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Questions and Answers
Which pancreatic enzyme hydrolyzes peptide bonds of aromatic amino acids?
What is the process by which di- and tri-peptides are taken up into intestinal mucosa?
What is the role of α-ketoglutarate in amino acid metabolism?
Which two amino acids do not participate in transamination?
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What is the fate of absorbed amino acids in the anabolic pathway?
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What is the primary function of carboxypeptidase?
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What is the coenzyme required for all transamination reactions?
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What is the outcome of the removal of amino group from amino acids?
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What is the enzyme responsible for oxidative deamination?
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What is the byproduct of oxidative deamination?
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Which enzyme has autocatalytic activity and activates the remaining amount of trypsinogen and other pancreatic zymogens?
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What is the primary function of elastase?
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What is the purpose of glutamine synthetase?
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What is the result of the complete oxidation of α-keto acids?
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Which enzyme cleaves glutamine into glutamate and free ammonia?
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What is the significance of elevated plasma AST and ALT levels?
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What is the primary function of enteropeptidase in the digestion of dietary proteins?
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What is the pH range at which pepsin is most active?
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What is the term used to describe the process of constant synthesis and degradation of proteins in the body?
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What is the primary characteristic of High Biological Value (HBV) proteins?
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Which of the following is an example of a Low Biological Value (LBV) protein?
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What is the estimated amount of body protein that is hydrolyzed and re-synthesized daily?
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What is the function of bicarbonate in the pancreatic juice?
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What is the term used to describe proteins that are found in animal sources such as meat, poultry, and seafood?
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Study Notes
Protein Metabolism
- Proteins are large, complex molecules that play critical roles in the body.
- There are 20 amino acids that occur in mammalian proteins.
- Amino acids are the fundamental units of proteins.
Types of Proteins
- High Biological Value (HBV) proteins:
- Provide all essential amino acids.
- Also known as complete proteins.
- Found in animal sources (meat, poultry, seafood, milk, and cheese) and plant sources (soya beans).
- Low Biological Value (LBV) proteins:
- Lack one or more essential amino acids.
- Also known as incomplete proteins.
- Found in plant sources (cereals, legumes, and nuts), except for soya beans.
Protein Turnover
- Proteins in the body are constantly synthesized and degraded.
- This process, called protein turnover, leads to the hydrolysis and re-synthesis of 300-400 gm of body protein each day.
- In healthy adults, the total amount of protein in the body remains constant.
Digestion of Dietary Proteins
- In the stomach:
- HCl initiates the activation of pepsinogen into active pepsin.
- Pepsin is an endopeptidase with an optimum pH of 1-2.
- In the intestine:
- Pancreatic juice rich in bicarbonate neutralizes gastric secretion.
- Pancreatic zymogens are released, including trypsinogen, chymotrypsinogen, pro-carboxypeptidase, and pro-elastase.
- Enteropeptidase initiates activation of trypsinogen into active trypsin.
- Active trypsin has autocatalytic activity, activating the remaining pancreatic zymogens.
Pancreatic Enzymes
- Trypsin: an endopeptidase that hydrolyzes peptide bonds of basic amino acids (e.g., lysine and arginine).
- Chymotrypsin: an endopeptidase that hydrolyzes peptide bonds of aromatic amino acids.
- Carboxypeptidase: an exopeptidase that hydrolyzes peptide bonds adjacent to the free COOH group of the polypeptide chain.
- Elastase: an endopeptidase that hydrolyzes peptide bonds of small amino acids (e.g., glycine, alanine, and serine).
Absorption of Proteins
- Free amino acids are taken into intestinal mucosa by sodium-dependent active transport through Na+-dependent co-transporters.
- Di- and tri-peptides are taken up by a H+-linked transport system and hydrolyzed in the cytosol to amino acids.
Fate of Absorbed Amino Acids
Anabolic Pathways
- Synthesis of proteins: tissue proteins, enzymes, and hormones.
- Synthesis of specialized products (e.g., creatine, choline, nitrogen bases of nucleic acids).
- Synthesis of small peptides (e.g., glutathione).
Catabolic Pathways
- Removal of amino group (NH2) from amino acids by transamination and deamination processes.
- The resulting products are ammonia, urea, and α-keto acids.
- α-Keto acids are further metabolized to be completely oxidized into CO2, H2O, or converted into glucose or ketones.
Removal of Nitrogen from Amino Acids
- Removing the amino group is essential to produce energy from amino acids.
- The nitrogen can be incorporated into other compounds or excreted, with the carbon skeletons being metabolized.
Transamination
- α-Ketoglutarate plays an essential role in amino acid metabolism by accepting the amino groups from most amino acids, thus becoming glutamate.
- Glutamate produced by transamination can be oxidatively deaminated or used as an amino group donor in the synthesis of nonessential amino acids.
- All amino acids, except lysine and threonine, participate in transamination.
- All transamination reactions are reversible.
- Pyridoxal phosphate (PLP) is a coenzyme in all transamination reactions.
Oxidative Deamination
- Oxidative deamination is catalyzed by Glutamate dehydrogenase.
- It results in the liberation of the amino group as free ammonia (NH3).
- These reactions occur primarily in the liver and kidney.
Diagnostic Importance of Plasma Aminotransferases
- Elevated plasma levels of ALT and AST indicate damage to cells rich in these enzymes.
- Liver disease: Plasma AST and ALT are elevated, with ALT being more specific than AST for liver disease.
- Myocardial infarction and muscle disorders: Elevated AST only, with normal ALT plasma levels.
Transport of NH3 to the Liver
- Glutamine synthetase combines ammonia (NH3) with glutamate to form glutamine (a nontoxic transport form of ammonia).
- Glutamine is transported in the blood to the liver, where it is cleaved by glutaminase to produce glutamate and free ammonia.
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Description
This quiz covers the basics of protein metabolism, including the structure and function of proteins, amino acids, and types of proteins.
