Biochemistry: Amino Acid Metabolism

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Questions and Answers

What are the two main functions of hydrochloric acid in the stomach?

To kill bacteria and denature proteins.

How is pepsinogen activated to pepsin?

Pepsinogen is activated by hydrochloric acid or autocatalytically by already activated pepsin.

What do pancreatic proteases do in the digestion process?

They cleave polypeptides into oligopeptides and amino acids.

What role does cholecystokinin and secretin play in digestion?

They mediate the release and activation of pancreatic zymogens. Signup and view all the answers

What is the function of aminopeptidase in the small intestine?

It cleaves the N-terminal residue from oligopeptides to produce smaller peptides and free amino acids. Signup and view all the answers

What are the main steps involved in the removal of nitrogen from amino acids?

Removing the α-amino group, transporting it to the liver, and entering the urea cycle. Signup and view all the answers

Why is the removal of the α-amino group considered obligatory in amino acid catabolism?

It is essential for producing energy from amino acids. Signup and view all the answers

In which part of the cellular process does the nitrogen from amino acids ultimately end up?

It is incorporated into the urea cycle for excretion. Signup and view all the answers

What are the three main sources that supply the amino acid pool in the body?

The three main sources are: degradation of body proteins, dietary protein, and synthesis of nonessential amino acids. Signup and view all the answers

Describe the process by which amino acids are catabolized for nitrogen metabolism.

Amino acids are catabolized primarily through transamination and deamination processes. Signup and view all the answers

How is amino-nitrogen transformed into urea for excretion?

Amino-nitrogen is converted to urea through the urea cycle, where ammonia generated from amino acid deamination is combined with carbon dioxide. Signup and view all the answers

What are the two important concepts involved in the role of body proteins in nitrogen metabolism?

The two important concepts are the amino acid pool and protein turnover. Signup and view all the answers

What is the primary role of transamination in amino acid catabolism?

Transamination primarily functions to funnel amino groups to glutamate. Signup and view all the answers

Where in the body are proteolytic enzymes produced, and why are they important?

Proteolytic enzymes are produced in the stomach, pancreas, and small intestine to hydrolyze proteins for absorption. Signup and view all the answers

What is produced as a result of transamination involving α-ketoglutarate and an amino acid?

An α-keto acid and glutamate are produced. Signup and view all the answers

What happens to the amino acid pool when amino acids are in excess?

Excess amino acids are rapidly degraded and not stored in the body. Signup and view all the answers

Which enzyme is responsible for catalyzing transamination reactions?

Aminotransferase is the enzyme responsible for transamination. Signup and view all the answers

What coenzyme is required for the function of aminotransferases?

Pyridoxal-5'-phosphate (PLP) is required as a coenzyme. Signup and view all the answers

Explain how proteins are digested in the body, particularly in relation to their size.

Proteins are too large to be absorbed directly, so they are hydrolyzed into smaller peptides and amino acids by enzymatic action. Signup and view all the answers

What roles do di- and tripeptides play in the digestion of protein?

Di- and tripeptides, formed from protein digestion, can be absorbed by the intestine and further broken down into amino acids. Signup and view all the answers

How does alanine aminotransferase (ALT) function during amino acid catabolism?

ALT facilitates the synthesis of glutamate during amino acid catabolism. Signup and view all the answers

What does aspartate aminotransferase (AST) do with amino groups?

AST transfers amino groups from glutamate to oxaloacetate, forming aspartate. Signup and view all the answers

What is the end product of oxidative deamination of glutamate?

The end product is ammonia, which contributes nitrogen to urea synthesis. Signup and view all the answers

Why does transamination not result in net deamination?

Transamination does not lead to net deamination because it involves the transfer of the amino group rather than its removal. Signup and view all the answers

What is the role of carbamoyl phosphate synthetase (CPS) in the urea cycle?

CPS catalyzes the condensation and activation of NH3 and HCO3- to form carbamoyl phosphate. Signup and view all the answers

How is citrulline formed in the urea cycle?

Citrulline is formed by the carbamoylation of ornithine, catalyzed by ornithine transcarbamoylase. Signup and view all the answers

What is the primary outcome of oxidative deamination by glutamate dehydrogenase?

The primary outcome is the conversion of glutamate into α-ketoglutarate and the release of free ammonia (NH3). Signup and view all the answers

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

N-acetylglutamate allosterically activates CPS I, enhancing its activity in the urea cycle. Signup and view all the answers

Which coenzymes can glutamate dehydrogenase utilize, and what does this imply?

Glutamate dehydrogenase can use either NAD+ or NADP+, indicating its versatility in different metabolic contexts. Signup and view all the answers

Identify the final products of the cleavage of arginine in the urea cycle.

The cleavage of arginine results in the formation of ornithine and urea. Signup and view all the answers

How does the availability of ADP influence the activity of glutamate dehydrogenase?

The presence of ADP activates glutamate dehydrogenase, signaling a need for energy generation. Signup and view all the answers

How do amino groups from amino acids enter the urea cycle?

Amino groups enter the urea cycle through the breakdown of amino acids into metabolic intermediates. Signup and view all the answers

Why is the regulation of glutamate dehydrogenase important in metabolic processes?

Regulation is crucial because it helps control the balance between energy production and amino acid metabolism based on cellular energy levels. Signup and view all the answers

Differentiate between glucogenic and ketogenic amino acids based on their degradation.

Glucogenic amino acids are degraded into glucose precursors, while ketogenic amino acids are broken down into acetyl CoA or acetoacetate. Signup and view all the answers

What is the effect of increased amino acid breakdown on glutamate concentration?

Increased amino acid breakdown leads to elevated glutamate concentration. Signup and view all the answers

Describe the role of α-ketoglutarate in amino acid catabolism.

α-Ketoglutarate serves as an intermediate in the TCA cycle and is involved in the transamination processes during amino acid catabolism. Signup and view all the answers

What are the nine essential amino acids?

Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, and Leucine. Signup and view all the answers

How is tyrosine synthesized from phenylalanine?

Tyrosine is synthesized from phenylalanine through a one-step hydroxylation process. Signup and view all the answers

Name the seven metabolic intermediates that amino acids degrade into.

Amino acids can degrade into pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, acetyl-CoA, and acetoacetate. Signup and view all the answers

What are the two nitrogen sources incorporated into urea during the urea cycle?

One nitrogen is supplied by free ammonia (NH3), and the other by aspartate. Signup and view all the answers

What metabolic pathway produces urea, and where is it synthesized?

Urea is produced in the liver through the urea cycle. Signup and view all the answers

List the four common metabolic intermediates used in the synthesis of nonessential amino acids.

Pyruvate, Oxaloacetate, α-Ketoglutarate, and 3-phosphoglycerate. Signup and view all the answers

Explain the process of transamination in amino acids metabolism.

Transamination involves the transfer of an amino group from an amino acid to an α-keto acid, forming a new amino acid. Signup and view all the answers

What is the significance of the TCA cycle in relation to glutamate dehydrogenase activity?

The TCA cycle's activity is stimulated by the flux through glutamate dehydrogenase, leading to enhanced ATP production. Signup and view all the answers

What is the role of amino-nitrogen in the body, specifically related to urea formation?

Amino-nitrogen is transformed into urea in the liver for excretion, helping to remove excess nitrogen from the body. Signup and view all the answers

Study Notes

Nitrogen Metabolism

Amino acids are not stored by the body
Amino acids are obtained from the diet, synthesized de novo, or produced from protein degradation
Amino acid catabolism is part of the larger process of nitrogen-containing molecule metabolism
Nitrogen enters the body through food
Nitrogen leaves the body as urea, ammonia, and other products of amino acid metabolism
Body proteins are involved in these transformations through the amino acid pool and protein turnover

Amino Acid Pool

Supplied by three sources: degradation of body proteins, dietary protein, and synthesis of nonessential amino acids from metabolic intermediates
Depleted by three routes: synthesis of body protein, use as precursors of essential nitrogen-containing molecules, and conversion to glucose, glycogen, fatty acids, ketone bodies, or CO2 + H2O

Dietary Protein Digestion

Proteins are generally too large for absorption by the intestines (except for maternal antibodies in breast milk)
Proteins must be hydrolyzed into di- and tripeptides and individual amino acids for absorption
Proteolytic enzymes are produced by the stomach, pancreas, and small intestine

Stomach's role in protein digestion

Gastric juice contains hydrochloric acid and the proenzyme pepsinogen
Hydrochloric acid kills bacteria and denatures proteins, making them more susceptible to protease hydrolysis
Pepsinogen is activated to pepsin, either by hydrochloric acid or autocatalytically by other pepsin molecules
Pepsin cleaves dietary proteins into peptides and a few free amino acids

Pancreas' role in protein digestion

Large polypeptides from the stomach are further cleaved into oligopeptides and amino acids by pancreatic proteases
Proteases exhibit different specificities for amino acid R-groups adjacent to the susceptible peptide bonds
Pancreatic zymogens are synthesized and secreted as inactive forms, and their activation is mediated by cholecystokinin and secretin

Small Intestine's role in protein digestion

The lumen of the small intestine contains aminopeptidases and other exopeptidases, further cleaving oligopeptides into smaller peptides and free amino acids

Overall Nitrogen Metabolism

The two key choices for amino acid catabolism are reuse and the urea cycle
Amino groups are removed from amino acids through transamination, followed by oxidative deamination by glutamate dehydrogenase
Ammonia is produced during oxidative deamination
Ammonia is transported to the liver from most tissues
Amino groups are used in synthesis, or the carbon skeletons are converted to other metabolic products for energy or biosynthesis
Nitrogen is excreted in the form of urea

Urea Cycle

One nitrogen of the urea molecule comes from free ammonia and the other from aspartate
The carbon and oxygen atoms of urea come from carbon dioxide
Urea is formed in the liver and transported to the kidneys for excretion

Nitrogen Removal from Amino Acids

Removing the a-amino group is crucial for energy production from amino acids
The removed nitrogen can be incorporated into other compounds or excreted
Amino acids are degraded to one of seven metabolites

Breakdown of Amino Acids

Amino acids are degraded to compounds that contribute to energy production or gluconeogenesis
Amino acids can be classified as glucogenic (degraded to glucose precursors) or ketogenic (degraded into acetyl CoA or acetoacetate, precursors of fatty acids or ketone bodies)

Amino Acid Biosynthesis

Essential amino acids cannot be synthesized in sufficient quantities by the body and must be obtained from the diet
The essential amino acids include phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, and Lysine.
Nonessential amino acids can be synthesized from common metabolites; tyrosine synthesis occurs by hydroxylation of phenylalanine.

Transamination

First step in amino acid catabolism; involved in transferring an amino group from amino acid to alpha-ketoglutarate
Produces an alpha-keto acid and glutamate
Enzymes are called aminotransferases (ALT, AST)

Oxidative Deamination

Glutamate is oxidatively deaminated by glutamate dehydrogenase, releasing ammonia (NH3)
Glutamate dehydrogenase can use NAD+ or NADPH as a coenzyme
This reaction occurs primarily in the liver and kidneys and plays a role in the production of urea

Regulation of Urea Cycle

The urea cycle is regulated by substrate availability
CPS I activity is allosterically regulated by N-acetylglutamate
N-acetylglutamate synthesis is stimulated by increasing glutamate concentration

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Description

This quiz covers key concepts in amino acid metabolism, including the functions of hydrochloric acid in digestion, the activation of pepsinogen, and the role of pancreatic proteases. It also explores nitrogen removal from amino acids and the transformation of amino-nitrogen into urea. Test your understanding of these essential biochemical processes.