NEET-PG 2013 - Biochemistry Practice Questions

Q: Serotonin is derived from -

Tryptophan. ***Tryptophan*** - **Serotonin**, also known as 5-hydroxytryptamine (5-HT), is synthesized from the essential amino acid **tryptophan** through a two-step enzymatic pathway. - Tryptophan is first hydroxylated by tryptophan hydroxylase to 5-hydroxytryptophan (5-HTP), which is then decarboxylated by L-amino acid decarboxylase to form serotonin. *Tyrosine* - **Tyrosine** is a precursor for the synthesis of **catecholamines** (dopamine, norepinephrine, and epinephrine) and thyroid hormones. - It is not involved in the synthesis pathway for serotonin. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is hydroxylated to form **tyrosine**. - Therefore, it is indirectly involved in catecholamine synthesis but not in serotonin synthesis. *Methionine* - **Methionine** is an essential amino acid primarily known for its role in protein synthesis and as a precursor for **S-adenosylmethionine (SAM)**, a methyl group donor in many biological reactions. - It does not serve as a direct precursor for serotonin.

Q: What is the approximate half-life of albumin in the human body?

20 days. ***20 days*** - The **half-life of albumin** in the human body is approximately **20 days**, reflecting the time it takes for half of the circulating albumin to be catabolized or excreted. - This relatively long half-life means that changes in albumin levels, such as those due to malnutrition or liver disease, may take several weeks to become evident. *3 days* - A half-life of 3 days is too short for albumin, which is a major, long-lasting plasma protein. - Proteins with such a short half-life typically include more rapidly turnover proteins or small peptides. *7 days* - A half-life of 7 days is also too short for albumin, which plays a critical role in maintaining plasma oncotic pressure and transporting various substances. - While some proteins have a 7-day half-life, albumin's is considerably longer. *30 days* - A half-life of 30 days is longer than the typical half-life of albumin. - While some proteins may have half-lives in this range, 20 days is the more commonly accepted value for albumin.

Q: Gluconeogenesis occurs in all except:

Muscle. ***Muscle*** - **Muscle tissue** lacks the enzyme **glucose-6-phosphatase**, which is essential for releasing free glucose into the bloodstream during gluconeogenesis. - While muscle can store glycogen, it primarily uses glucose for its own energy needs and does not contribute significantly to systemic glucose homeostasis through gluconeogenesis. *Liver* - The **liver** is the primary site of **gluconeogenesis**, producing glucose to maintain blood glucose levels during fasting and starvation. - It contains all the necessary enzymes, including **glucose-6-phosphatase**, to convert precursors like lactate, amino acids, and glycerol into glucose. *Kidney* - The **kidney** becomes a significant site of **gluconeogenesis** during prolonged fasting, contributing up to 10-20% of the body's glucose production. - Renal gluconeogenesis primarily utilizes **lactate** and **glutamine** as substrates. *Gut* - The **small intestine (gut)** has been identified as a site of **gluconeogenesis**, particularly following a meal rich in protein. - Its contribution is relatively smaller compared to the liver but plays a role in **postprandial glucose homeostasis**.

Q: Which amino acid has two chiral centers?

Threonine. ***Threonine*** - Threonine is unique among the standard 20 amino acids because it possesses **two chiral centers**: one at the **alpha-carbon** and another at the **beta-carbon**. - The presence of two chiral centers means that threonine can exist as **four stereoisomers** (2^n, where n is the number of chiral centers). *Tryptophan* - Tryptophan has only **one chiral center**, which is the **alpha-carbon** bonded to the amino group, carboxyl group, hydrogen atom, and the side chain. - Its side chain, an **indole ring**, does not contain an additional chiral center. *Tyrosine* - Tyrosine, like most amino acids, possesses only **one chiral center** at its **alpha-carbon**. - The aromatic ring system (phenol group) in its side chain does not introduce another chiral center. *Phenylalanine* - Phenylalanine also has only **one chiral center** located at its **alpha-carbon**. - Its benzyl side chain, consisting of a methylene group and a benzene ring, is not chiral.

Q: Which element is required by phosphofructokinase?

Magnesium. **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.

Q: Metabolic changes seen in starvation include all of the following except?

Increased glycolysis. ***Increased glycolysis*** - In starvation, the body's primary goal is to conserve **glucose** for essential organs like the brain, as glucose supply is limited. Therefore, glycolysis, the breakdown of glucose, is *decreased*, not increased. - The body shifts to using alternative fuels such as **fatty acids** and **ketone bodies** to spare glucose. *Increased gluconeogenesis* - **Gluconeogenesis**, the synthesis of glucose from non-carbohydrate precursors like amino acids and glycerol, is *increased* during starvation to maintain blood glucose levels. - This process is crucial for providing glucose to tissues that primarily rely on it, such as the brain and red blood cells. *Ketogenesis* - **Ketogenesis**, the production of ketone bodies from fatty acids, is significantly *increased* during prolonged starvation. - **Ketone bodies** become a major energy source for the brain and other tissues when glucose is scarce, helping to spare muscle protein. *Protein degradation* - **Protein degradation** (proteolysis) is *increased* during starvation, especially in the initial phases, to provide amino acids for gluconeogenesis. - Muscle protein is a primary source of these amino acids, contributing to muscle wasting observed in prolonged starvation.

Q: What is the end product of purine metabolism in humans?

Uric acid. ***Uric acid*** - **Uric acid** is the final breakdown product of **purine metabolism** in humans. - It is formed from the degradation of **adenosine** and **guanosine**, with xanthine oxidase playing a key role in its synthesis. *Allantoin* - **Allantoin** is the end product of **purine metabolism** in most mammals other than primates, as they possess the enzyme **uricase** to further break down uric acid. - Humans lack **uricase**, hence allantoin is not the end product in humans. *Carbon Dioxide* - **Carbon dioxide** is a major end product of **carbohydrate** and **fat metabolism** through cellular respiration. - It is not directly associated with the degradation pathway of purines. *None of the options* - This option is incorrect because **uric acid** is indeed the definitive end product of purine metabolism in humans.

Q: Which protein hormone is often referred to as the 'guardian angel against obesity' due to its role in regulating metabolism?

Adiponectin. ***Adiponectin*** - **Adiponectin** is a hormone secreted by **adipose tissue** that plays a crucial role in regulating glucose and fatty acid metabolism, increasing **insulin sensitivity**, and decreasing inflammation. - Its levels are inversely correlated with body fat percentage; individuals with obesity tend to have lower adiponectin levels, leading to its nickname as the 'guardian angel against obesity'. *Fibronectin* - **Fibronectin** is a glycoprotein involved in cell adhesion, growth, migration, and differentiation, and is a key component of the **extracellular matrix**. - It does not primarily function in metabolic regulation or body weight control, unlike adiponectin. *High-Density Lipoprotein (HDL)* - **HDL** is a type of lipoprotein that transports cholesterol from peripheral tissues back to the liver, a process known as **reverse cholesterol transport**. - While beneficial for cardiovascular health, HDL is a lipid-carrying particle, not a protein hormone, and its primary role is not in metabolic regulation or direct obesity prevention. *Insulin* - **Insulin** is a peptide hormone produced by the pancreas that regulates carbohydrate and fat metabolism, primarily by facilitating glucose uptake from the blood into cells. - While essential for metabolism, high levels of insulin in the context of insulin resistance can contribute to obesity, rather than act against it.

Q: Which kinetic parameter is primarily associated with enzyme specificity?

None of the options. ***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.

Q: What does salvage purine synthesis refer to?

Synthesis of purine nucleotides from purine bases. ***Synthesis of purine nucleotides from purine bases*** - **Salvage pathways** recycle pre-existing purine or pyrimidine bases (from nucleic acid degradation) by re-attaching them to a **ribose phosphate** to form a new nucleotide. - This process is energy-efficient as it bypasses several steps of the de novo synthesis pathway, utilizing enzymes like **adenine phosphoribosyltransferase (APRT)** and **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. *Synthesis of purine nucleotides from ribose-5-phosphate.* - While **ribose-5-phosphate** is a precursor, the complete synthesis from this molecule is part of the **de novo pathway**, which starts with PRPP (phosphoribosyl pyrophosphate) formation from ribose-5-phosphate. - This option does not specify the direct reuse of a pre-formed purine base, which is the hallmark of salvage. *Synthesis of purine nucleotides from simple precursors (de novo synthesis).* - **De novo synthesis** is the creation of nucleotides from scratch using simple metabolic precursors like amino acids (glycine, aspartate, glutamine), CO2, and THF derivatives. - This contrasts with salvage pathways, which recycle existing bases. *Synthesis of purine nucleotides from degraded RNA.* - Degraded RNA breaks down into **nucleotides**, which can then be further broken down into **purine bases** and ribose phosphates. - The direct synthesis of purine nucleotides from *degraded RNA* involves recovering the individual bases or nucleosides, then converting them to nucleotides via salvage, not directly using the entire degraded RNA.

Question 1

Serotonin is derived from -

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Phenylalanine

Answer

Methionine

Question 2

What is the approximate half-life of albumin in the human body?

Accepted Answer

20 days

Answer

30 days

Answer

3 days

Answer

7 days

Question 3

Gluconeogenesis occurs in all except:

Accepted Answer

Muscle

Answer

Kidney

Answer

Gut

Answer

Liver

Question 4

Which amino acid has two chiral centers?

Accepted Answer

Threonine

Answer

Tyrosine

Answer

Tryptophan

Answer

Phenylalanine

Question 5

Which element is required by phosphofructokinase?

Accepted Answer

Magnesium

Answer

Inorganic phosphate

Answer

Manganese

Answer

Copper

Question 6

Metabolic changes seen in starvation include all of the following except?

Accepted Answer

Increased glycolysis

Answer

Ketogenesis

Answer

Protein degradation

Answer

Increased gluconeogenesis

Question 7

What is the end product of purine metabolism in humans?

Accepted Answer

Uric acid

Answer

Carbon Dioxide

Answer

Allantoin

Answer

None of the options

Question 8

Which protein hormone is often referred to as the 'guardian angel against obesity' due to its role in regulating metabolism?

Accepted Answer

Adiponectin

Answer

Fibronectin

Answer

High-Density Lipoprotein (HDL)

Answer

Insulin

Question 9

Which kinetic parameter is primarily associated with enzyme specificity?

Accepted Answer

None of the options

Answer

Both

Answer

Km

Answer

Vmax

Question 10

What does salvage purine synthesis refer to?

Accepted Answer

Synthesis of purine nucleotides from purine bases

Answer

Synthesis of purine nucleotides from ribose-5-phosphate

Answer

Synthesis of purine nucleotides from simple precursors (de novo synthesis)

Answer

Synthesis of purine nucleotides from degraded RNA

NEET-PG 2013 — Biochemistry