Metabolism of amino acids I. Josef Fontana

Transcription

1 Metabolism of amino acids I Josef Fontana EC

2 Overview of the lecture Introduction to protein and amino acids metabolism Metabolic pathways of amino acids Transamination Conversion glutamate - glutamine Oxidative deamination of glutamate Urea cycle

3 Introduction to protein and amino acids metabolism Turnover of proteins in the human body, basic reactions of amino acids

4 Proteins Very intense metabolism - daily turnover: skeletal muscle - 10 % liver - 40 % mucosa of the small intestine - 80 % Daily intake g Daily oxidation g = % E

5 Proteins Proteolysis Proteosynthesis Proteins in diet Amino acids pool Purines, pyrimidins, heme Amino acids biosynthesis Amino acids degradation Carbon skeleton Urea

6 AAs metabolism Sources of AAs: 1) diet 2) degradation of body proteins 3) de novo synthesis AAs pool Use of AAs: 1) proteosynthesis 2) degradation (energy, glucose, FA) 3) synthesis of N-compounds

7 Protein turnover is strictly regulated AAs surplus can not be stored - no storage protein! AAs serve as a fuel

8 Nitrogen balance Reflects the balance between the intake of nitrogen in food and nitrogen losses Most healthy individuals will present with the nitrogen balance in equilibrium N intake = N losses Increased amount of protein in the diet - excess amino acids are catabolized and their amino group excreted as urea or ammonia

9 Positive nitrogen balance Protein intake in the diet exceeds the protein losses During the recovery after illness, during periods of growth or during an administration of anabolic hormones

10 Negative nitrogen balance Nitrogen losses exceed its intake During starvation, severe illness or during an administration of catabolic hormones 1 g N 6.25 g proteins

11 Degradation of cell proteins Cellular proteins have different halflife Ornithine decarboxylase - 11 minutes Hemoglobin survives as long as erythrocyte Υ-Crystallin (protein of the eye lens) - the whole life

12 Regulation of cellular proteolysis Protein ubiquitin Marker for cellular protein - label for destruction Polyubiquitinisation - degradation in proteasomes

13 The Nobel Prize in Chemistry 2004 Aaron Ciechanover, Avram Hershko, Irwin Rose "for the discovery of ubiquitin-mediated protein degradation"

14 Essential and non-essential amino acids Essential AAs branched: Val, Leu, Ile aromatic: Phe, Trp basic: Lys sulfur-containing: Met with hydroxy group: Thr Conditionally essential: Arg, His Non-essential AAs Gly, Ala, Ser, Pro, Cys, Tyr, Asn, Gln, Asp, Glu

15 Important reactions of AAs Decarboxylation biogenic amines Transamination 2-ketoacids Oxidative deamination 2-ketoacids Formation of peptide bonds peptides and proteins

16 Metabolic pathways of amino acids Transamination

17 Transamination Transaminases (aminotransferases) Specific for one pair of AA and the corresponding α-keto acid Reversible reaction Pyridoxal phosphate (vit. B6 derivative) Liver enzymes: 1) ALT (alanine aminotransferase) 2) AST (aspartate aminotransferase)

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19 Alanine aminotransferase (ALT) Aspartate aminotransferase (AST)

20 Metabolic pathways of amino acids Conversion glutamate - glutamine

21 Conversion glutamate - glutamine Conversion of the carboxyl group of glutamate (in the side chain) in the amide in glutamine Glutamine synthetase (cytosol) The most important transport form of amino nitrogen in the blood Opposite reaction: glutaminase (MIT - ammonia from Gln to the urea cycle)

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23 Metabolic pathways of amino acids Oxidative deamination of glutamate

24 Oxidative deamination of glutamate Glutamate dehydrogenase Mitochondria, mainly in the liver Amino group was previously transferred to αkg by transamination - glutamate synthesis Oxidative deamination releases -NH 2 as NH 3 - restoration of αkg - goes to a new transamination

25 Oxidative deamination of glutamate

26 Formation of ammonium α-amino groups are converted to ammonium by oxidative deamination of glutamate

27 Fate of amino nitrogen derived from AAs Extrahepatic tissues 1) Transamination: forms mainly Ala and Glu + 2- oxoacids 2) Amidation: Glu + NH 3 Gln In the liver 1) Same mechanisms as in extrahepatic tissues 2) Oxidative deamination of Glu (forms NH 3 and αkg): glutamate dehydrogenase

28 Metabolic pathways of amino acids Urea cycle

29 Ammonium Conversion to urea Plasma concentration below 35 µmol/l Toxic for brain - nonpolar - freely crosses the blood brain barrier Combines with α-kg - glutamate - block of KC

30 Urea cycle Substrates: NH 3, CO 2 and aspartate Liver, excreted in kidneys Mitochondria / cytosol Carbamoylphosphate synthetase I Needs lot of energy Connected with KC via fumarate

31 Synthesis of carbamoylphosphate Carbamoylphosphate synthetase I Mitochondria NH HCO ATP

32 Synthesis of citrulline Citrulline is transported to cytosol

33 Synthesis of arginosuccinate

34 Cleavage to arginine and fumarate

35 Urea Arginine hydrolysis urea and ornithine Transport of ornithine to matrix

36 Restoration of aspartate Close association with KC - aspartate formation from fumarate Each degraded AA gives its amino group to αkg - glutamate - AST transfer to OAA - aspartate - urea cycle - urea

37 Urea cycle - KC

38 Regulation of urea cycle Carbamoylphosphate synthetase I Activated by N-acetylglutamate produced in reaction: AcCoA + Glu N-acetylglutamate synthetase: activated by arginine Protonproductive reaction - inhibited during acidosis Increased transcription in high-protein diet

39 Metabolism of amino acids II Josef Fontana EC

40 Overview of the lecture Metabolic pathways of amino acids Utilization of the amino acids carbon skeleton Formation of nonessential amino acids Important derivatives of amino acids Organ specifics of amino acids metabolism

41 Metabolic pathways of amino acids Utilization of the amino acids carbon skeleton

42 Carbon skeleton of AAs Carbon skeleton of each AA is converted by an original pathway Degradation leads to a formation of 7 intermediates: acetyl-coa acetoacetyl-coa pyruvate α-ketoglutarate succinyl-coa fumarate oxaloacetate Ketogenic AAs Glucogenic AAs

43 Aminoacids Ketogenic: Lys and Leu (begin with L) Glugogenic: serine, threonine, cysteine, methionine, aspartate, glutamate, asparagine, glutamine, glycine, alanine, valine, proline, histidine and arginine Keto- and glucogenic: isoleucine, phenylalanine, tyrosine and tryptophan

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45 7 degradation products of AAs pyruvate Gly, Ala, Ser, Thr, Cys, Trp oxaloacetate Asp, Asn -ketoglutarate Glu, Gln, Pro, Arg, His succinyl-coa Val, Ile, Met, Thr fumarate Phe, Tyr acetyl-coa Ile glucogenic AAs ketogenic AAs acetoacetyl-coa Lys, Leu, Phe, Tyr, Trp

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47 It is easy to deduce Aspartate and asparagine OAA (transamination) Glutamine and glutamate αkg (glutaminase and transamination) Alanine pyruvate (transamination) Lysine and leucine are ketogenic AcCoA and acetoacetylcoa Glycine, serine and cysteine (small AAs) converted to pyruvate

48 Degradation of branched AAs 1 st step: transamination specific transaminase activity in skeletal muscle and heart, activity in liver product: 2-oxoacids 2 nd step: dehydrogenation + decarboxylation product: acyl-coa

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50 Metabolic pathways of amino acids Formation of nonessential amino acids

51 Synthesis of AAs Essential: Phe, Trp, Val, Leu, Ile, Met, Thr, Lys Conditionally essential: Arg, His Nonessential: oxalacetate Asp, Asn 2-ketoglutarate Glu, Gln, Pro, (Arg) pyruvate Ala 3-phosphoglycerate Ser, Cys, Gly Phe Tyr

52 Tyrosine from Phenylalanine

53 Phenylketonuria AR, absence or reduced activity of phenylalanine hydroxylase Degradation of Phe: phenylpyruvate (urine) phenyllactate, phenylacetate Degradation of Phe: phenylethylamine H 5 C 6 -CH 2 - CH 2 -NH 2 (brain damage) Screening in newborns

54 Metabolic pathways of amino acids Important derivatives of amino acids

55 Decarboxylation of AAs gives monoamines (= biogenic amines) Tyr catecholamines Trp serotonin (5-hydroxytryptamine) His histamine Ser etanolamine choline acetylcholine Cys cysteamine Asp β-alanine coenzyme A Glu γ-aminobutyrate (GABA)

56 Nitric oxide

57 Nitric oxide NO-synthase (NOS) in neurons: NOS-I: neurotransmission in macrophages: NOS-II: kills bacteria endothelial: NOS-III: vasodilation Clinical correlation: nitrates in the treatment of angina pectoris hypotension during septic shock

58 Thyroid hormones HO HO Tyr I MIT Thyreoglobulin CO C CH H2 NH Thyreoglobulin CO C CH H2 NH I HO O C H C COOH H2 NH 2 I I I HO O C C H COOH H2 NH 2 I Trijodthyronin (T3) I I HO I I DIT Thyreoglobulin CO C CH H2 NH Thyroxin (T4)

59 Melanin Pigment derived from tyrosine (its oxidation and polymerization) There are two types: oculocutaneous - skin melanocytes neuromelanin - in substantia nigra of the midbrain (mesencephalon)

60 Formation of activated methionine = S-adenosylmethionine (SAM) SAM is used as -CH 3 group donor in metabolic methylations Figure is found on

61 Synthesis of creatine

62 Organ specifics of amino acids metabolism

63 Blood Total blood concentration of AAs: mm Glutamine: 0.6 mmol/l (main transport form of ammonia) and alanine: 0.3 mmol/l Ammonia: 6-35 µmol/l Urea: mmol/l Creatinine: µmol/l

64 Liver Main organ of AAs metabolism Removal of amino group from Aas Detoxification of ammonia - urea cycle and systhesis of glutamine C-skeleton metabolism - glucose, FA or ketone bodies Synthesis of non-essential AAs

65 Intestine - enterocytes Change spectrum of ingested AAs - concentration in the portal blood vary (e.g. more proline and citrulline - formed from Glu / Gln) Glutamine is an essential energy substrate for rapidly dividing cells (e.g. immune cells and enterocytes) Gln amino group enters the formation of purines, oxidation of C-skeleton gives energy Skeletal muscle is a major source of glutamine during starvation and stress

66 Skeletal muscle The main "reservoir" of proteins - use during stress and starvation Muscle changes spectrum of AAs released into the blood also (in comparison with AAs obtained by proteolysis of muscle proteins) Branched AAs transaminated in the muscle - their α-ketoanalogues are released into the blood (or are oxidized to gain E), amino groups transferred to glutamine or alanine - released to the blood

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68 Kidneys Main place of N-catabolites excretion: urea, ammonia, creatinine, uric acid etc. Tubular cells: conversion of Gln - Glu - α-kg, ammonia excreted in the urine Gluconeogenesis Conversion citrulline - arginine

69 Marasmus Kwashiorkor Inadequate intake of carbohydrates, fats and proteins not covered energy requirements of the organism Patients in the Hospice Unit appearance of "skin and bones" Treatment: nutritional intervention (enteral or parenteral), treatment of the disease Inadequate protein intake (and essential AA) with adequate energy intake Symptoms: retarded growth, loss of skin and hair pigmentation, ascites, mental apathy