Enzymes that move a molecular group from one molecule to another are known as -
In the electron transport chain (ETC), which enzyme does cyanide inhibit?
Km value is defined as:
What is the classification of the Y chromosome?
Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
How do enzymes function in biochemical reactions?
Enzyme causing covalent bond cleavage without hydrolysis ?
The anticodon region is an important part of which type of RNA?
At which positions does pancreatic lipase hydrolyze the ester linkages of triacylglycerides?
Apoenzyme is ?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 11: Enzymes that move a molecular group from one molecule to another are known as -
- A. Transferases (Correct Answer)
- B. Ligases
- C. Dipeptidases
- D. Oxido-reductases
Explanation: ***Transferases*** - **Transferases** are a class of enzymes that catalyze the transfer of a specific functional group (e.g., methyl, acetyl, phosphate) from one molecule (the donor) to another (the acceptor). - This broad category includes enzymes vital for many metabolic pathways, such as **kinases** (transferring phosphate groups) and **transaminases** (transferring amino groups). *Ligases* - **Ligases** are enzymes responsible for joining two large molecules together, typically by forming a new chemical bond. - This process usually involves the concomitant hydrolysis of a small, energy-rich molecule such as **ATP**, to provide the necessary energy for bond formation. *Dipeptidases* - **Dipeptidases** are a type of hydrolase enzyme that specifically cleaves the peptide bond within a **dipeptide**, releasing two free amino acids. - They are crucial for the final stages of protein digestion, breaking down small peptides into absorbable **amino acid units**. *Oxido-reductases* - **Oxido-reductases** are enzymes that catalyze **oxidation-reduction reactions** (redox reactions), where electrons are transferred from one molecule to another. - This class includes enzymes like **dehydrogenases** and **oxidases**, which play critical roles in cellular respiration and energy production.
Question 12: In the electron transport chain (ETC), which enzyme does cyanide inhibit?
- A. Complex II (Succinate dehydrogenase)
- B. Cytochrome c oxidase (Complex IV) (Correct Answer)
- C. Complex I (NADH dehydrogenase)
- D. Complex III (Cytochrome bc1 complex)
Explanation: ***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.
Question 13: Km value is defined as:
- A. Substrate concentration at Vmax/2 (Correct Answer)
- B. Substrate concentration at which reaction rate is maximum
- C. Substrate concentration at Vmax
- D. Substrate concentration at which enzyme activity is optimal
Explanation: ***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.
Question 14: What is the classification of the Y chromosome?
- A. Metacentric
- B. Submetacentric (Correct Answer)
- C. Acrocentric
- D. None of the options
Explanation: ***Submetacentric*** - The **Y chromosome** is classified as submetacentric because its **centromere** is located off-center, resulting in two arms of unequal length. - The short arm (Yp) is smaller than the long arm (Yq), but not as disproportionate as in acrocentric chromosomes. - The **X chromosome** is also submetacentric, making both sex chromosomes belong to this category. *Metacentric* - A **metacentric chromosome** has its **centromere** located in the middle, resulting in two arms of approximately equal length. - Examples include chromosomes 1, 3, 16, 19, and 20, which have nearly equal arm ratios unlike the Y chromosome. *Acrocentric* - An **acrocentric chromosome** has its **centromere** located very close to one end, creating one very short arm and one very long arm. - The five acrocentric human chromosomes are **13, 14, 15, 21, and 22**, which possess satellite DNA and nucleolar organizing regions (NORs) on their short arms. - The **Y chromosome is NOT acrocentric** despite historical confusion; it has a more centrally positioned centromere than true acrocentric chromosomes. *None of the options* - This option is incorrect because the Y chromosome has a specific and well-established classification as **submetacentric** based on its centromere position and arm ratio.
Question 15: Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
- A. G6PD (Correct Answer)
- B. APDH
- C. α-keto glutarate dehydrogenases
- D. None of the options
Explanation: ***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.
Question 16: How do enzymes function in biochemical reactions?
- A. Increase in activation energy
- B. Decrease in activation energy (Correct Answer)
- C. Shift equilibrium constant
- D. Provide energy to the reaction
Explanation: ***Decrease in activation energy*** - Enzymes act as **biological catalysts** by providing an alternative reaction pathway with a lower **transition state energy**. - This reduction in the **activation energy** allows a higher proportion of reactant molecules to overcome the energy barrier and react, thereby increasing the reaction rate. *Increase in activation energy* - This statement is incorrect as increasing activation energy would slow down the reaction rate, which is contrary to the function of enzymes. - Enzymes are designed to accelerate reactions, not inhibit them, by making them energetically more favorable to proceed. *Shift equilibrium constant* - Enzymes catalyze both the forward and reverse reactions equally, meaning they accelerate the rate at which equilibrium is reached but **do not alter the equilibrium constant (Keq)** of a reaction. - The equilibrium constant is determined by the difference in free energy between reactants and products, which enzymes do not change. *Provide energy to the reaction* - This statement is incorrect because enzymes do **not provide energy** to reactions; they only lower the activation energy barrier. - Enzymes facilitate reactions by stabilizing the transition state, not by adding energy to the system, which would violate thermodynamic principles.
Question 17: Enzyme causing covalent bond cleavage without hydrolysis ?
- A. Lyase (Correct Answer)
- B. Ligase
- C. Hydrolase
- D. Transferase
Explanation: ***Lyase*** - **Lyases** are enzymes that catalyze the cleavage of **covalent bonds** (C-C, C-O, C-N, and others) by means other than hydrolysis or oxidation, often creating a new double bond or a ring structure. - They remove groups from substrates to form double bonds, or conversely, add groups to double bonds. - **Examples:** Aldolase (cleaves C-C bonds in glycolysis), carbonic anhydrase (reversible cleavage of C-O bond), fumarase (C-C bond cleavage in TCA cycle). *Ligase* - **Ligases** are enzymes that join two large molecules by forming a new chemical bond, usually accompanied by the **hydrolysis of ATP**. - They are involved in synthesis reactions, not the cleavage of bonds. *Hydrolase* - **Hydrolases** specifically catalyze the hydrolysis of a chemical bond, involving the **addition of water** across the bond. - They break down large molecules into smaller ones using water - this is the key difference from lyases. *Transferase* - **Transferases** catalyze the transfer of a **functional group** from one molecule (the donor) to another (the acceptor). - They do not cause covalent bond cleavage without hydrolysis but rather move existing groups between molecules.
Question 18: The anticodon region is an important part of which type of RNA?
- A. r-RNA
- B. m-RNA
- C. t-RNA (Correct Answer)
- D. hn-RNA
Explanation: **t-RNA** - The **anticodon region** is a critical component of **transfer RNA (tRNA)**, responsible for recognizing and binding to the complementary codon on mRNA during protein synthesis. - This interaction ensures that the correct **amino acid** is delivered to the growing polypeptide chain according to the genetic code. *r-RNA* - **Ribosomal RNA (rRNA)** is a structural and enzymatic component of **ribosomes**, which are the cellular machinery for protein synthesis. - While rRNA plays a crucial role in forming **peptide bonds** and facilitating translation, it does not possess an anticodon region. *m-RNA* - **Messenger RNA (mRNA)** carries the **genetic code** from DNA to the ribosomes in the form of codons, which specify the sequence of amino acids for protein synthesis. - mRNA molecules have codons, but they do not have an **anticodon region**; instead, they are read by the anticodons of tRNA. *hn-RNA* - **Heterogeneous nuclear RNA (hnRNA)** is a precursor to mRNA in eukaryotic cells, containing both exons and introns. - It undergoes extensive processing, including **splicing**, to become mature mRNA, but it does not have an **anticodon region**.
Question 19: At which positions does pancreatic lipase hydrolyze the ester linkages of triacylglycerides?
- A. 1 and 2
- B. 2 and 3
- C. Only 3
- D. 1 and 3 (Correct Answer)
Explanation: **Correct: 1 and 3** - Pancreatic lipase specifically targets the **ester bonds at the sn-1 and sn-3 positions** (primary alcohol positions) on the glycerol backbone of triacylglycerides. - This positional specificity results in the formation of **2-monoacylglycerol (2-MAG)** and **two free fatty acids**. - This is the characteristic action of pancreatic triacylglycerol lipase during fat digestion in the intestinal lumen. *Incorrect: 1 and 2* - Hydrolysis at positions 1 and 2 would produce a 3-monoacylglycerol and free fatty acids, which is not the physiological product of pancreatic lipase. - The enzyme's positional specificity favors the outer sn-1 and sn-3 positions, not the middle sn-2 position. *Incorrect: 2 and 3* - Hydrolysis at positions 2 and 3 would yield a 1-monoacylglycerol and free fatty acids, which does not reflect pancreatic lipase activity. - The enzyme specifically spares the sn-2 position due to its structural specificity. *Incorrect: Only 3* - If only position 3 were hydrolyzed, the product would be a 1,2-diacylglycerol and one free fatty acid. - This represents incomplete hydrolysis; pancreatic lipase typically hydrolyzes **both outer positions (sn-1 and sn-3)** due to its regiospecificity.
Question 20: Apoenzyme is ?
- A. Protein moiety (Correct Answer)
- B. Organic cofactor
- C. Inactive enzyme component
- D. Non-protein component required for enzyme activity
Explanation: ***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.