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Amino Acid Metabolism Notes

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Amino Acid Metabolism Revision

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-digestion of dietary proteins in the intestine and degradation of proteins within the cell provide a steady supply of amino acids to the cell
-misfolded or damaged proteins are broken down- marked by ubiquitin
-the main use of amino acids provided by breakdown is for synthesis of proteins and other nitrogenous compounds such as nucleotide bases
-Excess amino acids can't be stored/excreted so are used as metabolic fuel-but alpha amino group is removed and the carbon skeleton is used as the metabolic intermediate-acetyl coA, acetoacetyl coA, pyruvate, intermediates of TCAconverted. The type of intermediate it is converted into is determined by the type of amino acid Types of amino acids Gluconeogenic amino acid: is an amino acid that can be converted into glucose through gluconeogenesis which happens in the liver Via pyruvate Via TCA cycle intermediates
-alanine Aspartic acid
-glycine Asparagine
-cysteine Glutamate
-threonine Ketogenic amino acid: amino acid that can be degraded directly into acetoacetate or one of its precursors (acetyl coA or acetoacetyl CoA). These can give rise to ketone bodies or fatty acids Leucine, lysine: only two amino acids which are ketogenic, their carbon skeletons are not substrates for gluconeogenesis so can't give rise to net formation of glucose. Mixed amino acids: can give rise to glucose and ketone bodies. E.g isoleucine, phenylalanine, tyrosine, tryptophan "TTIP" Amino acids with specific roles: Gluconeogenic precursors: Alanine, glutamine Regulation of energy balance: Leucine, isoleucine, valine Nitrogen excretion, creatine: Arginine,
-the amino groups released are converted into urea

CLASSIFICATION OF AMINO ACIDS Essential amino acids: cannot be synthesised de novo and must be supplied from the diet. If they are not taken up they will not be available for protein synthesis The amino acids that are essential in diet are 'Any Help in Learning These Little Molecules Proves Truly Valuable Arginine Histidine Isoleucine Leucine Threonine Lysine Methionine Phenylalanine Tryptophan Valine
-identification of essential amino acids was done through various experimentsomition of essential amino acids from the diet resulted in a negative nitrogen balance. More nitrogen was excreted than nitrogen ingested. This is because there is a constant turnover of proteins with an obligatory loss of amino acids, so the essential amino acids have to be replaced via the diet. If there is a deficiency in the dietary intake of essential amino acids and intake doesn't match breakdown then the spare amino acids released from protein breakdown can't be re-utilised into other proteins due to the lack of essential amino acids. So, more amino acids are broken down, leading to negative nitrogen balance.
-positive nitrogen balance: is when more nitrogen is ingested than excreted. This occurs during childhood when protein is needed for growth and little protein is used for metabolism. It also occurs during pregnancy. Essential endogenous synthesis: Arginine- it has to made in the body as level in required in the body are not met by the diet AMINO ACID METABOLISM Digestion of protein
-70/100g protein are ingested per day
-30-200g digestive enzymes and instestinal cell proteins (in addition to the dietary intake of 90g of protein a day human gut hydrolyses 16g of digestive enzymes that are secreted into the lumen, 50g of proteins shed from the intestinal mucosa
-proteins are broken down by peptidases
-Endopeptidases: (break down from inside the chain)
-Exopeptidases (break peptide bonds from the end of the chain)- there are two types carboxypeptidases and aminopeptidases

-products are single amino acids/di and tripeptides which are taken up by intestinal cells which are then converted into free amino acids which are then released into circulation. Digestion of dietary proteins protein digestion begins in the stomach 1) Acid secretion by parietal cells (acid secreting cells of the stomach wall)
-K+,H+-ATPase is a specific pump in parietal cells that pump H+ into the lumen and allows high hydrogen gradient to be generated in the lumen. The lumen pH falls to less than 1
-The acidic environment dentarues the proteins into random coils which are more accessible for proteolysis 2) Pepsin
-The chief cells of the stomach secrete pepsinogen, inactive precursor f pepsin. The activation of pepsinogen is stimulated by hydrogen ions which cleaves peptide bond resulting in the formation of catalytically active pepsin. Autocatalytic activation follows with pepsin activating other pepsinogen molecules.
-pepsin is an endopeptidase Digestion of protein in the intestinal lumen
-movement of partially digested chime through the duodenum stimulates release of 2 hormones: secretin and cholecystokinin and enzyme enteropeptidase
-Secretin: stimulates the release of pancreatic juice-alkaline secretion that is enriched in bicarbonate. The bicarbonate neutralises the HCL in the chyme as it enters the small intestine
-cholecystokinin: stimulates the release of of several inactive zymogens from the exocrine pancreas. Some of the zymogens are precursors of endopeptidases (trypsinogen, chymotrypsinogen, proelastase) and other precursors of exopeptidases (procarboxypepidase A, procarboxypeptidase B)
-Enterpeptidase/ enterokinase - this enzyme initiase cascade of proteolytic events that leads to the activation of pancreatic zymogens. Enteropeptidase hydrolyse a single peptide bone in trypsinogen resultin in the release of small peptide from the amino terminus. The removal of peptide from trypsinogenconformational change- catalytically active trypsin- catalyses conversion of all remaining zymgoens to their active form Specificity of the pancreatic proteases:

-each of the pancreatic proteases has a different specificity- the products of some enzymes are substrates for others. Produces a mixture of free amino acids, small peptides. Intestinal aminopeptidases and dipeptidases:
-intestinal mucosal cells contain enzymes that catalyse the terminal steps in protein digestion. The intestinal enzymes are found both in the brush border membrane and cytosol of intestinal cells.
-The amino acids/ oligopeptides are transported into the intestinal cells from the lumen and then released into the blood for absorption by other tissues
-75% of the amino acids entering the portal circulation from the GI tract after a meal are metabolised in the liver and 25% enters the general circulation Fate of amino acids Body proteins Derivatives: Hormones, neurotransmitters, pigment Conversion to glucose/fatty acids for storage as glycogen or triglyceride
-The amino acids deaminated in the liver and their carbon skeletons are used for gluconeogenesis/Ketogenesis Oxidation for energy
-Branched chain amino acids are mainly deaminated in skeletal muscleenergy

Amino acids
-Liver (all exept leucine, isolecuine, valine)
-Gut - glutamine, glutamate, aspartate
-Renal cortex- Glutamine
-Muscle- Leucine, isoleucine, valine USE OF AMINO ACIDS FOR ENERGY a) Removal of the alpha amino group
-Only the carbon skeleton can be used as an energy substrate so the amino group must be removed first
-The amino group is removed through transamination Amino acid + alpha-ketoacid  alphaketoacid + amino acid

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