Term
| Explain the concept of the amino acid pool |
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Definition
It is assumed that dietary and endogenous proteins are degraded and the resulting amino acids enter a single theoretical amino acid pool. Within the pool there is continuous exchange of nitrogen between amino acids.
Nitrogen entering the pool from food and protein degradation equals nitrogen losses from the pool to protein synthesis, excretion in urine, plus fecal and dermal losses |
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Term
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Definition
"dynamic state of body proteins", in which there is continuous degradation and resynthesis of proteins (protein turnover),while the overall content and nature of the proteins remains constant.
This concept emphasizes the need for proteins to be continuously supplied from the diet to replenish amino nitrogen losses from the amino acid pool. |
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Term
| What are the essential amino acids? |
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Definition
LIL TV To PM HA - mnemonic
Leucine
Isoleucine
Lycine
Tryptophan
Valine
Threonine
Phenylanaline
Methionine
Histidine
Arginine (conditionally essential) - capacity for its synthesis is very limited |
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Term
| What are the nonpolar aliphatic AA's |
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Definition
Glycine
Alanine
Valine
Proline
Leucine
Isoleucine |
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Term
| What are the aromatic AA's? |
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Definition
Phenylanaline
Tryptophan
Tyrosine |
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Term
| What are the polar uncharged AA's? |
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Definition
Asparagine
Glutamine
Serine
Threonine
Methionine
Cysteine |
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Term
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Definition
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Term
| What are the acidic AA's? |
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Definition
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Term
| What is a positive nitrogen balance? |
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Definition
Nitrogen intake exceeds nitrogen excretion
Positive balance leads to increases in the amount of body protein
Growing children, pregnant women, and bodybuilders have positive balance |
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Term
| What is negative nitrogen balance? |
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Definition
Nitrogen excretion is higher than nitrogen intake
This occurs in dietary protein deficiency |
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Term
| Explain how dietary protein is an important source of amino acids |
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Definition
The supply of dietary nitrogen is usually in the form of protein. Digestion of proteins to amino acids for absorption begins in the stomach and is completed in the small intestine. The stomach, the pancreas and the small intestine produce the proteolytic enzymes, most as inactive proenzymes (zymogens), which are activated. Once trypsin is formed , it will activate all the other zymogens secreted by the pancreas.
The active enzymes have different substrate specificities and no single enzyme can completely
digest a protein. They act together to initiate and complete the digestion to amino acids and small peptides, which are cleaved by peptidases of the intestinal epithelium.
Free amino acids are found in the portal vein after a protein meal. They are transported there from small intestine |
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Term
| How are amino acids transported into the cells of the kidney tubule and small intestine? |
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Definition
Amino acids are transported into the cells of the small intestines and the kidney tubules principally by active sodium dependent transport. A low intracellular sodium, maintained by the Na+/K+ ATP ase pump, facilitates the co-transport of sodium and the amino acids into the cells.
At least six different Na+/amino acid carriers are known that have overlapping specificity for
different amino acids.
These include carriers for 1. Neutral amino acids, 2. Imino acids (e.g.Proline), 3. Acidic amino acids, 4. Basic amino acids (lysine, arginine, urea cycle intermediate ornithine) and cysteine. |
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Term
| What are some abnormalities of protein digestion? |
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Definition
Individuals with deficiency in pancreatic secretions (due to e.g. cystic fibrosis, chronic pancreatitis or pancreatectomy) have protein malabsorption. This causes
abnormal appearance of undigested protein in the feces. Fat absorption is affected and foul-smelling bulky stool results. Also causes malnutrition and poor growth
Treat with oral pancreatic enzymes |
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Term
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Definition
Cystinuria is a common genetic error of amino acid transport. The transport system responsible for the re-absorption in the kidney tubules of the basic amino acid group is defective. High levels of all four amino acids appear in the urine - arginine, lysine, ornithine (urea cycle product) + cysteine (not technically a basic AA)
The clinical symptoms are related to precipitation of cystine (disulfide product of two cysteines) to
form kidney stones. The kidney stones may block the urinary tract and cause bleeding and severe pain.
Treat with large water intake and drugs to increase urine pH |
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Term
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Definition
Hartnup disease is another genetically determined, relatively rare disorder of amino acid absorption.
The transport of neutral amino acids across intestinal and renal epithelial cells is defective.
Malabsorption of the amino acids from the diet plus low renal tubular re-absorption (amino aciduria) causes essential amino acids such as tryptophan to become limiting.
Pellagra-like symptoms develop due to a deficiency of tryptophan, which is a precursor for nicotinamide
formation.
Treat with niacin |
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Term
| Explain the fates of absorbed AA's (what tissues do they go to in what amount, what they're used for.etc.) |
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Definition
Amino acids absorbed from the small intestine are transported via the hepatic portal vein directly
to the liver.
The liver has the primary metabolic function of regulating the blood concentration
and distribution of most metabolites, particularly glucose and amino acids.
57% of dietary amino acids are oxidized in the liver, 23% pass through the liver intact (mainly branch chain amino acids) and are extracted by other tissues, predominantly by muscles. 14% are used for
synthesis of liver proteins and 6% to synthesize plasma proteins.
Therefore more than half of the dietary amino acids are catabolized in the liver. Excess amino acids are not stored. Regardless of source, those not immediately incorporated into new protein are rapidly degraded, with the formation of urea. |
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Term
| Explain the glutamate dehydrogenase reaction |
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Definition
Reversible reaction of
glutamate ---> alpha-ketoglutarate and free ammonium
The forward reaction also produces NADH from NAD+
The reverse reaction (formation of glutamate) requires NADPH |
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Term
| How is the glutamate dehydrogenase reaction regulated? |
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Definition
High energy charge (lots of ATP and GTP) stimulate the synthesis of glutamate from alpha-ketoglutarate and free ammonium - high energy stimulates AA synthesis as opposed to breakdown
Low energy charge (lots of ADP and GDP) stimulate the release of the amino group from glutamate and form alpha-ketoglutarate (remember alpha-ketoglutarate is a TCA cycle intermediate - can be used to eventually generate ATP) |
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Term
| How is glutamate converted to glutamine and vice versa? |
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Definition
The glutamine synthetase reaction converts glutamate and free ammonium to glutamine. Requires ATP
Ammonia is liberated from glutamine to form glutamate by the glutaminase reaction. Water is needed in this reaction |
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Term
| Why is glutamate so important in amino acid metabolism? |
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Definition
Glutamate (and glutamine) is the main way AA's release NH4+
Glutamate is a principal amino donor to other amino
acids in subsequent transamination reactions.
The multiple roles of glutamate in nitrogen balance make it a gateway between free ammonia and the amino groups of most amino acids.
AA's can be converted to glutamate (to release NH4+) and glutamate (synthesized from alpha-ketoglutarate when energy charge is high) can be converted to other AA's |
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Term
| What are transamination reactions and why are they important? |
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Definition
Transaminations are where AA's transfer their alpha-amino group to alpha-ketoglutarate forming glutamate and an alpha-keto acid
AA's (except threonine, lysine, and proline) can undergo trasnaminations and most use glutamate and alpha-ketoglutarate
The reaction is reversible
These reactions are important because glutamate is used to release NH4+ - most amino acids can not do this directly
Also the reverse reactions can be used to synthesize other AA's from glutamate |
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Term
| What is the importance of pyridoxal phosphate (PLP) in the transamination reactions? |
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Definition
PLP, the coenzyme form of vitamin B6, is a prosthetic group on all transaminases
It is bound to the active site of the enzyme by electrostatic interactions and a by Schiff base bond with a lysine side chain of the apoprotein.
It is a necessary coenzyme as it participates directly in the reaction |
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