NEET-PG 2012 - Biochemistry Practice Questions

Q: Which vitamin is considered the most potent antioxidant?

Vit E. ***Vit E*** - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from **oxidative damage** by scavenging free radicals. - Its ability to interrupt **lipid peroxidation** makes it highly effective in protecting tissues rich in polyunsaturated fatty acids, such as cell membranes. *Vit A* - **Vitamin A**, particularly in its carotenoid forms like **beta-carotene**, is an antioxidant, but its primary role is in **vision** and **immune function**. - While it can quench **singlet oxygen** and trap free radicals, it is generally considered less potent than vitamin E in protecting against lipid peroxidation. *Vit K* - **Vitamin K** is crucial for **blood coagulation** and **bone metabolism**, but it does not have significant antioxidant properties. - Its primary biological functions are unrelated to scavenging **free radicals** or preventing oxidative stress. *Vit C* - **Vitamin C** is a potent **water-soluble antioxidant** that works in aqueous environments, such as the cytoplasm and extracellular fluid. - While it can neutralize **reactive oxygen species** and regenerate other antioxidants like vitamin E, its solubility limits its direct activity in protecting lipid membranes, making vitamin E more potent in that specific context.

Q: In the context of cell membrane composition, what is the typical weight ratio of protein to lipid?

1 : 1. ***1 : 1*** - The **typical weight ratio of protein to lipid** in most cell membranes is approximately **1:1** (equal by weight). - While the **number of lipid molecules** far exceeds the number of protein molecules, proteins are much larger and heavier, resulting in roughly equal weight contributions. - This **1:1 ratio represents an average** for typical plasma membranes, though it can vary significantly depending on membrane type and function. *1 : 2* - This protein:lipid ratio would indicate **lipids contribute twice as much by weight** as proteins. - This is characteristic of **myelin membranes**, which are specialized for insulation and have exceptionally high lipid content. - However, this is **not typical** of most cell membranes. *2 : 1* - This ratio would suggest **proteins contribute twice as much by weight** as lipids. - While some protein-rich membranes exist, this is **higher than the average** for typical cell membranes. - The typical ratio is closer to 1:1 rather than being protein-dominant at 2:1. *4 : 1* - A 4:1 protein:lipid ratio represents an **extremely protein-rich membrane**. - This is characteristic of the **inner mitochondrial membrane**, which is packed with electron transport chain proteins. - This is a **specialized membrane**, not representative of typical cell membranes.

Q: Aldehyde dehydrogenase requires NAD as ?

Coenzyme. ***Coenzyme*** - **NAD** (nicotinamide adenine dinucleotide) acts as a **coenzyme** for aldehyde dehydrogenase, serving as the **most specific and accurate classification** for its role. - As a coenzyme, **NAD** is an **organic, non-protein molecule** that binds reversibly to the enzyme and acts as a **transient carrier of electrons** (hydride ions, H⁻) during aldehyde oxidation. - **NAD⁺** accepts electrons from the aldehyde substrate, becoming reduced to **NADH**, which then dissociates and transfers electrons elsewhere in metabolism. - This is the **preferred answer** because coenzyme precisely describes NAD's organic nature, vitamin origin (niacin/B3), and its role as a mobile electron carrier. *Cofactor* - While technically **NAD is a type of cofactor** (cofactors include coenzymes, prosthetic groups, and metal ions), this term is **too general** for this context. - In biochemistry nomenclature, when both a general and specific term apply, the **more specific term (coenzyme) is preferred** to demonstrate precise understanding. - Choosing "cofactor" would be like calling a "cardiologist" a "doctor" - true but less specific. *Apoenzyme* - An **apoenzyme** is the **protein component** of an enzyme without its cofactor - it refers to the enzyme itself, not to NAD. - In this case, **aldehyde dehydrogenase** (the protein) is the apoenzyme, and **NAD** is the coenzyme that binds to it. - Together they form the active **holoenzyme** (apoenzyme + coenzyme = holoenzyme). *None of the options* - Incorrect because **coenzyme** is the accurate and specific term for NAD's role in aldehyde dehydrogenase function.

Q: Type of inhibition of aconitase by trans-aconitate is?

Competitive. ***Competitive*** - **Competitive inhibition** occurs when the inhibitor (trans-aconitate) structurally resembles the enzyme's natural substrate (cis-aconitate) and binds to the **active site**, preventing the substrate from binding. - This type of inhibition can be overcome by increasing the concentration of the **substrate**. *Non-competitive* - **Non-competitive inhibitors** bind to a site on the enzyme other than the active site, causing a conformational change that reduces the enzyme's efficiency, regardless of substrate concentration. - Trans-aconitate's structural similarity to aconitate's substrate points away from a non-competitive mechanism. *Allosteric* - **Allosteric inhibition** involves an inhibitor binding to a regulatory site (allosteric site) on the enzyme, which is distinct from the active site, to alter enzyme activity. - While allosteric regulation is a type of non-competitive inhibition, trans-aconitate's direct structural resemblance to the substrate makes competitive inhibition the more specific and accurate description. *None of the options* - This option is incorrect because **competitive inhibition** accurately describes the mechanism by which trans-aconitate inhibits aconitase, given its structural similarity to the natural substrate. - The other options are less fitting due to the specific characteristics of trans-aconitate's action.

Q: In ETC NADH generates -

3 ATPs. ***3 ATPs*** - Each molecule of **NADH** donates electrons to **Complex I** of the electron transport chain (ETC), resulting in the pumping of enough protons to generate approximately **3 ATP molecules** via **oxidative phosphorylation**. - This high yield is due to NADH's ability to activate multiple proton pumps along the ETC, maximizing the **proton gradient** for ATP synthesis. *1 ATPs* - This is an incorrect yield for NADH; **FADH2** typically generates fewer ATPs (around 2) because it enters the ETC at a later stage, bypassing the initial proton pump. - Generating only 1 ATP from NADH would be very inefficient and is not physiologically accurate for oxidative phosphorylation. *2 ATPs* - While closer, 2 ATPs is the approximate yield for **FADH2**, which enters the ETC at **Complex II**, bypassing Complex I and thus pumping fewer protons. - NADH enters at Complex I, which provides enough energy for a higher ATP yield. *4 ATPs* - 4 ATPs is an overestimation of the ATP yield from NADH in the electron transport chain. - The maximum theoretical yield from NADH via oxidative phosphorylation is typically considered to be 3 ATPs.

Q: Methionine can enter the TCA cycle at which level?

Succinyl-CoA. ***Succinyl - CoA*** - Methionine is a **glucogenic amino acid** that is catabolized to propionyl-CoA, which is then converted to **methylmalonyl-CoA** and finally to **succinyl-CoA**. - **Succinyl-CoA** is an intermediate of the **TCA cycle**, allowing methionine-derived carbons to enter the cycle. *Fumarate* - Fumarate is an intermediate of the TCA cycle, but methionine catabolism does not directly produce **fumarate**. - Amino acids like **phenylalanine** and **tyrosine** can be catabolized to fumarate. *Oxaloacetate* - **Oxaloacetate** is a TCA cycle intermediate and can be formed from **pyruvate** (via pyruvate carboxylase) or from certain amino acids like **aspartate** and **asparagine**. - Methionine does not directly convert to oxaloacetate. *Citrate* - **Citrate** is the first intermediate formed in the TCA cycle when **acetyl-CoA** combines with **oxaloacetate**. - Methionine catabolism does not lead to the direct formation of citrate.

Q: What are isoenzymes?

Physically distinct forms of the same enzyme. ***Physically distinct forms of the same enzyme*** - Isoenzymes are **multiple forms of an enzyme** that catalyze the **same reaction** but differ in their **physical or biochemical properties**, such as electrophoretic mobility, optimal pH, or kinetic parameters. - These differences usually arise from **genetic variations** (different genes encoding isoforms) or **post-translational modifications** (e.g., phosphorylation, glycosylation). *Physically same forms of different enzymes* - This statement is incorrect as isoenzymes are forms of the **same enzyme**, not different enzymes. - While different enzymes can catalyze similar reactions in certain pathways, they are not referred to as isoenzymes if they are structurally identical. *Forms of same enzyme that catalyze different reactions* - This describes enzymes with **broad substrate specificity** or those that act on different substrates but are not necessarily isoenzymes. - Isoenzymes specifically catalyze the **same chemical reaction**, but they may do so with different efficiencies or under different regulatory controls. *Forms of different enzyme that catalyze same reactions* - This describes a scenario where different enzymes might exhibit **catalytic promiscuity** or broad specificity, but not isoenzymes. - Isoenzymes are always derived from the **same parent enzyme** and catalyze the identical reaction.

Question 1

Which one of the following statements concerning gluconeogenesis is correct?

Accepted Answer

It is important in maintaining blood glucose during the normal overnight fast.

Answer

It occurs primarily in the liver.

Answer

It is stimulated by elevated levels of acetyl CoA.

Answer

It is primarily inhibited by insulin.

Question 2

Which of the following best describes the difference between glucokinase and hexokinase?

Accepted Answer

Glucokinase has higher Km for glucose compared to hexokinase.

Answer

Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase.

Answer

Glucokinase has a low affinity for glucose.

Answer

Glucokinase activity increases with glucose concentration while hexokinase remains saturated.

Question 3

What metabolic changes occur during overnight fasting?

Accepted Answer

Glucose production increases

Answer

Blood glucose decreases slightly

Answer

Fat breakdown increases

Answer

Ketone levels rise slightly

Question 4

Which vitamin is considered the most potent antioxidant?

Accepted Answer

Vit E

Answer

Vit A

Answer

Vit K

Answer

Vit C

Question 5

In the context of cell membrane composition, what is the typical weight ratio of protein to lipid?

Accepted Answer

1 : 1

Answer

1 : 2

Answer

4 : 1

Answer

2 : 1

Question 6

Aldehyde dehydrogenase requires NAD as ?

Accepted Answer

Coenzyme

Answer

None of the options

Answer

Apoenzyme

Answer

Cofactor

Question 7

Type of inhibition of aconitase by trans-aconitate is?

Accepted Answer

Competitive

Answer

Non-competitive

Answer

Allosteric

Answer

None of the options

Question 8

In ETC NADH generates -

Accepted Answer

3 ATPs

Answer

1 ATPs

Answer

4 ATPs

Answer

2 ATPs

Question 9

Methionine can enter the TCA cycle at which level?

Accepted Answer

Succinyl-CoA

Answer

Fumarate

Answer

Oxaloacetate

Answer

Citrate

Question 10

What are isoenzymes?

Accepted Answer

Physically distinct forms of the same enzyme

Answer

Physically same forms of different enzymes

Answer

Forms of same enzyme that catalyze different reactions

Answer

Forms of different enzyme that catalyze same reactions

NEET-PG 2012 — Biochemistry