METABOLISM OF AMINO ACIDS

Transcription

1 Dr. M. Sasvari METABOLISM OF AMINO AIDS 1. The fate of the amino group OO - 3 N 24 1

2 Key structure s Glutamine (Gln), 5 Gln Glutamate (Glu), 5 Glu a-keto-glutarate, 5 akg AMIDE N 2 O - O N 2 a 3 N 2 a O 2 a AMINO OO - AMINO OO - OO - Asn Asp OXA Asparagine (Asn), 4 Aspartate (Asp), 4 Oxaloacetate,

3 INTESTINE Food proteins AA extrahepatic tissues glucose proteins MUSLE proteases peptides peptidases LIVE Gln Ala AA AA N 3 bacteria Gln urea KIDNEY N ingested N excreted urea, N 4 (creatinine, urate) Nitrogen balance Negative: N ex > N in (starvation, diseases, senescence) 24 3 Positive: N in > N ex (children, pregnancy, re-feeding)

4 Protein digestion Proteases (1) Digestion of exogen proteins (food) Proteases are produced in zymogen form (proteolitic activation) Endopeptidases Pepsin, enteropeptidase Ser proteases (trypsin, chymotrypsin, elastase) Exopeptidases - substrate specificity - proteolytic activation - catalytic triad in the active centre Early activation (gain-of-function mutations): pancreatitis arboxipeptidases, Aminopeptidases, tripeptidases, dipeptidases Transport of amino acids: glutathion (gamma-glutamyl cycle): 3ATP/aminoacid (2) Degradation of endogen proteins (protein turnover) Extracellular: ollagenases, metalloproteases, ellastase Intracellular: catepsins (lysosomes) ubiquitination: a tag for degradation loss-of-function mutations: wide range of disease (e.g. cancer, stroke, Alzheimer) 24 4

5 Dr. M. Sasvari METABOLISM OF AMINO AIDS 1. The fate of the amino group (summary) OO - 1. Transamination 3 N 2. Deamination 3. Incorporation of free ammonia into organic compound 24 5

6 Dr. M. Sasvari METABOLISM OF AMINO AIDS 1. The fate of the amino group (summary) OO - 1. Transamination 3 N 2. Deamination 3. Incorporation of free ammonia into organic compound 24 6

7 Transamination A reversible exchange of an amino- and an oxo group OO - OO - 3 N O a-amino acid (except: Tyr, Lys) Transaminases (Aminotransferases) a-keto acid OBLIGATE PAI: a- KG Glu 24 7

8 Example: Exchange of the amino group between amino acids Ala pyruvate ALAT: Ala aminotransferase ASAT: Asp aminotransferase ALAT a- KG Glu ASAT OXA Asp linical importance: Non-functional plasma enzymes Markers of tissue damage ALAT: liver ASAT: heart/muscle 24 8

9 structure s Prosthetic group of transaminases O N 3 2 O 2 O 3 N 2 O O 3 N 2 O - P O 3 N 2 O - P Vitamin B6 (pyridoxine) Pyridoxal-P Pyridoxamine-P O N 3 2 PLP PMP Note: this prostetic group is bound to a Lys residue of the enzyme as a Schiff base 24 9

10 structure s Transamination (mechanism) OO - OO - N 3 O OO - OO - O - 2 O N N 2 2 O N 3 2 PLP PLP PLP aldimine ketimine Schiff base tautomers PMP Glu akg 24 10

11 METABOLISM OF AMINO AIDS 1. The fate of the amino group (summary) 3 N OO - 1. Transamination: a reversible exchange of an amino and a keto group 2. Deamination e.g. Glu dehydrogenase 3. Incorporation of free ammonia into organic compound e.g. Glu dehydrogenase 24 11

12 Oxidative Deamination The glutamate dehydrogenase reaction OO - N 3 Glu NADP 2 O NAD ATP/GTP (high N 4 ) Glu DE ADP/GDP NADP N 4 NAD ATP OO - O a- KG Two isoenzymes: GLUD1 (liver, kidney and other tissues) and GLUD2 (brain, testis) 24 12

13 Transdeamination AA a-ketoacid glucose or energy Transaminases a- KG Glu Glu DE NAD NAD 4 N excretion energy 24 13

14 METABOLISM OF AMINO AIDS 1. The fate of the amino group 3 N OO - 1. Transamination: a reversible exchange of an amino and a keto group 2. Deamination e.g. Glu dehydrogenase 3. Incorporation of free ammonia into organic compound e.g. Glu dehydrogenase e.g. urea for excretion 24 14

15 The urea cycle: excretion of amino group pathway N 4 O O and free ammonia O - arbamoyl phosphate synthetase I arbamoyl group O 2 N O P 2ATP 2ADP P i X N Ornithine transcarbamoylase 2 N N O X citrulline citru OO - 3 N ornithine orn Arginosuccinate synthetase Asp ATP AMP PPi 24 15

16 pathway OO - 2 N N X N 2 OO - Arginosuccinate Arg-succ Arginosuccinate lyase OO - N 2 O Arginase 2 O N 2 N X N 2 OO - fumarate N 2 Arg Arg urea N 3 X ornithine orn

17 Gln Glu akg N 4 O 3 - P synthetase I P 2ATP Arg Nac Glu synthetase N-acetyl-Glu The urea cycle 2ADP P i Asp Mitochondrial matrix Acetyl-oA OXA malate A NAD fumarate akg urea orn UEA YLE 4 ATP/cycle citru Asp ATP AMP PP i ytoplasm Arg Arg-succ fumarate

18 Urea cycle and the citrate cycle Ornithine - synthesized from Glu/aKG see later anaplerotic reaction N-acetyl- Glu - from acetyloa and Glu (akg) One of the N donors: Asp synthesized from OXA Fumarate enters the citrate cycle Energy need (4 ATP/urea) If ammonia is not used for urea cycle: incorporates into Glu/aKG 24 18

19 EGULATION Urea cycle 1. Enzyme induction (10 to 20-fold more enzymes could be synthesized) (high protein diet or starvation) 2. Allosteric activation of P synthetase (N-acetyl-Glu) (presence of Glu and energy) 3. Intermediate concentration Arg - diet, Orn de novo synthesis (see later) YPEAMMONEMIA (insufficient urea cycle) Problem: akg is used for removal of ammonia, therefore the citrate cycle is depleted: low ATP level - coma Treatment: 1. limited protein intake (a-ketoacids of essential AA) insulin glucose: limited protein breakdown 2. inhibition of bacterial ammonia release in the colon (lactulose) 3. administration of amino acid binding chemicals: benzoate(gly) and phenylacetate(glu) 4. replacing the missing intermediates 24 19

20 Metabolic diseases of urea cycle yperammonemia Type I. P synthetase I. deficiency mental retardation Arg supplementation (stimulation of Nac-Glu synthesis) yperammonemia Type II. ornitine transcarbamoylase (X chromosome) deficiency mental retardation (early death) orotic aciduria (see Nucleotides) Arginosuccinate synthetase deficiency (benign) citrullinemia Arg supplementation Arginosuccinate lyase deficiency arginosuccinate excretion Arg supplementation Arginase deficiency high Arg level abnormalities in the development of NS Arg-free diet 24 20

21 Urea cycle enzymes in extrahepatic tissues Intestine: Glu ornitine citrulline de novo arginine Kidney: itrulline Arg circulation Other tissues: Arg ornitine polyamines Nitric oxide synthase (NOS) NADP NADP Nitric oxide (NO. ) citrulline 24 21

22 3 N METABOLISM OF AMINO AIDS 1. The fate of the amino group OO - 1. Transamination: a reversible exchange of an amino and a keto group 2. Deamination e.g. Glu dehydrogenase e.g. other deaminases 3. Incorporation of free ammonia into organic compound e.g. Glu dehydrogenase e.g. urea for excretion e.g. Gln and Asn synthesis 24 22

23 Deamination: release of free ammonia 1. Loss of the a-amino group oxidative deamination Glu DE (NAD/energy production) see before 2. Loss of the a-amino group during the catabolism Ser(Thr) dehydratase is and Gly (see later) 3. Loss of the a-amino group with flavoproteins L- aminoacid oxidases (FMN, 2 O, O 2 ) (minor significance?) D- aminoacid oxidases (FAD, 2 O, O 2 ) (AA from bacteria?) 4. Loss of the b or g amide group glutaminase asparaginase Gln(Asn) 2 O Glu(Asp) 5. See also: nucleotide metabolism! 4 N 24 23

24 structure s Serine dehydratase (coenzyme:plp) 2 O OO - N 3 PLP - 2 O 2 OO - N 3 2 O N 4 3 OO - O Ser Pyruvate Thr dehydratase (coenzyme:plp) 3 -O OO - N 3 Thr 3 2 OO - O a-ketobutirate

25 structure s Amino acid oxidases (flavoproteins) N 3 OO - FMN (FAD) O 2 FMN 2 (FAD 2 ) 2 O 2 detoxification OO - N 2 2 O OO - O N

26 3 N METABOLISM OF AMINO AIDS 1. The fate of the amino group OO - 1. Transamination: a reversible exchange of an amino and a keto group 2. Deamination e.g. Glu dehydrogenase e.g. other deaminases 3. Incorporation of free ammonia into organic compound e.g. Glu dehydrogenase e.g. urea for excretion e.g. Gln and Asn synthesis 24 26

27 Incorporation of free ammonia 1. Glu DE (NADP) at high ammonia conc. (see before) 2. arbamyl-p synthetase (ATP) urea cycle (see before) 3. Gln synthetase (ATP) Glu N 4 ATP ADPPi Gln amino group donor 24 27

28 Synthesis of asparagine (IEV) Gln Asp Asn synthetase Glu Asn for protein synthesis ATP AMP PP i amino group donor Asn synthetase deficient tumors - Asn from circulation Treatment: i.v. asparaginase low Asn conc. in blood Selective inhibition of tumor protein synthesis 24 28

29 The intercellular glutamine cycle out Perivenous area transam. Gln Glu akg N 4 urea UEA YLE O - 3 LIVE KIDNEY Gln Glu Glu akg Periportal area in Gln N 4 urea excretion 24 29

30 Interorgan transport of amino groups: Gln and Ala blood muscle glycolysis NAD proteins lactate NAD pyruvate akg AA Ala Glu aketoacids N 4 (aketoacids of Val, Leu, Ile) Gln 24 30