Protein and Amino Acid Metabolism - Lecture Notes PDF

PROTEIN AND AMINO ACID Dr Norsyahida Mohd Fauzi Lecturer in Biopharmacy and METABOLISM Pharmacology Discipline, Member of Drug and Herbal Research Centre ...

PROTEIN AND AMINO ACID Dr Norsyahida Mohd Fauzi Lecturer in Biopharmacy and METABOLISM Pharmacology Discipline, Member of Drug and Herbal Research Centre  AMINO ACID Amino acids (a.a) are not stored by the body. A.a must be obtained from: -diet -synthesized de novo -normal protein degradation Excess a.a are rapidly degraded. Amino acid as sources of energy: -glucose and fatty acids are the main source of energy within the body. -a.a oxidation can provide energy as well. -this increases with excessive dietary protein or during starvation/ disease where normal sources are not available.  NITROGEN METABOLISM A.a catabolism is part of the larger process of the metabolism of nitrogen-containing molecules. Nitrogen enters the body in a variety of compounds present in food, the most important being amino acids contained in dietary protein. Nitrogen leaves the body as urea, ammonia, and other products derived from a.a metabolism. The role of body proteins in these transformation involves 2 important concepts: 1) The amino acid pool 2) Protein turnover  AMINO ACID POOL This pool is supplied by 3 sources: - Degradation of body protein - Dietary protein (essential a.a) - Synthesis of nonessential a.a. Amino pool is depleted by 3 routes: - Synthesis of body protein - Consumed as precursor of N- containing small molecule - Conversion to glucose, glycogen, fatty acids and ketone bodies or CO2+H2O  N-BALANCE In healthy, well fed adult the amount of a.a contained in the pool is constant. -N ingested = N excreted -Synthesis = degradation Positive N balance Baby, growing children, pregnancy, sick individuals recovering from long illness: - N ingested > N excreted - Synthesis > degradation Negative N balance Lack of essential a.a, fasting, starvation, malnutrition and metabolic stress (trauma, infection, surgery, fracture, severe burn) - excretion > ingestion - catabolism > synthesis  PROTEIN TURNOVER Proteins are constantly being synthesized and degraded Rate of turnover - In healthy adult, the total amount of protein remain constant, because the rate of protein synthesis is just sufficient to replace the protein that is degraded (protein turnover). - Varies among individuals  PROTEIN TURNOVER Protein degradation: a) Ubiquitin-proteasome proteolytic pathway -protein covalently attached to ubiquitin (a small, globular non- enzymic protein) - The consequitive addition of ubiquitin moieties generates polyubiquitin - Recognized by proteasome - Protein is unfolds, deubiquitinates and cuts into fragments and eventually a.a, - A.a then enter a.a pool.  PROTEIN TURNOVER b)Chemical signals for protein degradation Proteins chemically altered ( by oxidation/ubiquitin) are preferred to be degraded. The half-life of protein is influenced by the nature of the N- terminal residue E.g., Proteins that have: –Serine as N-ter: long-lived, half-life> 20h –Aspartate as N-ter: half-life 3 min –PEST (proline, glutamate, serine, threonine)-rich sequence: rapidly degraded  DIGESTION OF DIETARY PROTEINS Most of nitrogen in the diet is consumed in the form of protein. Proteins are too large to be absorbed by intestine (except antibodies from breast milk). Therefore protein has to be metabolized to di- or tripeptide and a.a. Proteolytic enzymes are produced from different organs: stomach, pancreas and small intestine. DIGESTION OF DIETARY PROTEIN Stomach: secrete gastric juice- HCl+ pepsinogen Pancreas: secrete pancreatic enzymes. - deficiency in pancreatic secretion: steatorrhea and undigested protein in faeces  DIGESTION OF DIETARY PROTEIN Small intestine: - Aminopeptidase (exopeptidase) in luminal surface of intestine, cleaves N-terminal residue from oligopeptides to produce smaller peptides and free a.a. - Celiac disease: immune-mediated damage to small intestine in response to ingestion of gluten ( a protein found in barley, wheat and rye)  DIGESTION OF DIETARY PROTEIN Absorption of a.a and small peptides - Free a.a are taken into the enterocytes by Na+-linked secondary transport system of the apical membrane. - Di and tripeptides are hydrolyxzed in the cytosol to a.a before enter the portal system by facilitated diffusion.  REMOVAL OF NITROGEN FROM AMINO ACIDS The presence of the α-amino group keeps amino acids safely locked away from oxidative breakdown. Removing the α-amino gp is an obligotory step in catabolism of all a.a. Once removed, nitrogen can be incorporated into other compounds or excreted  REMOVAL OF NITROGEN FROM AMINO ACIDS Transamination 1.Transfer α-amino gp to α-ketoglutarate. -Reaction catalyzed by aminotransferase (transaminases-found in cytosol & mitochondria- liver, kidney, intestine and muscle) -Cofactor: pyridoxal phosphate 2.Product: α-ketoacid (derived from the original amino acid) and glutamate (will then undergo oxidative deamination) 3.Reversible process Involved in the degradation and synthesis of amino acids. 4.All a.a (except lysine and threonine) participate in transamination at some point in their catabolism  TWO MOST IMPORTANT AMINOTRANSFERASE Alanineaminotransferase(ALT) Aspartateaminotransferase(AST) Liver disease - Plasma AST and ALT are elevated Nonhepatic disease - Myocardial infarction - Muscle disorders  OXIDATIVE DEAMINATION The oxidative removal of the amino group from amino acids resulting in ketoacids and ammonia. Reaction (in liver and kidney) Catalyzed by : Glutamate dehydrogenase (GD) Coenzyme: NAD+ or NADP+ Allosteric regulators - GTP: allosteric inhibitors of GD - ADP: allosteric activators of GD - (when ATP↓, a.a degradation by GD ↑)  TRANSPORT OF AMMONIA TO THE LIVER TWO mechanisms FIRST MECHANISM In most tissues Use glutamine synthetase to combine ammonia (NH3) with glutamate to form glutamine. Glutamine is transported in the blood to the liver. Cleaved by glutaminase to produce glutamate & free NH3  TRANSPORT OF AMMONIA TO THE LIVER SECOND MECHANISM In muscle Involves transamination of pyruvate to alanine. Alanine is transported by the blood to the liver In the liver, the pathway of gluconeogenesis can use the pyruvate to synthesize glucose, which can enter the blood and be used by muscle (glucose-alanine cycle)  UREA CYCLE Urea is the major form of amino gp derived from a.a. Accounts for 90% of N-containing components of urine. Urea molecule: - 2 nitrogen:1 nitrogen from free ammonia, 1 nitrogen from aspartate. - 1 carbon and 1 oxygen: from CO2 Urea is produced by liver, and then is transported in the blood to the kidneys for excretion in the urine.  FATE OF UREA 1) Urea diffuses from the liver, transported in the blood to the kidneys–excreted in urine. 2)Diffuse from the blood into the intestine, cleaved to CO2 + NH3 by bacterial urease. Ammonia is partly lost in - Faeces - Reabsorbed into blood Hyperammonaemia in kidney failure -↑ plasma urea level, ↑ urea in gut, ↑urease activity. -Treatment: Neomycin. ↓number of intestinal bacteria ,↓ammonia WHAT HAPPENS DURING FASTING?  DURING FASTING During fasting, the liver maintains blood glucose levels. Amino acids from muscle protein are a major carbon source for the production of glucose by the pathway of gluconeogenesis. As amino acid carbons are converted to glucose, the nitrogens are converted to urea. Thus, the urinary excretion of urea is high during fasting.  DURING FASTING As fasting progresses, however, the brain begins to use ketone bodies, sparing blood glucose. Less muscle protein is cleaved to provide amino acids for gluconeogenesis. Decreased production of glucose from amino acids is accompanied by decreased production of urea.  METABOLISM OF AMMONIA Ammonia level must be kept low Hyperammonemia toxic to CNS Therefore, there must be a metabolic mechanism by which nitrogen is moved from peripheral tissue  liver urea, while at the same time maintaining the low levels of circulating ammonia. SOURCES OF AMMONIA 1. From glutamine 2. From bacterial action in intestine 3. From amines 4. From purine and pyrimidines  SOURCES OF AMMONIA 1. From glutamine Kidney and intestine: - ammonia is generated from glutamine by renal/intestinal glutaminase and glutamate dehydrogenase - Excreted in urine as NH + 4 2. From bacteria: - Bacterial urease in the lumen of intestine  SOURCES OF AMMONIA 3. From amines - Diet - Hormones and neurotransmitter monoamines – amine oxidase 4. From purine and pyrimidines - In the catabolism of purines and pyrimidines, amino gp attached to the rings are released as ammonia.  TRANSPORT OF AMMONIA IN THE CIRCULATION Although ammonia is constantly produced in tissues, it is present at very low levels in blood. This is due to: - Rapid removal of blood ammonia by the liver - Many tissues including muscle, release amino acid nitrogen in the form of glutamine or alanine, rather than free ammonia.  TRANSPORT OF AMMONIA 1. Urea - Formation of urea in the liver is the most important disposal route for ammonia. - Urea (liver)kidneyurine 2. Glutamine - Nontoxic storage and transport form of ammonia - Occurs primarily in muscle & liver. - Important for removal of ammonia in brain - In plasma: [glutamine] higher than other a.a - Removed by: liver and kidney - Deaminated by: glutaminase Synthesis of glutamine METABOLISM OF AMMONIA  HYPERAMMONEMIA Hyperammonaemia –Normal Serum ammonia level = 5-35 M –Hyperammonaemia: neurotoxic on CNS –Symptoms: tremors, slurring of speech, somnolence (sleepy, drowsy), vomiting, cerebral oedema, blurring of vision –High conc: coma & death TWO major types of hyperammonaemia are: 1)Acquired Hyperammonaemia -liver disease (viral hepatitis) 2)Congenital Hyperammonaemia -ornithine transcarbamoylase deficiency : X-linked -arginase deficiency -treatment: restriction of dietary protein, phenylbutyrate