Nutrition: Protein and Amino Acid Pool

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Questions and Answers

What is a typical amount of protein consumed in the U.S. diet per day?

200 g/day

100 g/day (correct)

300 g/day

50 g/day

Which statement correctly describes the amino acid pool in healthy individuals?

It is primarily derived from dietary sources only.

It remains constant over time. (correct)

It is maintained in a state of flux.

It is continuously increasing in size.

All of the following are routes for depleting the amino acid pool except:

Synthesis of body protein

Conversion of amino acids to glucose

Amino acids as precursors for small molecules

Conversion of amino acids to vitamins (correct)

What is typically the largest source of amino acids used to maintain the amino acid pool?

Dietary proteins Signup and view all the answers

Which of the following processes helps to maintain the amino acid pool?

Synthesis of neurotransmitters Signup and view all the answers

In the context of protein turnover, what portion of body protein is typically synthesized and degraded daily?

~400 g/day Signup and view all the answers

Which of the following options is NOT a function of the amino acid pool?

Storage of glucose Signup and view all the answers

Which small molecules can amino acids serve as precursors for?

Porphyrins and neurotransmitters Signup and view all the answers

What is the primary purpose of urea synthesis in the body?

Disposal of nitrogen from the body Signup and view all the answers

Which compound is not directly involved in the urea cycle?

Acetyl CoA Signup and view all the answers

Which metabolic pathways can the carbon skeletons of α-ketoacids be converted into?

CO2, water, glucose, fatty acids, or ketone bodies Signup and view all the answers

During amino acid catabolism, what is the role of ammonia?

It is used in urea synthesis Signup and view all the answers

Which of the following compounds is a product of urea biosynthesis?

Carbamoyl phosphate Signup and view all the answers

What is primarily the fate of free ammonia in the body?

Converted to urea for excretion Signup and view all the answers

Which amino acid is notably associated with the synthesis of urea?

Arginine Signup and view all the answers

Which type of compounds are primarily processed through the urea cycle?

Amino acids Signup and view all the answers

What is the primary function of D-Amino acid oxidase (DAO)?

To catalyze the oxidative deamination of amino acid isomers Signup and view all the answers

Which compound is produced by glutamine synthetase when ammonia combines with glutamate?

Glutamine Signup and view all the answers

How does alanine contribute to ammonia transport to the liver?

Through transamination to form pyruvate Signup and view all the answers

What enzyme cleaves glutamine in the liver to produce glutamate and free ammonia?

Glutaminase Signup and view all the answers

Which condition has been associated with increased activity of D-amino acid oxidase?

Schizophrenia Signup and view all the answers

What role do α-keto acids play in amino acid metabolism?

They enter pathways for catabolism or reamination Signup and view all the answers

What happens to alanine once it reaches the liver?

It is converted back to pyruvate by transamination Signup and view all the answers

What is the result of elevated glutamate levels in the liver after protein ingestion?

Enhanced energy production from carbon skeletons Signup and view all the answers

How is ammonia primarily transported in the blood from peripheral tissues to the liver?

In the form of glutamine Signup and view all the answers

Which molecule acts as an allosteric inhibitor of glutamate dehydrogenase?

Guanosine triphosphate (GTP) Signup and view all the answers

What is the role of glutamate dehydrogenase in amino acid metabolism?

It converts glutamate and α-keto acids to ammonia. Signup and view all the answers

Which reaction is facilitated by low energy levels in a cell?

Amino acid degradation Signup and view all the answers

What type of amino acids are not used in the synthesis of mammalian proteins?

D-Amino acids Signup and view all the answers

Which of the following statements about D-Amino acids is true?

They are metabolized by the kidney and liver. Signup and view all the answers

What is the function of aminotransferases in amino acid metabolism?

They catalyze the transfer of amino groups. Signup and view all the answers

What mechanism allows amino acids to be generated from α-keto acids?

Transamination Signup and view all the answers

What is the primary source of ammonia in Western diets?

Excess amino acids Signup and view all the answers

Which of the following reactions is involved in the production of ammonia during transdeamination?

Linking of aminotransferase and glutamate dehydrogenase reactions Signup and view all the answers

What role does renal glutaminase play in ammonia production?

It generates ammonia from glutamine. Signup and view all the answers

How does the body maintain low levels of circulating ammonia?

By moving nitrogen to the liver for urea disposal Signup and view all the answers

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

It diffuses into the kidney for excretion. Signup and view all the answers

Which process produces an important amount of ammonia from dietary sources?

Deamination of amino acids Signup and view all the answers

Which enzyme is responsible for cleaving urea into CO2 and NH3 in the intestine?

Urease Signup and view all the answers

What is ammonia primarily excreted as after being generated in the kidneys?

Ammonium (NH4+) Signup and view all the answers

Which of the following is an additional source of ammonia besides amino acids?

Hydrolysis of glutamine Signup and view all the answers

In patients with kidney failure, how does elevated plasma urea levels affect the gut?

It enhances the transfer of urea from blood into the gut. Signup and view all the answers

What important balance does ammonia excretion through the kidneys help maintain?

Acid-base balance Signup and view all the answers

What is one consequence of the intestinal action of urease in kidney failure patients?

Increased hyperammonemia. Signup and view all the answers

What is the stoichiometric result of the urea cycle?

4 high-energy phosphate bonds are consumed for each molecule of urea. Signup and view all the answers

Which nitrogen sources contribute to the formation of urea?

One from aspartate and the other from ammonia. Signup and view all the answers

What role does neomycin play in ammonia production in patients with kidney issues?

It reduces the number of intestinal bacteria that produce ammonia. Signup and view all the answers

What is the result of the irreversible nature of urea synthesis?

It leads to a necessity for replenishing ATP through oxidative phosphorylation. Signup and view all the answers

Study Notes

Amino Acid Disposal of Nitrogen

Amino acids are not stored by the body, requiring intake, synthesis, or breakdown of existing proteins.
Excess amino acids are rapidly broken down.
The first stage involves removing the α-amino groups, which forms ammonia and a-keto acids.
A significant portion of the ammonia is used to synthesize urea.
The second stage of amino acid catabolism involves breaking down the carbon skeletons of a-keto acids into common energy-producing metabolic pathways.

Overall Nitrogen Metabolism

Amino acid catabolism is part of a larger nitrogen-containing molecule metabolism.
Nitrogen intake mostly comes from amino acids in dietary protein.
Nitrogen excretion happens mostly through urea, ammonia, and other products from amino acid metabolism.
Body protein functions in transformations via the amino acid pool and protein turnover.

Protein Turnover

Protein turnover is the simultaneous synthesis and degradation of protein molecules.
In healthy adults, the total amount of protein remains constant since protein synthesis offsets the rate of protein degradation.
The rate of protein turnover varies widely for different proteins.
Short-lived proteins are rapidly degraded, while long-lived proteins (structural proteins) have longer half-lives.
Protein degradation is done by two major enzyme systems: the ATP-dependent ubiquitin-proteasome system and the ATP-independent degradative enzyme system of the lysosomes.

Amino Acid Pool

Free amino acids are present throughout the body (cells, blood, extracellular fluids).
The amino acid pool includes amino acids from protein degradation, dietary protein, and synthesis of nonessential amino acids.
These amino acids are used for body protein synthesis and consumed for glucose, glycogen, fatty acids, or ketone bodies.
In healthy individuals, the intake and output of the amino acid pool are balanced, maintaining a constant amount in steady state.

Digestion of Dietary Proteins

Protein digestion begins in the stomach with gastric secretions.
Hydrochloric acid denatures proteins.
Pepsinogen is converted to pepsin, which breaks proteins into smaller peptides and free amino acids.
Pancreatic enzymes enter the small intestine and continue cleaving large polypeptides into oligopeptides and amino acids.
Different enzymes (trypsin, chymotrypsin, elastase, and carboxypeptidases) have unique specificities for amino acid R-groups to cleave peptide bonds.
Enteropeptidase activates trypsinogen to trypsin, initiating a cascade of zymogen activation.
Abnormalities in pancreatic secretion can lead to incomplete digestion and absorption of fats and proteins, resulting in steatorrhea and undigested protein in feces.

Transport of Amino Acids into Cells

Free amino acids enter cells by Sodium (Na+)-linked secondary transport systems in the apical membrane.
Di- and tripeptides enter cells via H+-linked transport systems, which hydrolyze the peptides into amino acids in the cell cytoplasm.
Free amino acids are released into the portal system via facilitated diffusion.
Branched-chain amino acids are not metabolized by the liver but travel mostly to muscle.

Removal of Nitrogen from Amino Acids

The presence of the α-amino group prevents oxidative breakdown and is essential for amino acid catabolism.
Transamination is the first step, where the α-amino group is transferred to α-ketoglutarate, forming glutamate and an α-keto acid.
Oxidative deamination of glutamate releases ammonia (NH3), which can be used in urea synthesis.
Aminotransferases (formerly called transaminases) catalyze the transfer of the amino groups to form glutamate, which then can be oxidatively deaminated.
Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) are two crucial aminotransferases.

Urea Cycle

Urea is the major nitrogenous waste product in mammals.
The urea cycle occurs in the liver, with the first steps occurring in the mitochondria and the remaining steps in the cytosol.
Carbamoyl phosphate synthetase I is the rate-limiting step in the urea cycle, requiring ATP for the synthesis of carbamoyl phosphate.
Ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase are the other enzymes involved in the cycle.
Urea is transported to the kidneys for excretion in the urine.

Metabolism of Ammonia

Ammonia is a major byproduct of amino acid metabolism and is toxic to the central nervous system (CNS).
The liver is the primary organ for ammonia disposal through urea cycle synthesis.
Glutamine is an important non-toxic transport form for ammonia.
Glutamine synthetase synthesizes glutamine from glutamate.
Glutaminase catalyzes the breakdown of glutamine into glutamate and ammonia.
Urea synthesis in the liver excretes a significant portion of nitrogenous waste.

Metabolic Defects in Amino Acid Metabolism

Inborn errors of amino acid metabolism are caused by mutant genes, usually affecting enzymes.
These defects can result in the loss of enzyme activity or a partial deficiency.
Without treatment, some defects result in mental retardation or developmental issues due to harmful accumulation of metabolites.
Conditions like phenylketonuria (PKU), maple syrup urine disease (MSUD), albinism, and homocystinuria are examples of metabolic defects in amino acid metabolism.

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Description

This quiz explores essential aspects of protein consumption and the amino acid pool in the U.S. diet. Test your knowledge on typical protein intake, the amino acid pool characteristics, and sources of amino acids. Dive into understanding how these factors influence overall health.