Amino Acid Metabolism 1 2021 PDF

Dr Mahani Mahadi © 2019, University of Cyberjaya. Please do not reproduce, redistribute or share without the prior express permission of the author.  Alhamdulillahirabbil 'alamiin. All praises be to Allah, the Lord of all the worlds. Blessings and Peac...

Dr Mahani Mahadi © 2019, University of Cyberjaya. Please do not reproduce, redistribute or share without the prior express permission of the author.  Alhamdulillahirabbil 'alamiin. All praises be to Allah, the Lord of all the worlds. Blessings and Peace be upon Muhammad the Messenger of Allah, the Seal of the Prophets. O Allah, we thank You for the pleasure of having the knowledge that is blessed by You. We beseech You to bless our teachers and parents with Your Guidance. We pray for Your Guidance to become righteous students who are always close to You, and who also bring happiness to our teachers and parents. Amin Ya Rabbal 'alamiin. Amino Acid Metabolism 1: AA catabolism  Learning Outcomes At the end of this lecture, students should be able to: Explain how protein are digested and degraded. explain on urea is made and excreted. Describe the urea cycle and list the enzymes involved.  Amino acids and proteins Protein are the most abundant organic compounds, constitute around 10-12 kg of body dry weight. performed a wide variety of functions. Is a nitrogen-containing macromolecules. degraded into individual amino acids. Proteolysis is a complete protein degradation to free amino acids. Protease s and peptidas es (proteolytic enzymes) Degradation of protein Some proteins are degraded by ubiquitin- proteasome complex. Ubiquitin, a small protein, in all eucaryotic cells. Protein designated for degradation in the proteasomes is marked by ubiquitin. The tagging reaction is catalyzed by enzymes called ubiquitin ligases. Once a protein is tagged, this is a signal to ligases to attach and allows to degrade the  Metabolic Pool of Amino Acids Amino acids pool is maintained by input and output sources. A.A are not stored by the body!!!! Sources: turnover of body protein, dietary protein intake, synthesis of non-essential a.a. a) protein turnover b) Dietary protein c) Synthesis non - 300-400 g of body - regular loss of essential a.a protein per day nitrogen due to constantly degradation of a.a -11 out of 20 a.a degraded and - 30-50g of protein lost can be synthesized everyday synthesized by the - E.g digestive enzyme, plasma - Supply a.a for body. protein, collagen synthesis protein and other nitrogen - Protein turnover is compound. influenced by - No storage of a.a – many factors; e.g excess will converted ubiuquitin- small to glucose/fat to polypeptide tags provide energy, or with protein to loss as urea and facilitates excreted. degradation. Utilization of amino acid from body pools 1) Degradation of protein e.g enzymes, hormones, immunoglobulin 2) Production of most important nitrogenous compounds e.g porphyrins, purines, pyrimidines 3) Produces around 10-15% of body energy  Converted to carbohydrates and fats when protein consumption is in excess of the body requirements.  Major Functions of Amino Acids Derived from Dietary Protein Substrates (precursors) for biosynthesis of other nutrient/ energy supply Glycogenic amino acids: --Blood glucose--Energy Ketogenic amino acids: -Acetyl CoA-Stored fat-Energy Biosynthesis of nitrogen-containing metabolites Hemoglobin DNA/ RNA – purine / pyrimidine Neurotransmitter – GABA, Creatine Others Histamine Melanin/ melatonin Hormone – tyrosine Neurotransmitter – acetylcholine Antibody receptors  Overview of Amino Acid metabolism 1 2  anabolism catabolism Overview of Amino Acid metabolism  Significant Degradation of Protein- (Catabolism of Amino acids) Amino acids produced by catabolism under three circumstances: Protein amino-acid residues from normal turnover are recycled to generate energy and molecular components Dietary amino acids that exceed body’s protein synthesis needs are degraded Proteins in the body are broken down to supply amino acids when carbohydrates are in short supply e.g starvation, diabetes mellitus  Metabolism of A.A: 1)Removal of Amino group Removal of amino group is a crucial step in the amino acid catabolism. The nitrogen of the amino groups can not be used for energy production and must be removed from the body. Amino nitrogen conversion to a urea (about 95 %), followed by urea excretion from the body via the urine. The second way is amino nitrogen releasing from glutamine in the form NH3/NH4+ in the tubular cells of the kidney (about 5 %).  1)Removal of Amino group Amino acids undergo certain reactions such as transamination followed by deamination for the liberation of ammonia. amino group ( NH2/ NH3+)of amino acids is utilized for formation of urea –> end product of protein metabolism. The carbon skeleton of a.a is first converted to keto acid which meet one / more of the following fates: 1) Utilization of energy 2) Used for the synthesis of glucose 3) Diverted for the formation of fat/ketone bodies 4) involved in production of non essential a.a Transamination ?  What is Transamination ???????? process of transfer of an amino group ( -NH2) from amino acid to α-keto acids/ α-oxoacid. Involves the interconversion of a pair of amino acids and a pair of keto acids catalyzed by a group of enzymes called transaminases ( aminotransferase) Transaminase require pyridoxal phosphate (PLP), a coenzyme from vit B6 It is a reversible process. No free NH3 liberated, only the transfer of amino groups occurs.  Structure of Pyridoxal Phosphate and Pyridoxamine Phosphate Intermediate, enzyme- bound carrier of amino groups Transamination; Important for the redistribution of amino groups and production of non- essential amino acid Involves both catabolism ( degradation) and anabolism ( synthesis) of amino acids Diverts the excess amino acids towards energy production  Glutamate is the only amino acids that undergoes oxidative deamination to liberate free NH3 for urea synthesis all amino acids undergo transamination except lysine, threonine, proline and hydroxyproline Serum transaminase is important for diagnostic purposes.  Typically, -ketoglutarate accepts amino groups L-Glutamate acts as a temporary storage of nitrogen L-Glutamate can donate the amino group when needed for amino acid biosynthesis All aminotransferases rely on the pyridoxal phosphate cofactor Transamination; Deamination Defined as removal of amino group from amino acids as NH3 Transamination involves only the shuffling of amino groups among amino acids. Results in the liberation of ammonia for urea synthesis. May occur either oxidative or non-oxidative deamination Oxidative Deamination Is the liberation of free ammonia from the amino group of amino acids coupled with oxidation. Take place in liver and kidney. Purpose; to provide NH3 for urea synthesis and α-keto acids for a variety of reaction includes energy production. Role of glutamate dehydrogenase: most of a.a are transfered to a-keto to produce glutamate. Glutamate serves as a collection centre for amino groups. Glutamate undergoes oxidative deamination catalyzed by GDH ( glutamate dehydrogenase) to liberate ammonia, utilize NAD + or NADP+ as a coenzyme. Glutamate dehydrogenase (GDH) is involved in both catabolic and anabolic reaction. The Glutamate Dehydrogenase Reaction Two-electron oxidation of glutamate followed by hydrolysis Net process is oxidative deamination of glutamate Occurs in mitochondrial matrix in mammals Can use either NAD+ or NADP+ as electron acceptor  High concentration of ammonia are toxic, it will converted to urea to disposal. High intake of protein will increase glutamate level in liver. It is converted to a-ketoglutarate and NH3. When energy level are low, degradation of glutamate occurs  provide a-keto which will enter TCA cycle to liberate energy. Non-Oxidative Deamination Some amino acids can be deaminated to liberate NH3 without undergoing oxidation. 1) amino acid dehydrases : serine, threonine and homoserine undergo non-oxidative deamination catalyzed by PLP-dependent dehydrases 2) amino acid desulfhydrases: cysteine and homocysteine undergo deamination coupled with desulfhydration to give keto acids.  Metabolism of Ammonia: Urea Cycle Ammonia is constantly being produced in the metabolism of amino acids. exist as NH4+ ion Production of NH3 occurs from the amino acids ( transamination and deamination), biogenic amines, amino groups of purine, pyrimidine and the action of intestinal bacteria on urea. Transport of ammonia between tissues and liver mostly occurs in form of glutamine or alanine. Alanine is important for NH3 transport from muscle to liver by glucose-alanine cycle. glucose-alanine cycle. Role of Glutamine: a) is a storehouse of NH3. b) serves as storage and transport of NH3. c) synthesis occur in liver, brain and muscle. Un-needed d) Freely diffusible in tissue. glutamine is e) Is synthesize from glutamate processed in and ammonia. intestines, f) enzyme; glutamine kidneys and synthetase. liver Ammonia in transported in the bloodstream safely as glutamate Continue: Function of ammonia Disposal of ammonia Is a waste product of Has 3 difference types of nitrogen metabolism nitrogen excretory form: Ammoniotelic: e.g aquatic Used for synthesis of animal, disposed off NH3 non essential amino acids, purine, Uricotelic: e.g reptiles and pyrimidines birds, NH3 converted to uric acid Maintain acid-base Ureotelic: mammal include balance man, NH3 converted to urea Urea cycle  Urea is the end product of protein metabolism Nitrogen of amino acids converted to ammonia is toxic to body, converted to urea, excreted in urine. Urea is synthesized in liver , transported to kidney for excretion. Urea has 2 amino (-NH2) groups, one derived from NH3 and from aspartate. Carbon atom is supplied by CO2.There is interrelation between urea cycle and citric acid cycle. Biosynthesis of urea involved 5 cyclic steps and 5 enzymes: 1. Synthesis of carbomyl phosphate 2. Formation of citrulline 3. Synthesis of arginosuccinate 4. Cleavage of arginosuccinate 5. Formation of urea 1.Synthesis of carbamoyl phosphate 2.Formation of citrulline Carbamoyl phosphate Synthesized from synthase 1 catalyst carbamoyl phosphate condensation NH4 ion with and ornithine by ornithine CO2 to form carbomyl transcarbomylase phosphate. citrulline then Used 2 ATP and required N transported to cytosol by acetylglutamate for its a transporter system. activity 3. Synthesis of arginosuccinate 4. Cleavage of arginosuccinate Arginosuccinate synthase Arginosuccinase cleaves condenses citrulline with arginosuccinate to give aspartate to produce arginine and fumarate. arginosuccinate. Arginine is the precursor This step require ATP for urea which is cleaved to AMP Fumarate provide a and pyrophosphate ( PPi) connect link with TCA and later broken down to cycle. phosphate (Pi) Formation of Urea Arginase cleaved arginine to yield urea and ornithine. Ornithine enter mitochondria and reuse in the urea cycle. Arginase is mostly found in liver but other four enzymes can also present in other tissue. Glutamate is Metabolized in the Mitochondria of Hepatocytes Ammonia is Re-captured via Synthesis of Carbamoyl Phosphate This is the first nitrogen-acquiring reaction Overall Reaction and Energetics Urea cycle is irreversible, consumes 4 ATP; 2ATP are used to synthesized carbamoyl phosphate and 1 ATP is converted to AMP and PPi to produce arginosuccinate. Disposal of Urea Exported to kidney and excreted. Small amount enter intestine and broken down to NH3 and CO2 by bacteria urease  loss in feces or absorbed in blood Urea Cycle N-2 from Aspartate Entry of Aspartate into the Urea Cycle This is the second nitrogen-acquiring reaction Integration between urea cycle and TCA Production of fumarate is the integrating point with TCA cycle. Fumarate is converted to malate and oxaloacetate in TCA cycle. Oxaloacetate undergoes transamination to produce aspartate which enter urea cycle. Oxaloacetate is an important metabolite  precursor for the synthesis of glucose Aspartate –Arginosuccinate Shunt Links Urea Cycle and Citric Acid Cycle  Summary: The Reactions in the Urea Cycle 1 ornithine + carbamoyl phosphate => citrulline (entry of the first amino group). citrulline passes into the cytosol. 2a citrulline + ATP => citrullyl-AMP + PPi 2b citrullyl-AMP + Aspartate => argininosuccinate + AMP (entry of the second amino group). 3 argininosuccinate => arginine + fumarate fumarate enters the citric acid cycle. 4 arginine => urea + ornithine Ornithine passes to the mitochondria to continue the cycle