Post-absorption Processing of Protein (RCSI, 2024) PDF

Royal College of Surgeons in Ireland – Medical University of Bahrain Post absorption processing of protein Module : GIHEP Class: MedYear2 semester 1 Lecturer : Paul O’Farrell Date : September 2024  Learning objectives Describe the functions of amino-acids in the body Discuss...

Royal College of Surgeons in Ireland – Medical University of Bahrain Post absorption processing of protein Module : GIHEP Class: MedYear2 semester 1 Lecturer : Paul O’Farrell Date : September 2024  Learning objectives Describe the functions of amino-acids in the body Discuss the synthesis of essential and non-essential amino-acids Discuss the role played by transaminase enzymes Explain the role of amino-acids as substrates in gluconeogenesis, ketogenesis and in energy-yielding metabolism Describe ureogenesis and the elimination of amino nitrogen from the body Describe a pathophysiological example, eg OTC Deficiency (OMIM #311250) Amino acid structure α-carboxyl group Amino acid identity O depends on R-group R CH C O- NH3+ α-amino group Amino acid structure α-carboxyl group Amino acid identity O depends on R-group R CH C O- NH3+ α-amino group Amino acid structure α-carboxyl group Amino acid identity O depends on R-group R CH C O- NH3+ α-amino group Function of amino acids Precursors for or constituents of:- Proteins – enzymes, receptors, hormones and transport proteins Biologically active smaller compounds – haem glycine – nucleic acids – purines and pyrimidines aspartate, glycine & glutamine – hormones eg thyroxine – neurotransmitters eg dopamine, catecholamines (adrenaline, noradrenaline), serotonin, glutamate Protein turnover and the amino acid pool ~12 kg ~100g Meisenberg and Simmons 1st ed Protein turnover and the amino acid pool ~12 kg ~100g Meisenberg and Simmons 1st ed How are proteins degraded? Lysosomal degradation proteaseomal degradation Maintenance of circulating amino acid levels Steady-state concentration of amino acids in circulation between meals depends on net balance between: – utilisation by tissues – release from amino acid ‘stores’ Two principal organs involved in maintaining Amino acid levels: – Muscle – provides amino acids to circulation in the fasting state – Liver - Urea Cycle, utilisation of NH3 Amino acid degradation Glucogenic or O Ketogenic R CH C O- catabolism NH3+ NH4+ toxic Ammonium Urea Nitrogen Keto acid Amino acid Transamination – transfer of amino groups between amino acids Amino acid Keto acid Nitrogen Keto acid Amino acid Transamination – transfer of amino groups between amino acids Carried out be aminotransferase enzymes (transaminases) usually named after the NH2 donor aa – because acceptor is almost always α- ketoglutarate (eg alanine aminotransferase = glutamate pyruvate transaminase) Amino acid Keto acid Thus, glutamate is ‘collector’ of amino groups from many amino acids Equilibrium constant close to 1  these enzymes can function in both directions: degradation and synthesis Nitrogen Keto acid Amino acid Transamination – transfer of amino groups between amino acids Carried out be aminotransferase enzymes (transaminases) usually named after the NH2 donor aa – because acceptor is almost always α- ketoglutarate (eg alanine aminotransferase = glutamate pyruvate transaminase) Amino acid Keto acid Thus, glutamate is ‘collector’ of amino groups from many amino acids Equilibrium constant close to 1  these enzymes can function in both directions: degradation and synthesis Nitrogen Transamination – transfer of amino groups between amino acids Carried out be aminotransferase enzymes (transaminases) usually named after the NH2 donor aa – because acceptor is almost always α- Lippencotts 19.7 ketoglutarate (eg alanine aminotransferase = glutamate pyruvate transaminase) “carbon skeleton” Thus, glutamate is ‘collector’ of amino groups from many amino acids Equilibrium constant close to 1  these enzymes can function in both directions: degradation and synthesis Transamination Reactions of catabolism tend to generate glutamate [except aspartate aminotransferase – aspartate provides amide for synthesis of urea Lippencott’s 19.7 So AST usually runs in the other direction - making aspartate from oxaloacetate] Glutamate acts as something of a universal amino donor in anabolism Glucose-Alanine cycle Alanine synthesized in muscle through transamination circulates to the liver where it is used in gluconeogenesis and its nitrogen eliminated as urea Glutamate dehydrogenase α-ketoglutarate collects amides as glutamate Oxidative deamidation of glutamate in the liver by glutamate dehydrogenase releases ammonia and α- ketoglutarate to be recycled most amino acid nitrogen can be released as ammonia through this pathway; and thus releases ‘carbon backbones’ for gluconeogenesis Lippencotts 19.11 (mod) Direction of reaction depends on conc of reactants (ie can synthesise glutamate) Allosteric control : ATP, GTP inhibitors ADP, GDP activators Thus, when energy is low, increased aa degradation allows carbon skeletons to be used for energy Ammonia Ammonia is a neurotoxin Hyperammonaemia: Slurring of speech Blurred vision Tremor Mental confusion  coma  death Sources of ammonia: Amino acids via transamination and glutamate dehydrogenase From urea in gut via bacterial urease From amines, purines, pyrimidines ( glutamine) Glutamine via glutaminase Disposal : As Urea via the Urea cycle (Liver) Hyperammonaemia Acquired : liver disease Heriditary: deficiencies of enzymes in urea cycle Transport of nitrogen to Liver glutamine synthethase combines glutamate with ammonia in most tissues Transported in blood to liver  Converted to NH3 and glutamate by glutaminase in aspartate oxaloacetate liver AST a-ketoglutarate Alanine formed by transamination of pyruvate in muscle » Transported in blood to liver  Converted to pyruvate and glutamate by transamination in liver  Pyruvate used to make glucose by gluconeogenesis; glucose can re-enter bloodstream to be used by muscle :: the glucose-alanine cycle carbamoyl phosphate synthase: NH3 to carbamoyl phosphate  enters urea cycle Aspartate transaminase : Glutamate to aspartate  enters urea cycle The urea cycle Urea : Synthesised in liver Water soluble – transported in blood Excreted by kidneys The urea cycle Aspartate + NH3 +CO2 +3ATP  urea + fumarate +2ADP +Amp +2Pi +PPi +3H2O Five enzymes: –(Carbamoyl phosphate synthase I)* –Ornithine transcarbamoylase* –Argininosuccinate synthase –Argininosuccinate lyase –Arginase Carbamoyl phosphate synthase is rate limiting step: * In mitochondrion Lippencott’s 19.14 Pathophysiological Example: Ornithine Transacarbamoylase Deficiency OTC deficiency is the most common deficiency of the Urea Cycle X-linked ; approx. 20% female carriers exhibit symptoms Leads to hyperammonaemia Symptoms include Anorexia, Irritability, Lethargy, Disorientation, Coma, Death The disease varies in severity; worst cases – neonatal death Treatments include – Low protein diet – Medications to enhance nitrogen excretion – Liver transplant, in severe cases – Has been the target of gene therapy experiments What about the ‘carbon skeleton’? Glucogenic metabolism  pyruvate or TCA cycle intermediates: gluconeogenesis Ketogenic metabolism  acetoacetate or precursor : ketone bodies Lippencott’s 20.2 Essential and non-essential Essential Non-essential Arginine Alanine Histidine Asparagine Isoleucine Aspartate Leucine Cysteine* Lysine Glutamate Methionine Glutamine Phenylalanine Glycine Threonine Proline Tyrptophan Serine Valine Tyrosine* Amino acid biosynthesis and interconversion Essential Amino-acids: 8 essential Amino acids – Ile; Leu; Lys; Met; Phe; Thr; Try; Val 2 semi-essential Amino acids – Arg & His [essential for children] Non-essential Amino acids 2 formed from essential Amino acids – Cys Tyr 8 formed from metabolic intermediates - Ala, Asp, Glu, Gln, Asn, Pro, Ser, Gly Synthesis of non-essential aa alanine pyruvate transaminase aspartate oxaloacetate transaminase glutamate α-ketoglutarate transaminase, GDH asparagine aspartate synthase glutamine glutamate synthase proline glutamate cyclisation serine 3-phosphoglycerate series of reactions glycine serine Serine hydroxymethyl transferase tyrosine phenylalanine Phenylalanine hydroxylase cysteine Methionine, serine series of reactions Synthesis of non-essential aa from metabolic intermediates glucose glycine 3-PG serine Asparagine glutamine proline α-ketoglutarate NH3 (glutamate dehydrogenase) Tyrosine synthesis Tyrosine synthesised from (essential) phenylalanine If this reaction is NAD+ limited – restricted conversion or Dihydrobiopterin reductase restricted dietary availability of Phe - Tyr becomes NADH essential Inability to synthesise Tyr leads to phenylketonuria (PKU) (Inherited errors of metabolism lecture) Cysteine and methionine metabolism Cysteine synthesised ? from (essential) methionine and serine Sulphur from Met, and the rest from serine Homocysteine linked to heart disease Meisenberg and Simmons 26.14 Protein metabolism overview proteolysis: degradation of protein into amino acids Protein protein proteolysis protein synthesis: synthesis of synthesis proteins from amino acid amino acids glucose gluconeogenesis amino acid catabolism glucogenic amino acid degradation of amino acids into amino metabolism pyruvate group and carbon backbone glucogenic & ketogenic ketogenic amino acid metabolism amino acid synthesis acetyl coA synthesis of amino acids by gluconeogenesis transamination of metabolic transamination intermediates Kreb’s cycle transamination urea cycle urea cycle: removal of ammonia urea reading Lippencott’s Ch. 19 and 20 Meisenberg and Simmons Ch. 26