NEET-PG 2015 - Biochemistry Practice Questions

Q: What is the role of colipase in fat digestion?

Assists pancreatic lipase in fat digestion. ***Assists pancreatic lipase in fat digestion*** - Colipase **binds to pancreatic lipase** and the **lipid-water interface** of the fat droplet, providing a conformational change that enables lipase to access and hydrolyze triglycerides. - It also prevents bile salts from inactivating pancreatic lipase, ensuring efficient **fat emulsification and digestion**. *Is secreted in an inactive form* - Colipase is secreted as **procolipase** by the pancreas, which is then activated by **trypsin** in the duodenum. - While correct, this option describes its activation rather than its primary role in fat digestion. *Encoded by the gene CLPS* - The gene **CLPS** indeed encodes for colipase, but this is a genetic detail rather than its functional role in the digestive process. - Knowledge of the encoding gene is not directly relevant to understanding its biochemical function in fat digestion. *Is secreted by pancreatic cells* - Colipase is indeed synthesized and secreted by the **pancreas** into the small intestine. - This statement is true but describes the **origin** of colipase, not its specific functional role in fat digestion.

Q: Pruritus [Itching] associated with Congenital Erythropoietic Porphyria is caused by deficiency of -

Uroporphyrinogen - III synthase. ***Uroporphyrinogen - III synthase*** - Congenital Erythropoietic Porphyria (CEP) is caused by a **deficiency of uroporphyrinogen III synthase**, leading to the accumulation of uroporphyrinogen I and coproporphyrinogen I. - These accumulated **Type I porphyrinogens** are non-functional in heme synthesis and are highly **photoreactive**, causing the characteristic photosensitivity and skin symptoms, including intense pruritus. *5-ALA dehydratase* - Deficiency of **5-ALA dehydratase** (also known as porphobilinogen synthase) is associated with **ALA dehydratase deficiency porphyria (ADP)**, a very rare acute hepatic porphyria. - Symptoms primarily involve **neurovisceral attacks** and do not typically include pruritus or photosensitivity. *Uroporphyrinogen - I synthase* - **Uroporphyrinogen I synthase** is an outdated and incorrect term; the correct enzyme in the heme synthesis pathway is **hydroxymethylbilane synthase (HMB synthase)** or **porphobilinogen deaminase (PBG deaminase)**, which synthesizes HMB. - Deficiency in HMB synthase leads to **acute intermittent porphyria (AIP)**, characterized by acute neurological attacks, not severe pruritus. *HMB synthase* - **HMB synthase** (hydroxymethylbilane synthase), also known as **porphobilinogen deaminase (PBG deaminase)**, is deficient in **acute intermittent porphyria (AIP)**. - AIP is marked by intermittent neurological dysfunction and abdominal pain, with **no significant photosensitivity or pruritus**.

Q: Which of the following processes primarily utilizes lactate produced anaerobically?

Cori cycle. ***Cori cycle*** - The **Cori cycle** (lactic acid cycle) involves the transport of **lactate** produced during anaerobic metabolism in muscles to the liver. - In the **liver**, this lactate is then converted back to **glucose** via gluconeogenesis, which can be returned to the muscles. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of glucose from non-carbohydrate precursors, one of which is lactate. - While it uses lactate, it is only one component of the broader **Cori cycle**, which describes the inter-organ cooperation. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, which can then be converted to lactate under anaerobic conditions. - This process *produces* lactate but does not *utilize* it, acting upstream of lactate production. *TCA cycle* - The **TCA cycle** (Krebs cycle) is a central part of aerobic respiration that oxidizes acetyl-CoA to produce ATP, NADH, and FADH2. - It does not directly utilize lactate; instead, lactate is typically converted to pyruvate before potentially entering the TCA cycle under aerobic conditions.

Q: Rate limiting enzyme in bile acid synthesis?

7α-hydroxylase. ***7α-hydroxylase*** - This enzyme, specifically **cholesterol 7α-hydroxylase**, catalyzes the first and rate-limiting step in the classic pathway of **bile acid synthesis**, converting cholesterol to 7α-hydroxycholesterol. - Its activity is tightly regulated, primarily by the availability of cholesterol and feedback inhibition by bile acids, making it a key control point. *Desmolase* - **Cholesterol desmolase** (CYP11A1) is the rate-limiting enzyme in **steroid hormone synthesis** in the adrenal glands, converting cholesterol to pregnenolone. - It is not involved in the committed steps of bile acid synthesis from cholesterol. *21α-hydroxylase* - **21α-hydroxylase** (CYP21A2) is crucial in the synthesis of **cortisol and aldosterone** from progesterone and 17-hydroxyprogesterone, respectively. - Deficiency in this enzyme is the most common cause of **congenital adrenal hyperplasia**, but it has no direct role in bile acid synthesis. *12α-hydroxylase* - **12α-hydroxylase** (CYP8B1) is an enzyme involved in the later steps of bile acid synthesis, specifically in the formation of **cholic acid** from 7α-hydroxy-4-cholesten-3-one. - While essential for synthesizing primary bile acids, it is not the *rate-limiting enzyme* for the overall pathway; 7α-hydroxylase holds that distinction.

Q: Coenzyme for phenylalanine hydroxylase is?

Tetrahydrobiopterin. ***Tetrahydrobiopterin*** - **Tetrahydrobiopterin (BH4)** is an essential coenzyme for aromatic amino acid hydroxylases, including **phenylalanine hydroxylase (PAH)**. - PAH converts **phenylalanine** to **tyrosine**, and deficiencies in BH4 or PAH itself lead to *phenylketonuria (PKU)*. *Tetrahydrofolate* - **Tetrahydrofolate (THF)** is a coenzyme derived from **folic acid** and is primarily involved in **one-carbon metabolism**, including **purine** and **pyrimidine synthesis**, and various amino acid interconversions. - It does not directly act as a coenzyme for phenylalanine hydroxylase. *Pyridoxal phosphate* - **Pyridoxal phosphate (PLP)**, a derivative of **vitamin B6**, is a crucial coenzyme for many enzymes involved in **amino acid metabolism**, particularly in **transamination**, **decarboxylation**, and side-chain cleavage reactions. - It is not the coenzyme for phenylalanine hydroxylase. *S-adenosyl methionine* - **S-adenosyl methionine (SAM)** is a major **methyl donor** in various biochemical reactions, important for the synthesis of **neurotransmitters**, **hormones**, and **phospholipids**. - While essential for many metabolic pathways, it is not involved as a coenzyme for phenylalanine hydroxylase.

Q: Which of the following is an example of an exopeptidase?

Carboxypeptidases. ***Carboxypeptidases*** - **Carboxypeptidases** are enzymes that cleave the **C-terminal** (carboxyl end) amino acid from a polypeptide chain, making them a type of exopeptidase. - They are crucial in protein digestion, releasing individual amino acids from the end of protein chains. *Trypsin* - **Trypsin** is an **endopeptidase** that cleaves peptide bonds within protein chains, specifically at the carboxyl side of **lysine** or **arginine** residues. - It does not cleave amino acids from the ends of polypeptide chains. *Chymotrypsin* - **Chymotrypsin** is an **endopeptidase** that cleaves peptide bonds within a polypeptide chain, primarily at the carboxyl side of **tyrosine**, **tryptophan**, or **phenylalanine**. - Its action is internal to the protein sequence, not at the termini. *Elastase* - **Elastase** is also an **endopeptidase** that cleaves peptide bonds internally, specifically targeting small, uncharged amino acid residues like **alanine**, **glycine**, and **valine**. - Its primary role is to break down elastin, an elastic protein in connective tissues, but it does so by internal cleavage.

Q: Which protein is responsible for conserving iron in the body?

Ferritin. ***Ferritin*** - **Ferritin** is the primary intracellular protein that **conserves iron** in the body by storing it in a safe, non-toxic, and bioavailable form - It is found mainly in the liver, spleen, and bone marrow, serving as the body's **iron reserve** - When iron is abundant, ferritin stores it; when iron is needed, ferritin releases it, thus **conserving iron for future use** - Serum ferritin levels directly reflect total body iron stores *Hepcidin* - **Hepcidin** is a regulatory peptide hormone that controls iron homeostasis by inhibiting **ferroportin**, the iron export channel - It reduces iron absorption from the intestine and iron release from macrophages during inflammation or iron overload - While it regulates iron distribution, it is a hormone, not a storage protein, and does not directly conserve iron within cells *Hemopexin* - **Hemopexin** binds free **heme** in plasma, preventing oxidative damage and delivering it to the liver for catabolism - It helps recover iron from heme but does not store or conserve iron in the body *Transferrin* - **Transferrin** is a plasma protein that **transports iron** from absorption sites (intestine) and storage sites (liver, spleen) to tissues that need it - Its role is iron delivery, not conservation or storage

Q: Abnormal proteins which are bound to ubiquitin are degraded in -

Proteasomes. ***Proteasomes*** - **Proteasomes** are multi-subunit protein complexes responsible for degrading **ubiquitin-tagged proteins**. - This degradation is a tightly regulated process essential for cell cycle control, gene expression, and immune response. *Golgi apparatus* - The **Golgi apparatus** primarily functions in modifying, sorting, and packaging proteins and lipids synthesized in the Endoplasmic Reticulum. - It does not directly participate in the degradation of **ubiquitin-bound proteins**. *Smooth ER* - The **smooth endoplasmic reticulum (SER)** is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. - It lacks ribosomes and is not directly implicated in the degradation of misfolded proteins tagged with ubiquitin. *Lysosomes* - **Lysosomes** are organelles containing various hydrolytic enzymes that break down waste materials and cellular debris, as well as foreign invaders like bacteria. - While they degrade proteins, they primarily target **extracellular proteins** taken up by endocytosis or cellular components via **autophagy**, not specifically ubiquitin-bound proteins.

Q: Pyruvate dehydrogenase requires all cofactors except:

Pyridoxin (Vitamin B6). ***Pyridoxin (Vitamin B6)*** - **Pyridoxin** (vitamin B6) is a coenzyme for many enzymes involved in **amino acid metabolism**, but it is **not directly required** by the pyruvate dehydrogenase complex. - The pyruvate dehydrogenase complex uses **thiamine pyrophosphate**, **lipoic acid**, **FAD**, **NAD+**, and **Coenzyme A** as cofactors. *Thiamin* - **Thiamin pyrophosphate** (TPP), derived from thiamin (vitamin B1), is a crucial coenzyme for the **E1 subunit** of the pyruvate dehydrogenase complex. - It participates in the **decarboxylation** of pyruvate, releasing CO2. *Riboflavin* - **FAD** (flavin adenine dinucleotide), derived from riboflavin (vitamin B2), is a coenzyme for the **E3 subunit** (dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex. - It is involved in the **regeneration of oxidized lipoamide**. *Niacin* - **NAD+** (nicotinamide adenine dinucleotide), derived from niacin (vitamin B3), is a coenzyme for the **E3 subunit** of the pyruvate dehydrogenase complex. - It acts as an **electron acceptor** during the reoxidation of FADH2.

Q: Aminolevulinic acid is a metabolic product in the synthesis of -

Heme. ***Heme*** - **Aminolevulinic acid (ALA)** is the first committed precursor in the **heme biosynthesis pathway**. - Its formation from succinyl CoA and glycine is catalyzed by **ALA synthase**, which is the rate-limiting enzyme. *Tryptophan* - **Tryptophan** is an essential amino acid and a precursor for molecules like **serotonin**, **melatonin**, and **niacin**. - Its synthesis does not involve aminolevulinic acid. *Collagen* - **Collagen** is a structural protein primarily composed of amino acids such as **glycine, proline, and hydroxyproline**. - Its synthesis pathway is distinct and does not utilize aminolevulinic acid. *Glycosaminoglycans* - **Glycosaminoglycans (GAGs)** are long, unbranched polysaccharides consisting of repeating disaccharide units, significant components of the **extracellular matrix**. - Their synthesis involves various sugars and enzymes, but not aminolevulinic acid.

Question 1

What is the role of colipase in fat digestion?

Accepted Answer

Assists pancreatic lipase in fat digestion

Answer

Encoded by the gene CLPS

Answer

Is secreted in an inactive form

Answer

Is secreted by pancreatic cells

Question 2

Pruritus [Itching] associated with Congenital Erythropoietic Porphyria is caused by deficiency of -

Accepted Answer

Uroporphyrinogen - III synthase

Answer

Uroporphyrinogen - I synthase

Answer

5-ALA dehydratase

Answer

HMB synthase

Question 3

Which of the following processes primarily utilizes lactate produced anaerobically?

Accepted Answer

Cori cycle

Answer

Gluconeogenesis

Answer

TCA cycle

Answer

Glycolysis

Question 4

Rate limiting enzyme in bile acid synthesis?

Accepted Answer

7α-hydroxylase

Answer

Desmolase

Answer

21α-hydroxylase

Answer

12α-hydroxylase

Question 5

Coenzyme for phenylalanine hydroxylase is?

Accepted Answer

Tetrahydrobiopterin

Answer

Tetrahydrofolate

Answer

Pyridoxal phosphate

Answer

S-adenosyl methionine

Question 6

Which of the following is an example of an exopeptidase?

Accepted Answer

Carboxypeptidases

Answer

Trypsin

Answer

Chymotrypsin

Answer

Elastase

Question 7

Which protein is responsible for conserving iron in the body?

Accepted Answer

Ferritin

Answer

Hepcidin

Answer

Hemopexin

Answer

Transferrin

Question 8

Abnormal proteins which are bound to ubiquitin are degraded in -

Accepted Answer

Proteasomes

Answer

Golgi apparatus

Answer

Smooth ER

Answer

Lysosomes

Question 9

Pyruvate dehydrogenase requires all cofactors except:

Accepted Answer

Pyridoxin (Vitamin B6)

Answer

Thiamin

Answer

Riboflavin

Answer

Niacin

Question 10

Aminolevulinic acid is a metabolic product in the synthesis of -

Accepted Answer

Heme

Answer

Tryptophan

Answer

Collagen

Answer

Glycosaminoglycans

NEET-PG 2015 — Biochemistry