Biochemistry Urea Cycle Quiz

Questions and Answers

What role does N-acetyl glutamate play in the urea cycle?

It is a byproduct of the urea cycle.

It inhibits the activity of carbamoyl phosphate synthetase I.

It acts as an allosteric effector for carbamoyl phosphate synthetase I. (correct)

It provides a direct substrate for the urea cycle.

What does arginine become upon cleavage during the argininosuccinate process?

Carbamoyl phosphate

Oxaloacetate

Urea

Fumarate (correct)

How is malate produced from fumarate in the urea cycle?

Through condensation with acetyl CoA.

By a direct enzymatic reaction with urea.

By oxidative phosphorylation.

Through hydration of fumarate. (correct)

Which of the following amino acids significantly increases the activity of Acetylglutamate Synthetase?

Arginine Signup and view all the answers

What metabolic pathway does oxaloacetate enter after being oxidized from malate?

TCA Cycle Signup and view all the answers

What dietary intervention is critical for children with partial deficiencies to prevent mental retardation?

Low-protein diet supplemented with arginine Signup and view all the answers

Which of the following substances is administered to help reduce serum ammonia levels?

Sodium Benzoate Signup and view all the answers

What is one of the primary sources of ammonia in the blood?

Amino acids Signup and view all the answers

What enzyme is associated with the conversion of glutamate to ammonia in the liver?

Glutamate Dehydrogenase Signup and view all the answers

In metabolic conditions leading to elevated ammonia, how is some of the serum ammonia utilized?

To synthesize nonessential amino acids Signup and view all the answers

Why is it critical to keep the blood ammonia level low?

To prevent toxicity to the central nervous system Signup and view all the answers

Which compound is NOT mentioned as a source of ammonia?

Fatty Acids Signup and view all the answers

What must be done when protein intake is restricted in children with ammonia metabolism issues?

Supplement with arginine or citrulline Signup and view all the answers

What drives the formation of carbamoyl phosphate in the urea cycle?

Cleavage of 2 molecules of ATP Signup and view all the answers

Which enzyme is responsible for the initial step of carbamoyl phosphate formation?

Carbamoyl phosphate synthetase I Signup and view all the answers

Which metabolite is transported into the cytosol during the urea cycle?

Citrulline Signup and view all the answers

Which amino acid is regenerated with each turn of the urea cycle?

Ornithine Signup and view all the answers

What is formed as a product of the argininosuccinate synthesis reaction?

AMP and PPi Signup and view all the answers

What is the function of N-acetylglutamate in the urea cycle?

Required for activity of carbamoyl phosphate synthetase I Signup and view all the answers

Which reaction in the urea cycle requires the hydrolysis of a third molecule of ATP?

Argininosuccinate synthesis Signup and view all the answers

What is the general role of glutamate dehydrogenase in urea cycle reactions?

Provides ammonia for carbamoyl phosphate formation Signup and view all the answers

What is one consequence of high concentrations of ammonia in the body?

Coma and death Signup and view all the answers

Which condition is associated with acquired hyperammonemia?

Viral hepatitis Signup and view all the answers

Which metabolic reaction is shifted due to high ammonia levels?

Glutamate dehydrogenase reaction Signup and view all the answers

What is the role of renal glutaminase in ammonia transport?

It hydrolyzes glutamine to produce ammonia. Signup and view all the answers

What is the effect of liver cirrhosis on ammonia levels in the body?

Formation of collateral circulation Signup and view all the answers

How is ammonia primarily excreted from the body?

As NH4+ in urine. Signup and view all the answers

Why is the brain particularly vulnerable to hyperammonemia?

High energy production rate by the TCA cycle Signup and view all the answers

What happens to ammonia present at low levels in the blood?

It is rapidly removed by the liver. Signup and view all the answers

What effect does ammonia have on a-ketoglutarate levels?

Depletes a-ketoglutarate Signup and view all the answers

Which amino acids are primarily released by muscle during nitrogen transport?

Glutamine and alanine. Signup and view all the answers

Which of the following is NOT a cause of acquired hyperammonemia?

Regular physical activity Signup and view all the answers

What is the main pathway for ammonia disposal in the body?

Formation of urea in the liver. Signup and view all the answers

Which intermediate is depleted due to the shift towards glutamate formation in the presence of ammonia?

Alpha-ketoglutarate Signup and view all the answers

Where does bacterial action contribute to ammonia production?

In the intestinal lumen. Signup and view all the answers

What happens to urea after it is formed in the liver?

It travels from the liver to the kidneys. Signup and view all the answers

What is the significance of ammonia in maintaining acid-base balance?

It is excreted in urine as NH4+. Signup and view all the answers

Study Notes

Urea Cycle Overview

Urea cycle reactions are critical for ammonia detoxification and occur across mitochondria and cytosol.
Phosphate (Pi) promotes the forward reaction in the cycle.

Enzyme Functions and Reactions

Carbamoyl Phosphate Formation
- Initiated by carbamoyl phosphate synthetase I, requiring N-acetylglutamate.
- Two ATP molecules are utilized to drive the reaction.
Argininosuccinate Synthesis
- Involves the cleavage of a third ATP molecule, producing AMP and pyrophosphate (PPi).
- Uses aspartate generated from a transamination reaction between oxaloacetate and glutamate.
Argininosuccinate Cleavage
- Produces arginine (urea precursor) and fumarate.
- Fumarate can convert to malate, linking to other metabolic pathways.

Role of Ornithine and Citrulline

Ornithine is regenerated in each cycle turn and serves as an intermediary.
Citrulline must be transported to the cytosol from mitochondria.

Allosteric Regulation

N-acetyl glutamate acts as a crucial allosteric activator for carbamoyl phosphate synthetase I.
Higher levels of N-acetylglutamate are stimulated by amino acids, especially arginine.

Treatment for Urea Cycle Disorders

Low-protein diets supplemented with arginine or citrulline are crucial for managing ammonia levels.
Sodium benzoate and sodium phenylacetate help reduce serum ammonia by facilitating excretion.

Ammonia Metabolism

Blood ammonia must be maintained at low levels due to its neurotoxic potential.

Ammonia Sources

From Amino Acids
- Synthesized by the liver through aminotransferase and glutamate dehydrogenase reactions.
From Glutamine
- Renal glutaminase converts glutamine into ammonia, primarily excreted as NH4+.
From Intestinal Bacterial Action
- Bacteria in the intestine produce ammonia, which can be reabsorbed and transported via the portal system.

Toxicity Mechanisms

High ammonia levels may lead to coma or death.
Chronic acquired hyperammonemia often results from liver diseases, including viral hepatitis and cirrhosis.
An increase in glutamate formation from high ammonia depletes alpha-ketoglutarate, disrupting TCA cycle processes, crucial for ATP production in brain cells.

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Description

This quiz focuses on the urea cycle, including the reactions that occur in both mitochondria and cytosol. It explores the roles of key substances like citrulline and ornithine, and the importance of phosphate in driving the reactions forward. Test your knowledge on this fundamental biochemical pathway!