Which enzyme polymerises Okazaki fragments?
Which element is required by phosphofructokinase?
Carboxypeptidase contains which mineral?
Which kinetic parameter is primarily associated with enzyme specificity?
Which of the following is NOT a characteristic of the genetic code?
What is the typical Q10 value for enzymatic reactions?
What type of enzyme is hexokinase?
According to IUB system, hydrolases belong to which class?
Which of the following enzymes is classified as a serine protease?
Which is the primary energy molecule that gives approximately 7.3 kcal/mol?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 31: Which enzyme polymerises Okazaki fragments?
- A. DNA polymerase I
- B. DNA polymerase II
- C. DNA polymerase III (Correct Answer)
- D. RNA polymerase
Explanation: ***DNA polymerase III*** - **DNA polymerase III** is the primary replicative enzyme in **prokaryotes (bacteria)** responsible for synthesizing new DNA strands, including the **polymerization of Okazaki fragments** on the lagging strand. - It possesses high processivity (can add ~500 nucleotides without dissociating), essential for rapid and efficient DNA synthesis during replication, adding nucleotides in a **5' to 3' direction**. - In **eukaryotes**, DNA polymerase δ (delta) performs the analogous function of polymerizing Okazaki fragments. *DNA polymerase I* - **DNA polymerase I** in prokaryotes primarily functions in **removing RNA primers** left by primase and **filling the resulting gaps** with DNA nucleotides. - It has 5' to 3' exonuclease activity for primer removal and polymerase activity for gap filling, but is **not the main enzyme for elongating Okazaki fragments**. - Its role is in **DNA repair and finishing replication**, not the extensive synthesis of Okazaki fragments. *DNA polymerase II* - **DNA polymerase II** in prokaryotes is primarily involved in **DNA repair mechanisms**, particularly in **restarting stalled replication forks** and responding to DNA damage. - It is not the main enzyme responsible for the polymerization of **Okazaki fragments** during normal DNA replication. *RNA polymerase* - **RNA polymerase** (specifically **primase**, a specialized RNA polymerase) synthesizes short **RNA primers** (8-12 nucleotides) during DNA replication, which provide the 3'-OH group necessary to initiate DNA synthesis. - It does not synthesize DNA or polymerize **Okazaki fragments**; its function is to create RNA primers, not extend DNA strands.
Question 32: Which element is required by phosphofructokinase?
- A. Magnesium (Correct Answer)
- B. Inorganic phosphate
- C. Manganese
- D. Copper
Explanation: **Magnesium** - **Phosphofructokinase** (PFK) is an enzyme in **glycolysis** that catalyzes the phosphorylation of fructose-6-phosphate. - This reaction requires **ATP**, and like many enzymes that utilize ATP, PFK requires **magnesium ions (Mg²⁺)** as a cofactor, typically forming a complex with ATP (MgATP²⁻). *Inorganic phosphate* - **Inorganic phosphate** is a substrate for some kinase reactions, but not a direct cofactor requirement for the *activation* of phosphofructokinase itself. - While phosphate is incorporated into molecules during phosphorylation, it does not act as a metal ion cofactor to facilitate the enzyme's activity. *Manganese* - While **manganese (Mn²⁺)** can sometimes substitute for magnesium in certain enzyme reactions, it is not the primary or required cofactor for phosphofructokinase under normal physiological conditions. - Many enzymes have a preference for specific metal ions based on their active site structure and coordination chemistry. *Copper* - **Copper (Cu²⁺)** is a cofactor for a variety of enzymes, particularly those involved in **redox reactions** (e.g., cytochrome c oxidase, superoxide dismutase). - However, copper is not a required metallic cofactor for the activity of **phosphofructokinase** in glycolysis.
Question 33: Carboxypeptidase contains which mineral?
- A. Copper
- B. Zinc (Correct Answer)
- C. Iron
- D. None of the options
Explanation: ***Zinc*** - **Carboxypeptidase** is a **metalloenzyme**, meaning it requires a metal ion for its catalytic activity. - **Zinc** acts as a crucial cofactor in the active site of carboxypeptidase, enabling its proteolytic function. *Copper* - **Copper** is a component of enzymes like **cytochrome c oxidase** and **superoxide dismutase**, but not carboxypeptidase. - Its presence is essential for processes like **electron transport** and **antioxidant defense**. *Iron* - **Iron** is a central component of **hemoglobin** and **myoglobin** for oxygen transport, and in enzymes like **catalase** and **peroxidase**. - It is not involved in the catalytic mechanism of carboxypeptidase. *None of the options* - This option is incorrect because **Zinc** is a known and essential mineral for the function of carboxypeptidase. - Carboxypeptidase is a metalloenzyme, and a metal cofactor is required for its activity.
Question 34: Which kinetic parameter is primarily associated with enzyme specificity?
- A. Both
- B. Km
- C. Vmax
- D. None of the options (Correct Answer)
Explanation: ***None of the options*** - **Enzyme specificity** is primarily determined by the unique three-dimensional **active site structure** of the enzyme, which allows it to bind only to specific substrates through complementary shape and chemical interactions. - This structural complementarity involves steric fit and specific non-covalent interactions (hydrogen bonds, van der Waals forces, electrostatic interactions) between the enzyme and its substrate. - **Neither Km nor Vmax are determinants of enzyme specificity**—they are kinetic parameters that describe enzyme behavior, not structural selectivity. *Km (Michaelis constant)* - Represents the substrate concentration at which the reaction rate is half of Vmax. - Indicates the **affinity** of an enzyme for its substrate (lower Km = higher affinity). - While enzymes may show different Km values for different substrates, **Km reflects binding affinity, not the structural basis of specificity**. *Vmax (Maximum velocity)* - The maximum rate of reaction when the enzyme is saturated with substrate. - Reflects **catalytic efficiency** and the amount of active enzyme present. - Does not relate to the enzyme's ability to discriminate between different substrate molecules. *Both* - Incorrect because neither Km nor Vmax determines which substrates an enzyme can recognize and bind. - Enzyme specificity is a **structural property** of the active site, while Km and Vmax are **kinetic properties** that describe reaction rates.
Question 35: Which of the following is NOT a characteristic of the genetic code?
- A. Overlapping (Correct Answer)
- B. Universal
- C. Degeneracy
- D. Nonambiguous
Explanation: ***Overlapping*** - The genetic code is generally **non-overlapping**, meaning each nucleotide is part of only one codon, and codons are read sequentially. - An overlapping code would mean that a single nucleotide could be part of multiple codons, which is not how protein synthesis typically occurs. *Nonambiguous* - This statement IS a characteristic; each codon specifies **only one amino acid**, meaning there is no ambiguity about which amino acid will be added. - While multiple codons can specify the same amino acid, a single codon never specifies more than one different amino acid. *Universal* - This statement IS a characteristic; the genetic code is largely **universal** across almost all organisms, from bacteria to humans. - The same codons typically specify the same amino acids in different species, which supports the idea of common ancestry. *Degeneracy* - This statement IS a characteristic; the genetic code is **degenerate**, meaning that most amino acids are specified by more than one codon. - This redundancy helps protect against the effects of single-nucleotide mutations.
Question 36: What is the typical Q10 value for enzymatic reactions?
- A. 2 (Correct Answer)
- B. 3
- C. 4
- D. 5
Explanation: ***2*** - The **Q10 value** represents the factor by which the rate of a reaction increases for every 10°C rise in temperature. - For most enzymatic and biological reactions, the **Q10 value** is typically around **2 to 3**. *3* - While **3** is within the typical range for some biological reactions, **2** is often considered the most common or average value cited for enzymatic reactions. - A **Q10 of 3** means the reaction rate triples with a 10°C increase, which is observed in certain cases but is not the most general "typical" value. *4* - A **Q10 value of 4** indicates a significantly higher temperature sensitivity than what is commonly observed for most enzymatic reactions. - Such a high Q10 would imply that the reaction rate quadruples for every 10°C increase, which is less typical. *5* - A **Q10 value of 5** is exceptionally high and rarely observed for common enzymatic reactions under physiological conditions. - This would suggest an extreme sensitivity to temperature changes, which is not characteristic of most enzyme kinetics.
Question 37: What type of enzyme is hexokinase?
- A. Ligase
- B. Transferase (Correct Answer)
- C. Oxidoreductase
- D. Reductase
Explanation: ***Transferase*** - Hexokinase catalyzes the transfer of a **phosphate group** from **ATP** to glucose, forming glucose-6-phosphate. - Enzymes that catalyze the transfer of functional groups from one molecule to another are classified as **transferases**. *Ligase* - **Ligases** are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, usually accompanied by the hydrolysis of a small pendant chemical group on one of the larger molecules or the less-stable of the two products. - This activity usually involves reactions like **DNA ligation**, not phosphate group transfer to a sugar. *Oxidoreductase* - **Oxidoreductases** catalyze **oxidation-reduction reactions**, involving the transfer of electrons from one molecule to another. - Hexokinase does not perform redox reactions; it transfers a phosphate group. *Reductase* - **Reductases** are a specific type of **oxidoreductase** that catalyze reactions where a molecule is reduced (gains electrons). - This is a subset of oxidation-reduction chemistry and is not the function of hexokinase.
Question 38: According to IUB system, hydrolases belong to which class?
- A. EC-1
- B. EC-2
- C. EC-3 (Correct Answer)
- D. EC-4
Explanation: ***EC-3*** - **Hydrolases** catalyze the **hydrolysis** of chemical bonds, which involves the addition of water to break the bond. - This class includes enzymes like **esterases**, **peptidases**, and **glycosidases**, all of which use water to cleave molecules. *EC-1* - **EC-1** refers to **oxidoreductases**, which catalyze **oxidation-reduction reactions**. - These enzymes are involved in the transfer of electrons or hydrogen atoms, not the hydrolysis of bonds. *EC-2* - **EC-2** represents **transferases**, enzymes that catalyze the **transfer of a functional group** from one molecule to another. - Examples include **kinases** and **transaminases**, which are distinct from hydrolytic enzymes. *EC-4* - **EC-4** encompasses **lyases**, which catalyze the **cleavage of various bonds** by means other than hydrolysis or oxidation, often forming double bonds. - This class includes enzymes like **decarboxylases** and **aldolases**, which are not primarily involved in breaking bonds with water.
Question 39: Which of the following enzymes is classified as a serine protease?
- A. Pepsin
- B. Trypsin (Correct Answer)
- C. Carboxypeptidase
- D. None of the options
Explanation: ***Trypsin*** - **Trypsin** is a digestive enzyme belonging to the **serine protease** family, characterized by a crucial **serine residue** in its active site. - It plays a vital role in protein digestion in the small intestine, cleaving peptide bonds on the carboxyl side of **lysine** or **arginine** residues. *Pepsin* - **Pepsin** is an aspartic protease, meaning it utilizes an **aspartate residue** in its active site for catalysis. - It primarily functions in the stomach, digesting proteins into smaller peptides in an **acidic environment**. *Carboxypeptidase* - **Carboxypeptidase** is a **metalloexopeptidase** that contains a zinc ion in its active site. - It removes amino acids one by one from the **carboxyl-terminal** end of polypeptide chains. *None of the options* - This option is incorrect because **trypsin** is indeed a well-known example of a serine protease.
Question 40: Which is the primary energy molecule that gives approximately 7.3 kcal/mol?
- A. ATP (Correct Answer)
- B. GTP
- C. Glucose-6-phosphate
- D. Creatine phosphate
Explanation: ***ATP*** - **Adenosine triphosphate (ATP)** is the primary energy currency of the cell, providing approximately **7.3 kcal/mol** upon hydrolysis of its terminal phosphate group. - This energy is released when ATP is converted to **ADP (adenosine diphosphate)** and an inorganic phosphate (Pi), driving various cellular processes. *GTP* - **Guanosine triphosphate (GTP)** is another nucleotide triphosphate that carries energy, but it is primarily involved in specific processes like **protein synthesis** and **signal transduction**, not as the ubiquitous primary energy molecule like ATP. - While it also releases energy upon hydrolysis, its standard free energy change is similar to ATP but it's not the main universal energy carrier. *Glucose-6-phosphate* - **Glucose-6-phosphate** is an important intermediate in **glycolysis** and **gluconeogenesis**, but it is not an energy-storing molecule in the same way as ATP. - Its high-energy phosphate bond is used in metabolic pathways, but it doesn't directly release 7.3 kcal/mol as a direct energy source for cellular work. *Creatine phosphate* - **Creatine phosphate** serves as an energy reserve in muscle and nerve cells, rapidly generating ATP from ADP during periods of intense activity. - While it is a high-energy phosphate compound, it functions to **replenish ATP** rather than being the direct energy molecule that performs cellular work.