NEET-PG 2012 - Biochemistry Practice Questions

Q: Km value is defined as:

Substrate concentration at Vmax/2. ***Substrate concentration at Vmax/2*** - The **Michaelis constant (Km)** is defined as the **substrate concentration** at which the reaction velocity is **half of the maximum velocity (Vmax/2)**. - It reflects the **affinity of an enzyme for its substrate**; a lower Km indicates higher affinity. *Substrate concentration at which reaction rate is maximum* - The **maximum reaction rate (Vmax)** is achieved when the enzyme is **saturated with substrate**, meaning all active sites are occupied. - Km specifically refers to the substrate concentration needed to reach **half of this maximum rate**, not the maximum rate itself. *Substrate concentration at Vmax* - At **Vmax**, the enzyme is fully saturated with substrate, and the reaction rate cannot increase further by adding more substrate. - The **Km value** is a measure related to the **efficiency of substrate binding** at conditions below saturation, specifically at half Vmax. *Substrate concentration at which enzyme activity is optimal* - **Optimal enzyme activity** is generally influenced by factors such as **pH and temperature**, which affect the enzyme's structure and catalytic efficiency. - Km is specifically related to the **substrate concentration** required to achieve a specific reaction rate, not the overall optimal environmental conditions for the enzyme.

Q: In the electron transport chain (ETC), which enzyme does cyanide inhibit?

Cytochrome c oxidase (Complex IV). ***Cytochrome c oxidase (Complex IV)*** - Cyanide binds to the **ferric iron (Fe3+)** in the heme a3 component of cytochrome c oxidase, blocking the final transfer of electrons to oxygen. - This inhibition effectively halts the entire **electron transport chain** and **oxidative phosphorylation**, leading to rapid cellular energy depletion. *Complex I (NADH dehydrogenase)* - While other toxins can inhibit Complex I (e.g., rotenone, amytal), **cyanide specifically targets Complex IV**. - Inhibition here prevents the entry of electrons from **NADH** into the ETC, but it's not cyanide's primary site of action. *Complex III (Cytochrome bc1 complex)* - Complex III is involved in transferring electrons from **ubiquinol** to cytochrome c, but it is not directly inhibited by cyanide. - Antimycin A is a well-known inhibitor of Complex III. *Complex II (Succinate dehydrogenase)* - Complex II directly receives electrons from **succinate** in the citric acid cycle and passes them to ubiquinone, bypassing Complex I. - Cyanide does not inhibit Complex II; inhibitors of this complex include malonate.

Q: Enzymes of glycolysis are found in:

Cytosol. ***Cytosol*** - Glycolysis is a metabolic pathway that occurs in the **cytosol** of cells. - All the enzymes required for the conversion of glucose to pyruvate are freely dissolved in the **cytoplasm**. *Cell membrane* - The cell membrane is primarily involved in **regulating the passage of substances** into and out of the cell, as well as cell signaling. - Glycolytic enzymes are not associated with the cell membrane. *Mitochondria* - Mitochondria are the primary site of **oxidative phosphorylation** and the **citric acid cycle**, not glycolysis. - While pyruvate (the end product of glycolysis) moves into the mitochondria for further metabolism, the initial glycolytic steps do not occur there. *Ribosomes* - Ribosomes are responsible for **protein synthesis** (translation). - They do not contain enzymes for metabolic pathways like glycolysis.

Q: Which glycogen storage disease also presents as a lysosomal storage disease?

Pompe's disease. ***Pompe's disease*** - Also known as **glycogen storage disease type II**, it is caused by a deficiency of **acid alpha-glucosidase (GAA)**, a *lysosomal enzyme*. - This deficiency leads to the accumulation of **glycogen in lysosomes**, particularly affecting muscle tissue, thereby earning its classification as both a glycogen storage disease and a lysosomal storage disease. *Von Gierke's disease* - This is **glycogen storage disease type I** and is due to a deficiency in **glucose-6-phosphatase**. - It primarily affects the **liver and kidneys**, causing severe **hypoglycemia** and **lactic acidosis**, but it is not classified as a lysosomal storage disease. *McArdle's disease* - This is **glycogen storage disease type V**, caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. - It manifests as **exercise intolerance** and muscle pain, but it does not involve lysosomal enzyme defects or glycogen accumulation in lysosomes. *Andersen's disease* - This is **glycogen storage disease type IV**, caused by a deficiency in the **glycogen branching enzyme**. - It leads to the formation of **abnormal glycogen structures**, primarily affecting the liver and causing early liver failure, but it is not a lysosomal storage disorder.

Q: Glucose oxidase converts glucose to?

Gluconic acid. ***Gluconic acid*** - **Glucose oxidase** specifically catalyzes the oxidation of glucose, producing **gluconic acid** and hydrogen peroxide. - This reaction forms the basis for many common **glucose diagnostic tests**, such as those used in blood glucose monitors. *Glucuronic acid* - **Glucuronic acid** is formed from the oxidation of glucose at carbon 6, typically through the **uronic acid pathway**. - It is known for its role in **detoxification** and conjugation reactions in the liver, not as a direct product of glucose oxidase. *Galactonic acid* - **Galactonic acid** is an oxidized form of galactose, a different monosaccharide from glucose. - Its formation is not associated with the action of **glucose oxidase**, an enzyme specific to glucose. *Iduronic acid* - **Iduronic acid** is a C5 epimer of glucuronic acid and is a common component of various **glycosaminoglycans** like dermatan sulfate and heparan sulfate. - It is not produced by the action of **glucose oxidase** on glucose.

Q: What are digestive enzymes classified as?

Hydrolases. ***Hydrolases*** - Digestive enzymes like **amylase**, **lipase**, and **proteases** break down complex food molecules by adding water, a process known as **hydrolysis**. - This class of enzymes catalyzes the cleavage of a chemical bond with the concurrent addition of a water molecule. - All major digestive enzymes belong to this class according to the **EC enzyme classification system**. *Oxidoreductases* - These enzymes catalyze **redox reactions**, involving the transfer of electrons from one molecule to another. - Examples include **dehydrogenases** and **oxidases**, which are not primarily involved in breaking down food molecules in digestion. *Transferases* - Transferases catalyze the transfer of functional groups (such as methyl, acyl, or phosphate groups) from one molecule to another. - Examples include **kinases** and **transaminases**, which are involved in metabolic pathways but not in the digestive breakdown of food. *Ligases* - Ligases are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, typically with the concomitant hydrolysis of ATP. - They are involved in **DNA repair** and **biosynthetic reactions**, not in the breakdown of food during digestion.

Q: Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?

G6PD. ***G6PD*** - **Glucose-6-phosphate dehydrogenase (G6PD)** catalyzes the first committed step in the pentose phosphate pathway, converting **glucose-6-phosphate** to **6-phosphogluconolactone**. - This reaction involves the reduction of **NADP+ to NADPH**, making G6PD the primary enzyme for NADPH production in this pathway. *APDH* - **APDH (adenosine phosphosulfate reductase)** is involved in sulfur metabolism and has no direct role in the pentose phosphate pathway or NADPH production. - This enzyme primarily functions in prokaryotes for the **reduction of APS (adenosine 5'-phosphosulfate)**. *α-keto glutarate dehydrogenases* - **Alpha-ketoglutarate dehydrogenase** is a mitochondrial enzyme part of the **Krebs cycle**, converting **alpha-ketoglutarate to succinyl-CoA**. - This enzyme is crucial for ATP production and generates **NADH**, not NADPH, in its reaction. *None of the options* - This option is incorrect because **G6PD** is indeed the primary enzyme responsible for NADPH generation in the pentose phosphate pathway.

Q: Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?

LDH-5. ***LDH-5 isoenzyme most significant in hepatic conditions*** - **LDH-5** is the predominant isoenzyme found in the **liver** and skeletal muscle. - An elevation of **LDH-5** is highly indicative of **hepatocellular damage** or injury. *LDH-1 isoenzyme associated with cardiac tissue* - **LDH-1** is primarily present in the **heart** and red blood cells. - Its elevation suggests conditions like **myocardial infarction** or hemolytic anemia, not liver injury. *LDH-3 isoenzyme typical in respiratory system* - **LDH-3** is found in the **lungs**, kidneys, and other tissues. - While it can be elevated in **pulmonary embolism** or renal disease, it is not specific for liver injury. *LDH-2 isoenzyme linked to erythrocyte metabolism* - **LDH-2** is abundant in **red blood cells** and also found in the heart and kidneys. - Elevations are often seen in conditions involving **hemolysis** or myocardial damage, similar to LDH-1.

Q: What is the specific activity of an enzyme?

Enzyme units per mg of protein. ***Enzyme units per mg of protein*** - **Specific activity** is defined as the number of **enzyme units** (representing catalytic activity) per milligram of total protein in the sample. - It is a measure of **purity**, indicating the amount of active enzyme relative to other proteins in a preparation. - Formula: Specific activity = Units of enzyme activity / mg of total protein - Used to track enzyme purification progress during isolation procedures. *Concentration of substrate transformed per minute* - This describes the **reaction velocity** or rate of catalysis, but not the specific activity of the enzyme. - While related to enzyme activity, it does not normalize the activity to the amount of **total protein**. - This would be expressed as reaction rate or velocity (V), not specific activity. *Enzyme units per mg of substrate* - This is an incorrect formulation that confuses substrate with protein. - **Specific activity** is normalized to the amount of **protein** in the enzyme preparation, not the substrate. - This option represents a common misconception in enzyme kinetics terminology. *Limit of enzyme per gram of substrate* - This phrase does not correspond to any standard biochemical measure of enzyme activity or concentration. - It does not provide information about the **catalytic efficiency** or **purity** of the enzyme preparation. - The term "limit" is not used in the context of specific activity measurements.

Q: Apoenzyme is ?

Protein moiety. ***Protein moiety*** - An **apoenzyme** is the **protein component of an enzyme** that is catalytically inactive by itself. - It requires a **non-protein cofactor** (either an inorganic ion or an organic molecule) to become active. *Organic cofactor* - An **organic cofactor** is also known as a **coenzyme**, which binds to the apoenzyme to form a functional holoenzyme. - While essential for enzyme activity, the apoenzyme itself is the protein part, not the organic cofactor. *Inactive enzyme component* - While an apoenzyme is **inactive on its own**, this description is too broad and doesn't specify its chemical nature. - It is specifically the **protein component** that is inactive until bound to its cofactor. *Non-protein component required for enzyme activity* - This describes a **cofactor** (either inorganic or organic), not the apoenzyme itself. - The apoenzyme is the **protein portion**, which *requires* the non-protein component for activity.

Question 1

Km value is defined as:

Accepted Answer

Substrate concentration at Vmax/2

Answer

Substrate concentration at which reaction rate is maximum

Answer

Substrate concentration at Vmax

Answer

Substrate concentration at which enzyme activity is optimal

Question 2

In the electron transport chain (ETC), which enzyme does cyanide inhibit?

Accepted Answer

Cytochrome c oxidase (Complex IV)

Answer

Complex II (Succinate dehydrogenase)

Answer

Complex I (NADH dehydrogenase)

Answer

Complex III (Cytochrome bc1 complex)

Question 3

Enzymes of glycolysis are found in:

Accepted Answer

Cytosol

Answer

Cell membrane

Answer

Mitochondria

Answer

Ribosomes

Question 4

Which glycogen storage disease also presents as a lysosomal storage disease?

Accepted Answer

Pompe's disease

Answer

Von Gierke's disease

Answer

McArdle's disease

Answer

Andersen's disease

Question 5

Glucose oxidase converts glucose to?

Accepted Answer

Gluconic acid

Answer

Glucuronic acid

Answer

Galactonic acid

Answer

Iduronic acid

Question 6

What are digestive enzymes classified as?

Accepted Answer

Hydrolases

Answer

Oxidoreductases

Answer

Transferases

Answer

Ligases

Question 7

Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?

Accepted Answer

G6PD

Answer

APDH

Answer

α-keto glutarate dehydrogenases

Answer

None of the options

Question 8

Which isoenzyme of lactate dehydrogenase (LDH) is predominantly elevated in liver injury?

Accepted Answer

LDH-5

Answer

LDH-3

Answer

LDH-1

Answer

LDH-2

Question 9

What is the specific activity of an enzyme?

Accepted Answer

Enzyme units per mg of protein

Answer

Concentration of substrate transformed per minute

Answer

Enzyme units per mg of substrate

Answer

Limit of enzyme per gram of substrate

Question 10

Apoenzyme is ?

Accepted Answer

Protein moiety

Answer

Organic cofactor

Answer

Inactive enzyme component

Answer

Non-protein component required for enzyme activity

NEET-PG 2012 — Biochemistry