Harper's Biochemistry Chapter 28 - Catabolism of Proteins & of Amino Acid Nitrogen

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary role of carbamoyl phosphate in the urea cycle?

It plays a role only in the energy generation processes.

It functions primarily in the synthesis of herbal compounds.

It serves as a precursor for urea synthesis. (correct)

It acts solely as a source of nitrogen for amino acids.

Which factor is essential for the activation of carbamoyl phosphate synthetase I?

Bicarbonate

Ornithine

ADP

N-acetylglutamate (correct)

How many moles of ATP are required for the synthesis of one mole of carbamoyl phosphate?

1 mole

3 moles

4 moles

2 moles (correct)

What role do ornithine and lysine play in the urea cycle?

They are inhibitors of arginase and compete with arginine. Signup and view all the answers

What is one function of the mitochondria in the context of the urea cycle?

They are where carbamoyl phosphate is synthesized. Signup and view all the answers

What is the primary substrate for the synthesis of N-acetylglutamate in the urea cycle?

Acetyl-CoA Signup and view all the answers

Which enzyme catalyzes the conversion of citrulline and aspartate to argininosuccinate?

Argininosuccinate synthetase Signup and view all the answers

How does starvation affect the levels of urea cycle enzymes?

Increases enzyme levels significantly Signup and view all the answers

What role does N-acetylglutamate play in the urea cycle?

It serves as an enzyme activator for carbamoyl phosphate synthetase I Signup and view all the answers

Which of the following describes the entry and exit of intermediates in the urea cycle?

Ornithine enters the cytosol, while citrulline exits mitochondria Signup and view all the answers

What is required for the formation of argininosuccinate from citrulline and aspartate?

ATP and citrulline-AMP intermediate Signup and view all the answers

In the context of the urea cycle, what is the significance of mitochondrial inner membrane carriers like ORC1 and ORC2?

They transport ornithine and citrulline across mitochondrial membranes Signup and view all the answers

Which of the following is a product of the hydrolysis reaction involving N-acetyl-L-glutamate?

Acetate and L-glutamate Signup and view all the answers

What are the stages of urea biosynthesis?

Transamination, oxidative deamination of glutamate, ammonia transport, reactions of the urea cycle Signup and view all the answers

Which amino acid is primarily released by muscles during protein metabolism?

Valine Signup and view all the answers

How does the liver's capacity for gluconeogenesis from alanine change in starvation?

It increases substantially Signup and view all the answers

What is the role of transamination in amino acid metabolism?

It transfers nitrogen to α-ketoglutarate, forming glutamate Signup and view all the answers

Why is alanine a key gluconeogenic amino acid?

It can be easily converted into glucose Signup and view all the answers

What is the primary metabolic role of ornithine in mammals?

Urea synthesis Signup and view all the answers

What enhances the expression of urea cycle enzymes in the liver during starvation?

Enhanced protein degradation for energy Signup and view all the answers

What is primarily transported by mitochondria in the urea cycle?

Ammonia and carbon dioxide Signup and view all the answers

Which of the following substrates is required for the synthesis of one mole of urea?

3 moles of ATP Signup and view all the answers

What role does N-acetylglutamate play in the urea cycle?

It functions as an enzyme enhancer Signup and view all the answers

Which enzyme is NOT typically active in the urea cycle?

Aldolase Signup and view all the answers

Where does the majority of the urea synthesis cycle occur within the cell?

Matrix of mitochondria and cytosol Signup and view all the answers

What role do branched-chain amino acids play in nitrogen metabolism?

They participate in nitrogen transport Signup and view all the answers

Which enzyme initiates the biosynthesis of urea?

Carbamoyl phosphate synthetase I Signup and view all the answers

What concentration level of alanine indicates saturation for gluconeogenesis in the liver?

20 to 30 times normal physiological levels Signup and view all the answers

What adaptation does the body use to maintain acid-base balance relating to ammonia production?

Enhanced ammonia secretion into urine Signup and view all the answers

Which of the following is NOT involved in urea synthesis?

Citrate Signup and view all the answers

What contributes nitrogen to the formation of urea?

Both ammonium ion and aspartate Signup and view all the answers

In which part of cellular metabolism do the reactions of the urea cycle primarily occur?

In both mitochondria and cytosol Signup and view all the answers

Which substance is consumed in the urea cycle but is ultimately regenerated by the end of the cycle?

Ornithine Signup and view all the answers

Which compound is a precursor in the biosynthesis of urea that contributes nitrogen atoms?

Aspartate Signup and view all the answers

What role does N-acetylglutamate play in the urea cycle?

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

Where in the cell is carbamoyl phosphate synthetase I primarily located?

Mitochondrial matrix Signup and view all the answers

Which enzyme defect would most likely impair the early steps of the urea cycle leading to hyperammonemia?

Carbamoyl phosphate synthetase I Signup and view all the answers

What is the primary analytical method employed for newborn screening of metabolic disorders in the United States?

Tandem mass spectrometry Signup and view all the answers

Which of the following disorders would NOT typically be detected by newborn metabolic screening?

Inherited immunodeficiencies Signup and view all the answers

Which of the following compounds is produced as a result of nitrogen metabolism in the context of the urea cycle?

Urea Signup and view all the answers

What type of metabolic diseases are primarily associated with deficiencies in the urea cycle?

Hyperammonemia Signup and view all the answers

Which amino acid is crucial for providing nitrogen in the synthesis of urea?

Glutamine Signup and view all the answers

Which organ is primarily responsible for the synthesis of urea in the urea cycle?

Liver Signup and view all the answers

Which amino acid is released by muscles and taken up predominantly by the brain during protein metabolism?

Alanine Signup and view all the answers

What physiological condition significantly increases the expression of enzymes in the urea cycle?

Starvation Signup and view all the answers

Which of the following stages is NOT a part of urea biosynthesis?

Krebs cycle reaction Signup and view all the answers

What is the primary role of branched-chain amino acids in nitrogen metabolism?

Release nitrogen for gluconeogenesis Signup and view all the answers

What concentration level of alanine triggers the liver's capacity for gluconeogenesis to reach saturation?

20 to 30 times normal Signup and view all the answers

Which enzyme is involved in the transamination process that interconverts pairs of α-amino acids and α-keto acids?

Aminotransferase Signup and view all the answers

What happens to the rate of gluconeogenesis from alanine as the alanine concentration increases?

It decreases until saturation Signup and view all the answers

What role does pyridoxal phosphate (PLP) play during transamination?

It functions as a carrier for amino groups. Signup and view all the answers

What is a direct product of the enzyme-bound Schiﬀ base rearrangement during transamination?

α-keto acid Signup and view all the answers

Which metabolites are produced during the conversion of an amino acid via l-amino acid oxidase?

α-keto acid and hydrogen peroxide Signup and view all the answers

How is ammonia typically removed from circulation?

It is rapidly cleared by the liver as urea. Signup and view all the answers

What is the maximum concentration of ammonia normally present in peripheral blood?

10-20 μg/dL Signup and view all the answers

Which enzyme is primarily associated with the transamination process involving the addition and release of substrates?

Aminotransferase Signup and view all the answers

What is formed after the removal of the α-amino nitrogen from an amino acid during transamination?

α-keto acid Signup and view all the answers

What substance is produced when hydrogen peroxide is split by catalase?

Oxygen and water Signup and view all the answers

Which of the following reflects the fate of reduced flavin in the process described?

It is reoxidized by molecular oxygen. Signup and view all the answers

Which enzyme deficiency is characterized by a phenotype indistinguishable from that caused by a defect in carbamoyl phosphate synthetase I?

N-acetylglutamate synthetase Signup and view all the answers

What is the primary metabolic consequence of defects in the urea cycle enzymes characterized by the accumulation of ammonia?

Hyperammonemia Signup and view all the answers

What is a key characteristic of the syndrome resulting from mutations in the ORC1 gene?

Hyperornithinemia and hyperammonemia Signup and view all the answers

Which of the following genetic diseases related to the urea cycle is the most rare, with an estimated frequency of 1:62,000?

Carbamoyl phosphate synthetase I deficiency Signup and view all the answers

Which compound is essential for the enzymatic activity of carbamoyl phosphate synthetase I and is also involved in its regulation?

N-acetylglutamate Signup and view all the answers

What is the primary product formed when glutamate is synthesized through the action of glutamate dehydrogenase?

Alpha-ketoglutarate Signup and view all the answers

In the 'ping-pong' mechanism for transamination, which compound serves as the amino group donor?

Amino acids Signup and view all the answers

Which oxido-reductant can be utilized in the reaction catalyzed by glutamate dehydrogenase?

NAD+ Signup and view all the answers

What type of reaction is primarily associated with the enzyme pyridoxal phosphate?

Transamination Signup and view all the answers

Which of the following amino acids can be directly synthesized from pyruvate?

Serine Signup and view all the answers

What type of enzyme is glutamate synthase considered to be in amino acid metabolism?

Transferase Signup and view all the answers

Which key intermediate is involved in the formation of L-glutamate during the reaction catalyzed by glutamate dehydrogenase?

Ammonia Signup and view all the answers

What effect does the reaction catalyzed by glutamate dehydrogenase strongly favor?

Formation of glutamate Signup and view all the answers

Which biomolecule serves as a cofactor in the transamination reactions described in the ping-pong mechanism?

Pyridoxal phosphate Signup and view all the answers

How does glutamate contribute to the urea cycle?

As a nitrogen donor in the formation of urea Signup and view all the answers

What metabolic principle explains the fact that various mutations can lead to identical clinical symptoms in patients?

Allelic heterogeneity Signup and view all the answers

Which of the following methods is used to identify intermediates in a metabolic pathway before a block occurs?

Metabolic screening tests Signup and view all the answers

Which deficiency is associated with impaired activity of carbamoyl phosphate synthetase I?

N-acetylglutamate synthase deficiency Signup and view all the answers

Definitive diagnosis of a metabolic disorder often requires what specific step?

Quantitative enzyme activity assay Signup and view all the answers

How do metabolic perturbations need to be managed when administering therapies for inherited metabolic diseases?

Simultaneously ensure adequate substrate availability Signup and view all the answers

What type of inheritance pattern is often evident in genetic mutations affecting enzyme-catalyzed reactions in metabolic pathways?

Autosomal recessive inheritance Signup and view all the answers

Which reaction in the urea cycle is analogous to a reaction in the citric acid cycle?

Conversion of oxaloacetate to malate Signup and view all the answers

An exponential increase in DNA sequencing has contributed to the identification of what in the context of inherited metabolic diseases?

Mutations connected to disorders Signup and view all the answers

What is the role of aspartate in the reaction catalyzed by argininosuccinate lyase?

It acts as a substrate Signup and view all the answers

In assessing a patient for a urea cycle disorder, which test is commonly prioritized first?

Newborn screening test for metabolic disorders Signup and view all the answers

Match the following enzymes with their corresponding substrates:

Alanine aminotransferase = Pyruvate Glutamate aminotransferase = α-Ketoglutarate Urease = Urea Aminopeptidase = Proteins Signup and view all the answers

Match the following amino acids with their transamination involvement:

Glutamate = Forms α-ketoglutarate Alanine = Forms pyruvate Ornithine = Involves δ-amino group Lysine = Does not participate Signup and view all the answers

Match the following nitrogen metabolism processes with their outcomes:

Transamination = Transfer of amino groups Deamination = Formation of ammonia Amino acid biosynthesis = Synthesis of protein components Oxidative deamination = Conversion of glutamate to ammonia Signup and view all the answers

Match the following coenzymes with their respective reactions:

NAD+ = Reductive deamination NADP+ = Biosynthesis reactions FAD = Fatty acid oxidation Coenzyme A = Acetylation reactions Signup and view all the answers

Match the following metabolic terms with their definitions:

Transdeamination = Conversion of nitrogen to ammonia Aminotransferase = Enzyme for amino group transfer Reversible reaction = Can proceed in both directions Biosynthesis = Formation of complex molecules Signup and view all the answers

Match the following terms related to nitrogen metabolism with their descriptions:

Transaminases = Enzymes that facilitate the transfer of amino groups Proteasome = Cellular structure that degrades proteins Urea = Main nitrogenous waste product in mammals Glutamine = Non-toxic amino acid that carries ammonia Signup and view all the answers

Match the following pathways in protein turnover with their characteristics:

ATP-dependent pathways = Require energy for protein degradation ATP-independent pathways = Use proteolytic enzymes without energy Proteasomal degradation = Targeted protein degradation process Lysosomal degradation = General degradation process for cellular components Signup and view all the answers

Match the following amino acids with their roles in nitrogen metabolism:

Glutamate = Key amino acid in nitrogen transfer reactions Aspartate = Contributes nitrogen in urea synthesis Alanine = Portable nitrogen source from muscles to liver Citrulline = Intermediate in the urea cycle Signup and view all the answers

Match the following enzymes with their functions in urea biosynthesis:

Carbamoyl phosphate synthetase I = Catalyzes the first step in the urea cycle Argininosuccinate synthetase = Combines citrulline and aspartate to form argininosuccinate Glutamate dehydrogenase = Catalyzes the oxidative deamination of glutamate Urease = Hydrolyzes urea into ammonia and carbon dioxide Signup and view all the answers

Match the following metabolic products with their origins:

Urea = Final product of nitrogen metabolism in mammals Ammonia = Byproduct of amino acid deamination Creatinine = Waste product from muscle metabolism Bilirubin = Byproduct of heme degradation Signup and view all the answers

Study Notes

Protein Turnover

Protein turnover is the mean rate of protein turnover in healthy individuals
Some human proteins are degraded at rates higher than the average rate
Protein turnover involves ATP-dependent and ATP-independent pathways
Proteasomes, ubiquitin, cell surface receptors, circulating asialoglycoproteins, and lysosomes play roles in protein degradation

Nitrogen Catabolism

The end products of nitrogen catabolism in mammals differ from those in birds and fish
Transaminases (aminotransferases), glutamate dehydrogenase, and glutaminase are central to human nitrogen metabolism
Reactions convert ammonia, carbon dioxide, and aspartate into urea
Urea synthesis occurs in different cellular locations
Allosteric regulation and acetylglutamate regulate early steps of urea synthesis
Defects in urea cycle enzymes result in similar clinical signs and symptoms but distinct at a molecular level

Biomedical Importance

Nitrogen intake equals nitrogen excretion in normal adults
Positive nitrogen balance (ingested nitrogen > excreted nitrogen) occurs during growth and pregnancy
Negative nitrogen balance (excreted nitrogen > ingested nitrogen) occurs in conditions such as surgery, advanced cancer, and certain nutritional disorders
Genetic disorders affecting ubiquitin, ubiquitin ligases, or deubiquitinating enzymes lead to diseases like Angelman syndrome, juvenile Parkinson disease, von Hippel-Lindau syndrome, and congenital polycythemia
Ammonia, highly toxic, arises primarily from amino acid a-amino nitrogen
Tissues convert ammonia to glutamine, then to urea for excretion
Liver dysfunction leads to elevated blood ammonia, causing clinical symptoms
Urea cycle enzymes are examples of metabolic defects linked to various physiologic consequences
Urea cycle provides a molecular model for studying other human metabolic defects

Protein Degradation

Protein degradation occurs at different rates depending on tissue and protein type
Proteins not immediately used for new protein synthesis are rapidly degraded
Carbon skeletons of amino acids convert to intermediates for metabolic use, and nitrogen is converted to urea for excretion
Proteases hydrolyze internal peptide bonds
Peptides are further degraded to amino acids by endopeptidases, aminopeptidases, and carboxypeptidases

ATP-Independent Degradation

Blood glycoproteins are degraded by losing sialic acid, followed by internalization and lysosomal breakdown by proteases
Extracellular, membrane-associated, and long-lived intracellular proteins are also degraded in lysosomes

ATP & Ubiquitin-Dependent Degradation

Abnormal or misfolded proteins are degraded
Ubiquitin, a small polypeptide, targets intracellular proteins for degradation
Ubiquitin is highly conserved in structure
Ubiquitin molecules attach to target proteins via non-peptide bonds
Proteins are marked for degradation in the cytosol

Proteasome

Ubiquitin-tagged proteins are degraded in the proteasome, a macromolecule
Proteasomes contain proteolytic enzymes
Proteins tagged with multiple ubiquitin molecules enter the proteasome core
Proteasomes help clear abnormal or misfolded proteins

Interorgan Exchange

Muscle is a major source of amino acids for the body
Liver plays a critical role in urea synthesis and gluconeogenesis from alanine
Kidney removes waste nitrogen and glutamine

Urea synthesis

Urea is the primary nitrogenous waste product in humans
Urea synthesis occurs in the liver in four stages: transamination, oxidative deamination of glutamate, ammonia transport, and urea cycle reactions
Synthesis of 1 mole of urea requires 3 moles of ATP and utilizes several enzymes
Urea synthesis involves mitochondrial and cytosolic steps

Metabolic Disorders

Defects in the urea cycle are characterized by hyperammonemia
Clinical symptoms include various neurological problems and severe mental retardation
Early detection and dietary intervention can improve the outcome for these disorders
Tandem mass spectrometry is a powerful technique used in newborn screening for metabolic disorders

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the dynamics of protein turnover and nitrogen catabolism in humans. This quiz covers the rates of protein degradation, metabolic pathways, and the significance of nitrogen in mammalian physiology. Understand the critical enzymes and regulatory mechanisms involved in these processes.