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Questions and Answers
What is the first step in amino acid degradation?
What is the first step in amino acid degradation?
The urea cycle begins with the formation of carbamoyl phosphate.
The urea cycle begins with the formation of carbamoyl phosphate.
True
What role does the ubiquitin system play in protein metabolism?
What role does the ubiquitin system play in protein metabolism?
It tags proteins for destruction.
In the glucose-alanine cycle, alanine is transported from ______ to the liver.
In the glucose-alanine cycle, alanine is transported from ______ to the liver.
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Match the following components of protein metabolism with their functions:
Match the following components of protein metabolism with their functions:
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What is one source of the amino acid pool?
What is one source of the amino acid pool?
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The amino group of amino acids is transferred to pyruvate to yield alanine.
The amino group of amino acids is transferred to pyruvate to yield alanine.
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Which of the following methods is NOT involved in nitrogen removal from amino acids?
Which of the following methods is NOT involved in nitrogen removal from amino acids?
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What happens to the ammonia produced during amino acid degradation?
What happens to the ammonia produced during amino acid degradation?
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The process of removing the amino group from amino acids is called __________.
The process of removing the amino group from amino acids is called __________.
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Transaminations are irreversible reactions.
Transaminations are irreversible reactions.
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What is the role of pyridoxal phosphate (PLP) in transaminase reactions?
What is the role of pyridoxal phosphate (PLP) in transaminase reactions?
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Match the following terms with their descriptions:
Match the following terms with their descriptions:
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What determines the half-life of a normal, non-damaged protein according to the N-end rule?
What determines the half-life of a normal, non-damaged protein according to the N-end rule?
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The enzyme __________ is responsible for transferring the amino group from aspartate to an alpha-keto acid.
The enzyme __________ is responsible for transferring the amino group from aspartate to an alpha-keto acid.
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Match the transaminase with the correct amino acid it transfers:
Match the transaminase with the correct amino acid it transfers:
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Impaired protein degradation can lead to diseases such as Alzheimer's and certain types of cancer.
Impaired protein degradation can lead to diseases such as Alzheimer's and certain types of cancer.
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What is the role of ubiquitin in protein degradation?
What is the role of ubiquitin in protein degradation?
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Free amino acids found throughout the body are known as the _____ pool.
Free amino acids found throughout the body are known as the _____ pool.
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Match the following diseases to their associated protein degradation issue:
Match the following diseases to their associated protein degradation issue:
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Study Notes
Protein and Amino Acid Metabolism
- The body degrades proteins through food digestion and intracellular degradation (lysosomes, ubiquitin system).
- Amino acids are not used as energy storage; they're directly used in energy metabolism, often through the urea cycle.
- Protein turnover in cells ranges from 10 minutes to over 150 hours.
Protein Digestion
- Dietary protein is initially broken down in the mouth and stomach (denatured and partially hydrolyzed by HCl and pepsin).
- Further digestion occurs in the small intestine, where proteases break down large polypeptides into amino acids and some dipeptides.
- These amino acids are absorbed into the bloodstream.
Extraction of Amino Acids from Food
- Pancreatic enzymes (chymotrypsin, trypsin, carboxypeptidase, elastase) break down proteins into amino acids, oligopeptides, and dipeptides.
- These smaller molecules are transported across the small intestine lining and into the bloodstream.
- Peptidases further break down dipeptides and tripeptides into individual amino acids for absorption.
Amino Acid Catabolism (Liver)
- Absorbed amino acids are transported to the liver.
- The liver is the primary site for both carbon skeleton and nitrogen metabolism.
- Most nitrogen is removed by liver enzymes (transamination or oxidative deamination) to produce ammonia (NH3).
Amino Acid Transport
- Amino acids are taken into cells via facilitated diffusion, often coupled to sodium ion transport.
- Some transport systems use Na+ cotransport (symport) or Na+- dependent counter-transport (antiport).
γ-Glutamyl Cycle
- Primarily functions in kidney renal epithelial cells.
- The enzyme γ-Glutamyl transpeptidase is membrane-bound, interacting with glutathione (GSH) and amino acids.
- The cycle removes amino acids from glutathione, transports them into cells and generates γ-Glutamyl amino acids.
- Three ATP molecules are utilized in the process.
Protein Degradation (Proteases & Lysosomes)
- Proteases are enzymes that degrade proteins.
- Four major groups of proteases are: serine proteases (e.g., trypsin), aspartate proteases (e.g., pepsin), zinc proteases (e.g., carboxypeptidase), and cysteine proteases (e.g., cathepsins).
- Lysosomes are organelles containing proteases that function in an acidic environment.
- Proteins are marked for lysosomal degradation by signal sequences (like KFERQ) or glycosylation.
Ubiquitin-dependent Protein Degradation
- Proteins have specific half-lives (ranging from minutes to years).
- Misfolded or defective proteins are tagged with ubiquitin (a small protein).
- Ubiquitin is linked to the target protein, and eventually, chains of ubiquitins signal degradation in the proteasome.
Impaired Protein Degradation and Disease
- Defects in protein degradation can lead to diseases such as Alzheimer's, spongiform encephalopathies, and some forms of cancer.
- Accumulation of misfolded proteins can form aggregates that damage neurons.
Amino Acid Pool
- Free amino acids in the body are collectively called the "amino acid pool."
- Sources of amino acids include dietary proteins, hydrolysis of body proteins, and synthesis of non-essential amino acids.
- The amino acid pool is used for synthesizing proteins,specialized products, or catabolized into other products.
Fate of Amino Acids
- Amino acids can be used in anabolic pathways (synthesizing proteins, hormones, enzymes, creatine, heme, etc.)
- Or amino acids can be used in catabolic pathways through transamination and deamination to remove nitrogen and other processes to entirely degrade them into other compounds.
- Degradation products may be used in energy production or to form other metabolites like glucose, fatty acids, and ketone bodies.
Ammonia
- Ammonia is a toxic substance, primarily to the central nervous system (CNS).
- The body primarily eliminates ammonia by converting it into urea in the urea cycle, taking place in the liver.
Urea Cycle
- The urea cycle in the liver removes nitrogen from amino acids, converting them to urea for excretion.
- This cycle involves several steps and enzymes, consuming four ATP molecules per urea molecule produced.
- The cycle connects to other metabolic pathways (such as the citric acid cycle) through fumarate and aspartate.
Genetic Defects of Urea Cycle
- Defects in the urea cycle lead to elevated ammonia in the blood (hyperammonemia), which can cause severe health problems.
- Treatment strategies for these defects involve antibiotics (to reduce intestinal ammonia) and special diets.
Methods for removing excess amino acids
- The body can eliminate excess amino acids by conjugation reactions.
- Examples include reactions with benzoate or phenylacetate. Compounds formed from these reactions are excreted.
Glucose-Alanine Cycle
- The glucose-alanine cycle allows muscles to eliminate nitrogen during periods of fasting or exertion.
- This involves transamination of pyruvate to alanine, transport of alanine to the liver, and reconversion of alanine to pyruvate for gluconeogenesis and glucose production.
Non-Oxidative Deamination
- Serine and threonine can be deaminated through non-oxidative mechanisms, removing nitrogen without hydrogen removal, with the use of PLP as a coenzyme.
- Serine is converted to pyruvate.
Hydrolytic Deamination
- Glutamine and asparagine are hydrolytically deaminated by glutaminase and asparaginase respectively in the kidney.
- This process removes nitrogen from glutamine and asparagine as ammonia, helpful with maintaining acid-base balance.
Transamination
- Amino groups are transferred from amino acids to α-keto acids, catalyzed by aminotransferases.
- This process produces glutamate and an α-keto acid.
Oxidative Deamination
- Glutamate undergoes oxidative deamination by glutamate dehydrogenase, releasing ammonia.
- This process forms NADH or NADPH, and α-ketoglutarate.
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Description
Test your knowledge on protein digestion, amino acid extraction, and the metabolism of proteins in the body. This quiz covers various processes including the role of enzymes and the urea cycle. Sharpen your understanding of how dietary proteins are utilized.
