NEET-PG 2015 - Biochemistry PYQs Practice Questions

Q: Which of the following enzymes is not part of the fatty acid synthase complex?

Acetyl-CoA carboxylase. ***Acetyl-CoA carboxylase*** - **Acetyl-CoA carboxylase (ACC)** is a crucial enzyme in fatty acid synthesis, catalyzing the committed and rate-limiting step of converting **acetyl-CoA to malonyl-CoA**. - While essential for providing the substrates for fatty acid synthase, ACC is a **separate, distinct enzyme** and not structurally part of the fatty acid synthase complex itself. *Ketoacyl reductase* - **Ketoacyl reductase** is an integral enzymatic domain of the fatty acid synthase complex. - It catalyzes the **first reduction step** in the fatty acid synthesis cycle, converting a $\beta$-ketoacyl group to a $\beta$-hydroxyacyl group using NADPH. *Enoyl reductase* - **Enoyl reductase** is an intrinsic enzymatic domain of the fatty acid synthase complex. - It catalyzes the **second reduction step**, converting a trans- $\alpha$, $\beta$-enoyl group to a saturated acyl group using NADPH. *Ketoacyl synthase* - **Ketoacyl synthase (or $\beta$-ketoacyl-ACP synthase)** is a core enzymatic domain within the fatty acid synthase complex. - It catalyzes the **condensation reaction** between the growing acyl chain and malonyl-ACP, forming a $\beta$-ketoacyl-ACP. [Practice more Biochemistry PYQs on OnCourse]

Q: What is the immediate source of energy for cellular processes?

ATP. ***ATP*** - **Adenosine triphosphate (ATP)** is the direct and immediate source of energy for almost all cellular processes, including **muscle contraction**, **active transport**, and **biosynthesis**. - Its high-energy phosphate bonds release energy upon hydrolysis, driving various cellular functions. *Cori's cycle* - The **Cori cycle** involves the interconversion of **lactate** and **glucose** between the muscle and the liver to regenerate glucose stores. - It is an important metabolic pathway for glucose homeostasis during anaerobic conditions, but it does not directly provide immediate energy for cellular processes. *HMP* - The **Hexose Monophosphate Pathway (HMP)**, also known as the **pentose phosphate pathway**, primarily produces **NADPH** and **ribose-5-phosphate**. - While it generates NADPH for reductive biosynthesis and protects against oxidative stress, it is not an immediate source of energy. *TCA cycle* - The **Tricarboxylic Acid (TCA) cycle**, or Krebs cycle, is a central metabolic pathway that oxidizes **acetyl-CoA** to produce **ATP**, **NADH**, and **FADH2**. - While it is a major producer of ATP, it is not the *immediate* source; instead, it generates the precursors that fuel oxidative phosphorylation to produce ATP. [Practice more Biochemistry PYQs on OnCourse]

Q: Which is the first steroid intermediate formed in the conversion of cholesterol to steroid hormones?

Pregnenolone. ***Pregnenolone*** - **Pregnenolone** is the **first steroid intermediate** formed from **cholesterol** in steroidogenesis - The conversion occurs in mitochondria via the **cholesterol side-chain cleavage enzyme (P450scc/CYP11A1)** - This is the **rate-limiting step** in steroid hormone biosynthesis - From pregnenolone, all other steroid hormones are subsequently synthesized *Progesterone* - Progesterone is the **second intermediate**, formed from pregnenolone - It serves as a precursor for glucocorticoids, mineralocorticoids, and androgens - Not the first intermediate from cholesterol *Glucocorticoid* - Glucocorticoids (e.g., cortisol) are **end products**, not intermediates - Formed several steps downstream from cholesterol via pregnenolone and progesterone *Mineralocorticoid* - Mineralocorticoids (e.g., aldosterone) are **end products**, not intermediates - Synthesized from progesterone through multiple enzymatic steps *Estradiol* - Estradiol is a **late-stage product** synthesized from androgens - Requires aromatase enzyme for conversion from testosterone - Multiple steps removed from the initial cholesterol conversion [Practice more Biochemistry PYQs on OnCourse]

Q: Level of which of the following is not elevated in heart disease

5-nucleotidase. ***5-nucleotidase*** - While other enzymes like LDH, SGOT, and ALP can be elevated in various conditions including heart disease (especially in the context of tissue damage), 5-nucleotidase is **not typically elevated in heart disease**. - Its elevation is more commonly associated with **biliary obstruction** or certain liver pathologies. *LDH* - **Lactate dehydrogenase (LDH)** is a marker of **cellular damage** and can be elevated in myocardial infarction, though it is less specific than troponins. - LDH levels rise later than CK-MB and remain elevated longer, indicating persistent tissue injury. *SGOT* - **Serum glutamic oxaloacetic transaminase (SGOT)**, also known as **aspartate aminotransferase (AST)**, is elevated in acute **myocardial infarction** due to cardiac muscle damage. - While it's a marker for cardiac injury, it's not specific as it's also highly concentrated in the liver. *ALP* - **Alkaline phosphatase (ALP)** can be mildly elevated in heart failure due to **hepatic congestion** caused by reduced cardiac output. - While its primary diagnostic significance is in bone and liver disease, its elevation in advanced heart disease is usually a secondary consequence. [Practice more Biochemistry PYQs on OnCourse]

Q: Chymotrypsinogen is activated into chymotrypsin by:

Trypsin. ***Activation of Chymotrypsinogen by Trypsin*** - **Trypsin** is the primary enzyme responsible for the activation of **chymotrypsinogen** into its active form, **chymotrypsin**, by cleaving a specific peptide bond. - This activation is part of a cascade of proteolytic enzyme activations in the **pancreatic juice**, crucial for protein digestion in the small intestine. *Pepsin* - **Pepsin** is a protease active in the **stomach**, requiring an acidic environment for its activity, and is involved in the initial breakdown of proteins. - It does not play a role in the activation of pancreatic zymogens like chymotrypsinogen; its primary function is protein digestion in the gastric lumen. *Renin* - **Renin** is an enzyme primarily involved in the **renin-angiotensin-aldosterone system** (RAAS), which regulates blood pressure and fluid balance. - Its action involves cleaving **angiotensinogen** to form angiotensin I, and it has no role in the activation of digestive enzymes like chymotrypsinogen. *HCl* - **Hydrochloric acid (HCl)** is produced in the stomach and is essential for providing the acidic environment required for **pepsin's activity** and for denaturing proteins. - While HCl is crucial for digestion, it does not directly activate chymotrypsinogen; this activation is an enzymatic process carried out by another protease. [Practice more Biochemistry PYQs on OnCourse]

Question 1

Which of the following enzymes is not part of the fatty acid synthase complex?

Accepted Answer

Acetyl-CoA carboxylase

Answer

Enoyl reductase

Answer

Ketoacyl synthase

Answer

Ketoacyl reductase

Question 2

What is the immediate source of energy for cellular processes?

Accepted Answer

ATP

Answer

Cori's cycle

Answer

HMP

Answer

TCA cycle

Question 3

Which is the first steroid intermediate formed in the conversion of cholesterol to steroid hormones?

Accepted Answer

Pregnenolone

Answer

Glucocorticoid

Answer

Mineralocorticoid

Answer

Estradiol

Question 4

Level of which of the following is not elevated in heart disease

Accepted Answer

5-nucleotidase

Answer

SGOT

Answer

ALP

Answer

LDH

Question 5

Chymotrypsinogen is activated into chymotrypsin by:

Accepted Answer

Trypsin

Answer

Pepsin

Answer

Renin

Answer

HCl

Question 6

Enzyme activity is expressed as?

Accepted Answer

Micromoles/min

Answer

Millimoles/lit

Answer

Milli gm/lit

Answer

Mg/dl

Question 7

Which of the following is an example of an exopeptidase?

Accepted Answer

Carboxypeptidases

Answer

Trypsin

Answer

Chymotrypsin

Answer

Elastase

Question 8

How many isoenzymes does lactate dehydrogenase (LDH) have?

Accepted Answer

5, based on H and M polypeptide subunits

Answer

7, based on H and M polypeptide subunits

Answer

9, based on H and M polypeptide subunits

Answer

3, based on H and M polypeptide subunits

Question 9

Vitamin D absorption is decreased by ?

Accepted Answer

Fat malabsorption

Answer

Proteins

Answer

Acid

Answer

Lactose

Question 10

What is the mechanism of conversion of trypsinogen to trypsin?

Accepted Answer

Removal of part of protein

Answer

Hydrolysis

Answer

Phosphorylation

Answer

Removal of Carboxyl group

NEET-PG 2015 — Biochemistry