M1 - Biochemistry Biochemical Pathways; If you know all of this you can get an A Amino Acid Metabolism Parts I-III Dr. Diegelmann 10/2, 10/3, 10/7 1 2 Introduction and Summary of topics to be covered: OBJECTIVES Several Overviews will be presented showing various After studying the material presented in these lectures, pathways (Do Not Memorize) you should appreciate that: Amino Acids as a source of energy 1. Amino acids are important sources for energy Digestion; the 1st step in the process of utilization The 3 general reactions 2. Amino acids are the precursors of many essential bioactive amines 1) Transfer or removal of the NH3 group 3. Excess NH3 produced during the metabolism of amino acids is excreted 2) Removal of the Carboxyl group as Urea formed in the Urea cycle 3) Transfer of Methyl groups 4. Some amino acids are ketogenic, some are glucogenic and some are both Specific pathways for amino acid carbon metabolism 5. Many metabolic diseases are the result of abnormal Amino Acid Metabolism One carbon metabolism Metabolic processing of amino acid nitrogen The Urea Cycle 3 Phosphocreatine, an important energy reservoir 4 Ammonium Ion Oxidation of α Keto Acids * for ENERGY * * Here is the BIG PICTURE * 5 DO Not Memorize 6 The LIVER is the central organ responsible for AA metabolism Contribution to metabolic energy varies depending on the organism. Carnivores may obtain up to 90% of energy requirements from amino acid metabolism Vegetarians may obtain only a small fraction of their energy CENTRAL needs from amino acids. ROLE Microorganisms can also use amino acids for an energy source if they are present in their environment. Plants using photosynthesis for energy rarely, if ever, use amino acids for energy. In animals, amino acids undergo oxidative degradation during 3 different metabolic circumstances. 7 8 1) During normal synthesis & degradation of cellular proteins… some amino acids will undergo oxidative degradation if they are not Under these different circumstances, amino acids lose their amino groups needed for new protein. forming alpha-keto acids, which in turn undergo oxidation to CO2 and water. In the process ammonia is also generated and is available for the biosynthesis 2) If a person has a diet rich in protein and has excess amino acids, of amino acids, nucleotides and other biological amines. they can not be stored and will be degraded. For example, people on the Atkins diet eat abundant amounts of protein and very little carbohydrates. In addition, the carbon skeleton can eventually be converted to glucose through the citric acid cycle to provide energy. (Fig. 1) 3) During starvation or in patients with diabetes when carbohydrates are not available for energy, then protein must be used for an energy source. (Fig.1) 9 10 When protein is broken down for energy, then MOST of the energy is derived from oxidation of alpha-keto acids derived from amino acids ENERGY (Need to know) 11 12 DIGESTION Functions of Saliva • Digestion Bolus Formation Lubrication Dissolves Food Aids in Taste • Protection • Soft Tissue Repair • Moistens Mouth & Throat • Aids in Speech 13 14 Gastrin Proteins are cleaved on the amino-terminal side of aromatic amino residues Tyrosine, Phenylalanine Low pH * and Tryptophan by Pepsin. As the contents pass into the small intestine, the pancreas secretes bicarbonate to neutralize the acid and allow other protein degrading enzymes to function. 15 16 Gastrin Secretin: produced in the upper * Low pH portion of the small intestine (duodenum) and inhibits gastric acid secretion & stomach motility; Bicarbonate stimulates the pancreas to release bicarbonate ions and stimulates the gall bladder to secrete bile. 17 18 Trypsinogen (inactive) is converted to Trypsin (active) by Secretin the action of Enteropeptidase (Enterokinase), a proteolytic enzyme secreted by intestinal cells. 19 20 The zymogen Trypsinogen is converted to the active protease called Trypsin by an enzyme called enteropeptidase (enterokinase). Trypsin then cleaves proteins at sites of Lysine and Arginine on the Chymotrypsinogen (inactive Zymogen) carboxy-terminal side. Chymotrypsinogen is converted to Chymotrypsin by Trypsin. The Chymotrypsin then cleaves on the carboxy-terminal side of Tyrosine, is converted to Chymotrypsin Phenylalanine and Tryptophan. Carboxypeptidase is a "broad spectrum" (active enzyme) by the action of enzyme and makes multiple hits on remaining small peptides. Trypsin 21 22 ACUTE PANCREATITIS 23 24 Melamine Melamine is a trimer of cyanamide, and contains 66% nitrogen by mass and has been implicated in the Chinese protein export contaminations. It was added to baby formula to replace Protein and thus save money. Unfortunately, it is very toxic and has caused death and illness. Pvt. Tim Hall = Essential Amino Acids 25 26 1. TRANSAMINATION 1. Reactions Involving the Amino Group 27 28 Mechanism: PYRIDOXAL PHOSPHATE IS A CO-FACTOR Enzyme picks up amino group & transfers it to PLP Aldehyde form Amino group Aminated can then be form transferred to alpha-Keto Glu. Movie 29 30 2. Deamination Example of Non Oxidative Deamination Serine Pyruvate + Serine NH3 dehydratase & PLP L Serine Dehydratase Example of Oxidative Deamination Catalyses the irreversible non-oxidative deamination of L-serine to equimolar amounts of pyruvate and ammonia. 31 32 3. Deamidation (Direct removal of the NH3) Direct Removal 33 34 2. Reactions Involving the Carboxyl Group 35 36 General Reaction: Specific decarboxylases, utilizing PLP as a co-factor, remove the carboxyl group from the amino acid thus forming a new biological amine and liberating CO2 Examples of Decarboxylation of Amino Acids to form important biological amines 37 38 Clinical Notes: Histamine is formed from Histidine by this General Reaction of Decarboxylation. Specific decarboxylases, utilizing PLP as Recently it has been reported that patients with a co-factor, remove the carboxyl group from the amino acid thus Alzheimer's disease & Parkinson's disease have reduced forming a new biological amine and liberating CO levels of Dopamine. 2 Overproduction of Dopamine in the brain may be linked with psychological disorders such as schizophrenia. 39 40 Clinical Note: Histamine stimulates acid secretion in the stomach. Cimetidine (Tagamet), an H2 receptor antagonist and an analog of Histamine is used to treat duodenal ulcers because it blocks gastric acid secretion. 41 42 Melatonin, a sleep aid, has a very similar structure related to Clinical Note: Serotonin is a neurotransmitter in the brain Tryptophan & and causes contraction of smooth muscle of arterioles Serotonin. and bronchioles. Melatonin is also formed from Tryptophan and is a "sleep inducing" molecule. Ingestion of foods rich in Tryptophan (meat & milk) leads to sleepiness because the resulting Serotonin is also sleep-inducing. Tryptophan also reduces anxiety & depression and has been called "Nature's Prozac". Turkey meat is especially rich in Tryptophan and is the cause for that very tired feeling after Thanksgiving dinner… Melatonin 43 44 “Where our Sex Drive Comes From” TIME, Jan. 19, 2004 http://www.time.com/time/2004/sex/flash.html Tyrosine Dopamine Spermine & Spermidine are also formed by Decarboxylation reactions…. Tryptophan Serotonin They are involved in DNA packaging. Precursors are Methionine & Ornithine. 45 46 3. Transmethylation: Amino acids can also be metabolized by the transfer of methyl groups. An example would be the ultimate transfer of a methyl group Reactions from Methionine to the methyl donor S-Adenosylmethionine. A variety of enzymes called methyl transferases will then transfer the Involving methyl group to other molecules. the Methyl Group 47 48 TRANSMETHYLATION SAM Branched Chain Amino Acids: Leucine, Isoleucine & Valine, are not degraded in the liver. They are oxidized as fuels primarily in muscle, adipose, kidney & brain tissue. These tissues contain an enzyme called Branched-Chain Aminotransferase that converts these 3 amino acids to corresponding alpha keto acids. A defect in the complete catabolism of branched chain amino acids leads to the metabolic defect called Maple syrup urine disease. S-Adenosyl Methionine; an important methyl donor 49 50 Branched Chain Amino Acid Metabolism Glutamate has a Central Role 51 in many AA metabolic pathways 18-25 52 * * Degradation of Tryptophan is the most complex of all the Tryptophan degradation pathways of AA metabolism is very complex Another Example… 53 54 Conversion of Tryptophan to Serotonin Phenylalanine & Tyrosine Clinical Note: A deficiency of phenylalanine hydroxylase, the enzyme required to convert Phenylalanine to Tyrosine, can lead to the pathologic condition known as phenylketonuria PKU… Decarboxylation In the past a "diaper test" was used to detect this metabolic defect and the diet was step adjusted to contain low phenylalanine to prevent mental retardation during the first 10 years of life. Now a "neo natal" screen is used. There are 8 to 10 cases per 100,000 births. Genetic defects in other enzymes in this pathway can cause several inheritable human diseases. 55 56 PKU due to deficiency of Phenylalanine hydroxylase Dietary Sources of “diaper test” Phenylalanine Can cause mental retardation Neonatal Cheeses Screen Nuts and seeds Milk chocolate Meat (excluding fat) Poultry (excluding skin) Fish, including shellfish Milk Eggs Aspartame (NutraSweet) 57 58 Normal Metabolism of Phenylalanine Arti Pandya, M.D. Board certified in clinical genetics, pediatrics and clinical molecular genetics. 59 * 66 “Black Urine” if it accumulates “Alkaptonuria”