Upload
stella-andrews
View
248
Download
2
Tags:
Embed Size (px)
Citation preview
Chapter 18
Amino Acid Oxidation and The Production of Urea
Amino Acid Oxidation
Dependency of amino acid as energy source
Carnivores> herbivores> microorganism >> plant
Amino acid degradation in animals Amino acids for oxidation
Extra amino acid during protein turnover Protein-rich diet (no storage) During starvation or in uncontrolled diabetes
Removal of amino group (NH4+)
-keto acid (C skeleton of amino acids)
Oxidation to CO2 & H2O
Sources of C3 or C4 units for gluconeogenesis or fuels
Pathways of amino acid catabolism
18.1 Metabolic Fates of Amino Groups
Amino Group Catabolism
Amino Group Catabolism
Amino acid metabolism amino group (= nitrogen metabolism)
Liver is a major site Recycle for biosynthetic pathways Excretion; ammnonia, urea, uric acid
Glutamate & glutamine General collection point for amino group NH3 from amino acids + -ketoglutarate glutamate
into mitochondria, release of NH4+
Source of ammonia Dietary protein (major source) Muscle & other tissues
NH4+ + glutamate glutamine
mitochondria in hepatocytes
NH4+ + pyruvate alanine hepatocytes
Digestion of Dietary Protein
In stomach Entry of diet secretion of gastrin from gastric mucosa secretion of HCl (parietal cells), pepsinogen (chief cells)
Acidic gastric juice (pH 1.0 to 2.5) Antiseptic & denaturing agent (protein unfolding)
Pepsinogen : zymogen Conversion to active pepsin by autocatalytic cleavage (at low pH) Digestion of peptide bonds at Phe, Trp, Tyr mixture of small peptides
In small intestine Low pH secretion of secretin stimulation of bicarbonate secretion from pancreas neutralization Arrival in the upper part of intestine (duodenum) release of cholecystokinin into blood
stimulation of pancreatic zymogens Trypsinogen : activated by enteropeptidase Chymotrypsinogen, procarboxypeptidase A and B : activated by trypsin
c.f.) Protection of pancreas from proteolytic digestion Production of zymogens Pancreatic trypsin inhibitor
Protein digestion by trypsin, chymotrypsin, carboxypeptidase, aminopeptidase Uptake of amino acids by the epithelial cells
Digestion of Dietary Protein
Blood capillariesLiver
Transamination
1st step of amino acid catabolism Transfer of -amino group to -ketoglutarate
Generation of L-glutamate & -ketoacid
Aminotransferase (transaminase)
Amino acid specificity (named after amino acids)
Reversible reaction
; ∆G’° ≈ 0 kJ/mol
Pyridoxal phosphate (PLP)
Bimolecular Ping-Pong reactions
Pyridoxal phosphate (PLP)
Coenzyme form of pyridoxine (vitamin B6)
Intermediate carrier of amino group
Electron sink for carbanion (resonance stabilization) Transamination Racemization (L- & D-form interconversion) Decarboxylation
PLP-mediated transamination at -carbon
PLP-mediated transamination: Ping-Pong mechanism
amino acid -ketoglutarate
pyridoxal phosphate pyridoxamine phosphate pyridoxal phosphate
-keto acid glutamate
PLP-mediated amino acid transformations at -carbon
Oxidative Deamination of Glutamate
Oxidative deamination Mitochondrial matrix of hepatocytes Glutamate dehydrogenase
Generation of -ketoglutarate & ammonia NAD+ or NADP+ as electron acceptor Allosteric regulation
By ADP (inhibition) By GTP (activation)
Transdeamination Transamination of A.a. + oxidative deamination of Glu A few amino acids undergoes direct oxidative deamination
Glutamine as Ammonia Carrier in the Bloodstream
Ammonia generated in extrahepatic tissues
Glutamine synthetase Incorporation of ammonia into
glutamate glutamine
Transport of gln to the liver via blood
Higher gln concentration than other amino acids in blood
Glutaminase in the liver, intestine, and kidney
Glutamine Glutamate + NH4+
Alanine Transports Ammonia from Skeletal Muscles to the Liver
Glucose-alanine cycle In muscle
Glycolysis & degradation of amino acids Alanine aminotransferase
Transfer amino group of glutamate to pyruvate alanine + -ketoglutarate
Transport of alanine to the liver
In the liver Alanine aminotransferase
Transfer amino group of alanine to -ketoglutarate glutamate + pyruvate
Gluconeogenesis Pyruvate , lactate glucose
Transport of glucose to muscle
Ammonia is toxic to animals.
Comatose state of brain (high brain’s water content)
1. NH3: alkalization of cellular fluid 2. -ketoglutarate, NADH, ATP:
citric acid cycle & ATP production3. glutamate and GABA (-aminobutyrate):
neurotransmitter depletion
18.2 Nitrogen Excretion and the Urea Cycle
Produced in liver
Blood
Kidney urine
Urea Cycle in Mitochondria
Formation of carbamoyl phosphate; preparatory step
NH4+ + HCO3
- + 2 ATP carbamoyl phosphate + 2 ADP + Pi
Carbamoyl phosphate synthetase I
- ATP-dependent reaction
1st step in the urea cycle;
Ornitine + carbamoyl phosphate citrulline + Pi
Ornitine transcarbamoylase
Urea Cycle in Cytosol
2nd step; formation of argininosuccinate Incorporation of the second N from aspartate Argininosuccinate synthetase ATP requirement Citrullyl-AMP intermediate
3rd step; formation of arginine & fumarate Argininosuccinase; only reversible step in the cycle
4th step; Cleavage of arginine to urea & ornithine Arginase
Asparatate-argininosuccinate shunt
Metabolic links between citric acid and urea cycles In cytosol
Fumarate to malate citric acid cycle in mitochondria In mitochondria
OAA + Glu -ketoglutarate + Asp urea cycle in cytosol
Energetic cost
• Consumption
3 ATP for urea cycle
• Generation
Malate to OAA
1 NADH = 2.5 ATP
Regulation of the Urea Cycle
Long term regulation
Regulation in gene expression
Starving animals & very-high protein diet Increase in synthesis of enzymes
in urea cycle
Short term regulation
Allosteric regulation of a key enzyme
Carbamoyl phosphate synthetase I Activation by N-acetylglutamate
Treatment of genetic defects in the urea cycle
Genetic defect in the urea cycle
ammonia accumulation; hyperammonemia Limiting protein-rich diet is not an option Administration of aromatic acids; benzoate or phenylbutyrate Administration of carbamoyl glutamate Supplement of arginine
18.3 Pathways of Amino Acid Degradation
Amino Acid Catabolism
Carbon skeleton of 20 amino acids
Conversion to 6 major products
- pyruvate
- acetyl-CoA
- -ketoglutarate
- succinyl-CoA
- fumarate
- oxaloacetate
Glucogenic or Ketogenic Amino Acids
Ketogenic amino acids
Conversion to acetyl-CoA or acetoacetyl-CoA
ketone bodies in liver
Phe, Tyr, Ile, Leu, Trp, Thr, Lys Leu : common in protein
Contribution to ketosis under starvation conditions
Glucogenic amino acids
Conversion to pyruvate, -ketoglutarate, succinyl-CoA, fumarate, and OAA
glucose/glycogen synthesis
Both ketogenic and glucogenic Phe, Tyr, Ile, Trp, Thr
Enzyme cofactors in amino acid catabolism
One-carbon transfer reactions ; common reaction type, involvement of one of 3 cofactors
Biotin ; one-carbon tranfer of most oxidized state, CO2
Tetrahydrofolate (H4 folate) ; One-carbon transfer of intermediate oxidation states or methyl groups S-adenosylmethionine ; one-carbon transfer of most reduced state, -CH3
Tetrahydrofolate
folate (vitamin) to H4 folate Dihydrofolate reductase
Primary source of one-carbon unit
Carbon removed in the conversion of Ser to Gly
Oxidation states of H4 folate ; One-carbon groups bonded to
N-5 or N-10 or both- Methyl group (most reduced)- Methylene group- Methenyl, formyl, formimino group
(most oxidized) Interconvertible & donors of one-
carbon units (except N5-methyl-tetrahydrofolate)
S-adenosylmethionine (adoMet)
Cofactor for methyl group transfer Synthesized from Met and ATP
Methionine adenosyl transferase Unusual displacement of triphosphate from ATP
Potent alkylating agent Destabilizing sulfonium ion inducing nucleophilic attack on methyl group
Six amino acids are degraded to pyruvate
Ala, Trp, Cys, Ser, Gly, Thr pyruvate acetyl-CoA citric acid cycle or gluconeogenesis
Interplay of PLP and H4folate in Ser/Gly metabolism
3rd pathway of glycine degradation - D-amino acid oxidase detoxification of D-amino acid high level in kidney
- Oxalate crystals of calcium oxalate (kidney stones)
Seven Amino Acids Are Degraded to Acetyl-CoA
Trp, Lys, Phe, Tyr, Leu, Ileu, Thr acetoacetyl-CoA acetyl-CoA
Intermediates of Trp catabolism be precusors for other biomolecules
Catabolic pathways for Phe & Tyr
Phe & Tyr are precusors dopamine norephinephrine, epinephrine melanin
Phenylalanine hydroxylase
Mixed function oxidase ; Substrate hydroxylation + oxygen reduction to H2O Tetrahydrobiopterin as a cofactor
Example of A.a. metabolism defects; Phe catabolism
Phe degradaion fumarate + acetoacetyl-CoA Defects in Phe catabolism
Phenylketonuria (PKU) Genetic defect in Phe hydroxylase or
dihydrobiopterin reductase Elevated levels of Phe & phenylpyruvate Mental retardation
Alkaptonuria Genetic defect in homogentisate dioxygenase Oxidation of accumulated homogentisate
Black urine Arthritis
Genetic defects of A.a. metabolism defective neural development & metal retardation
Five Amino Acids Are Converted to -ketoglutarate
Pro, Glu, Gln, Arg, His; amino acids with five C skeleton converging to Glu
Four Amino Acids Are Converted to Succinyl-CoA
Met, Ileu, Thr, Val converging to propionyl-CoA
Branched-Chain Amino Acids Are Not Degraded in the Liver
Leu, Ile, Val
Primarily oxidized as fuels in muscle, adipose, kidney, brain
Branched-chain aminotransferases (not in liver)
Branched-chain -keto acid dehydrogenase complex
Asn and Asp are Degraded to Oxaloacetate