases.

Question 8: Chaperones are:

A. Mediators of post-translational assembly of protein complexes (Correct Answer)
B. Antigen presenting cells
C. Purine metabolism mediators
D. None of the above

Explanation: ***Mediators of post-translational assembly of protein complexes*** - **Chaperones** are proteins that assist in the proper folding of other proteins, especially new polypeptides, and in the assembly of **protein complexes** after translation. - They prevent **misfolding** and aggregation of proteins, ensuring their correct functional conformation. *Antigen presenting cells* - **Antigen-presenting cells (APCs)** are immune cells (e.g., macrophages, dendritic cells) that present **antigens** to T cells for recognition. - Their primary function is in the **immune response**, not protein folding or assembly. *Purine metabolism mediators* - **Purine metabolism mediators** are enzymes or molecules involved in the synthesis, breakdown, and recycling of **purines (adenine and guanine)**. - This function is entirely distinct from the role of chaperones in **protein folding**. *None of the above* - This option is incorrect because the first option accurately describes the function of **chaperones**.

Question 9: Which organelle contains its own DNA apart from the nucleus?

A. Mitochondria (Correct Answer)
B. RER
C. Golgi complex
D. SER

Explanation: ***Mitochondria*** - Mitochondria contain their own **circular DNA (mtDNA)**, which is inherited maternally, and their own ribosomes. - This DNA encodes for some proteins essential for **cellular respiration** and its own replication, supporting the endosymbiotic theory. *RER* - The **Rough Endoplasmic Reticulum (RER)** is characterized by the presence of **ribosomes** on its surface. - It plays a crucial role in the **synthesis and modification of proteins** designated for secretion or insertion into membranes, but does not contain DNA. *Golgi complex* - The **Golgi complex** is involved in **modifying, sorting, and packaging proteins and lipids** for secretion or delivery to other organelles. - It consists of flattened sacs called cisternae but does not possess DNA. *SER* - The **Smooth Endoplasmic Reticulum (SER)** is involved in **lipid synthesis**, **detoxification** of drugs and poisons, and **calcium ion storage**. - Unlike mitochondria, it does not contain its own genetic material.

Question 10: Western blot is done for:

A. Protein (Correct Answer)
B. RNA
C. Lipid
D. DNA

Explanation: ***Protein*** - **Western blot** is a laboratory technique used to detect specific **proteins** in a sample of tissue homogenate or extract. - It involves separating proteins by **electrophoresis**, transferring them to a membrane, and then detecting them using **antibodies**. *RNA* - **Northern blot** is the technique specifically designed for the detection and analysis of **RNA** molecules. - It involves separating RNA fragments by **electrophoresis**, transferring them to a membrane, and querying with a labeled probe. *Lipid* - There is no direct "lipid blot" technique analogous to Western, Northern, or Southern blots. - **Lipids** are typically analyzed using techniques such as mass spectrometry, thin-layer chromatography, or gas chromatography. *DNA* - **Southern blot** is the molecular biology method used for the detection of specific **DNA** sequences in DNA samples. - It involves fragmenting DNA, separating by **electrophoresis**, and then hybridizing with a labeled DNA probe.