fch 532 lecture 24 chapter 26: amino acid metabolism wed. urea cycle quiz friday: ketogenic vs....
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FCH 532 Lecture 24
Chapter 26: Amino acid metabolism
Wed. Urea cycle quiz
Friday: Ketogenic vs. glucogenic (or both) amino acids-what common metabolites do this amino acids go towards?
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Figure 26-11 Degradation of amino acids to one of seven common
metabolic intermediates.
Met degradation
• Met reacts with ATP to form S-adenosylmethionine (SAM).• SAM’s sulfonium ion is a highly reactive methyl group so this compound is
involved in methylation reactions.• Methylation reactions catalyzed by SAM yield S-adenosylhomocysteine
and a methylated acceptor molecule.• S-adenosylhomocysteine is hydrolyzed to homocysteine.• Homocysteine may be methylated to regenerate Met, in a B12 requiring
reaction with N5-methyl-THF as the methyl donor.• Homocysteine can also combine with Ser to form cystathionine in a PLP
catalyzed reaction and -ketobutyrate. -ketobutyrate is oxidized and CO2 is released to yield propionyl-CoA.• Propionyl-CoA proceeds thorugh to succinyl-CoA.
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1. Methionine adenosyltransferase
2. Methyltransferase
3. Adenosylhomocysteinase
4. Methionine synthase (B12)
5. Cystathionine -synthase (PLP)
6. Cystathionine -synthase (PLP)
7. -ketoacid dehydrogenase
8. Propionyl-CoA carboxylase (biotin)
9. Methylmalonyl-CoA racemase
10. Methylmalonyl-CoA mutase
11. Glycine cleavage system or serine hydroxymethyltransferase
12. N5,N10-methylene-tetrahydrofolate reductase (coenzyme B12 and FAD)
NADH, H+
Branched chain amino acid degradation
• Degradation of Ile, Leu, and Val use common enzymes for the first three steps
1. Transamination to the corresponding -keto acid2. Oxidative decarboxylation to the corresponding acyl-CoA3. Dehydrogenation by FAD to form a double bond.
First three enzymes1. Branched-chain amino acid aminotransferase2. Branched-chain keto acid dehydrogenase (BCKDH)3. Acyl-CoA dehydrogeanse
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branched-chain amino acids (A) isoleucine, (B) valine, and (C)
leucine.
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After the three steps, for Ile, the pathway continues similar to fatty acid oxidation (propionyl-CoA carboxylase, methylmalonyl-CoA racemase, methylmalonyl-CoA mutase).
4. Enoyl-CoA hydratase - double bond hydration
5. -hydroxyacyl-CoA dehydrogenase- dehydrognation by NAD+
6. Acetyl-CoA acetyltransferase - thiolytic cleavage
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For Valine:
7. Enoyl-CoA hydratase - double bond hydration
8. -hydroxy-isobutyryl-CoA hydrolase -hydrolysis of CoA
9. hydroxyisobutyrate dehydrogenase - second dehydration
10. Methylmalonate semialdehyde dehydrogenase - oxidative carboxylation
Last 3 steps similar to fatty acid oxidation
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For Leucine:
11. -methylcronyl-CoA carboxylase-carboxylation reaction (biotin)
12. -methylglutaconyl-CoA hydratase-hydration reaction
13. HMG-CoA lyase
Acetoacetate can be converted to 2 acetyl-CoA
Leucine is a ketogenic amino acid!
Leu and Lys are ketogenic• Leu proceeds through a typical branched amino acid breakdown but the
final products are acetyl-CoA and acetoacetate.• Most common Lys degradative pathway in liver goes through the
formation of the -ketoglutarate-lysine adduct saccharopine.• 7 of 11 reactions are found in other pathways.• Reaction 4: PLP-dependent transamination• Reaction 5: oxidative decarboxylation of an a-keto acid by a multienzyme
complex similar to pyruvate dehydragense and a-ketoglutarate dehydrogenase.
• Reactions 6,8,9: fatty acyl-CoA oxidation.• Reactions 10 and 11 are standard ketone body formation reactions.
Figure 26-23The pathway of lysine degradation in
mammalian liver.
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1. Saccharopine dehydrogenase (NADP+, Lys forming)
2. Saccharopine dehydrogenase (NAD+, Glu forming)
3. Aminoadipate semialdehyde dehydrogenase
4. Aminoadipate aminotransferase (PLP)
5. -keto acid dehydrogenase
6. Glutaryl-CoA dehydrogenase
7. Decarboxylase
8. Enoyl-CoA hydratase
9. -hydroxyacyl-CoA dehydrogenase
10. HMG-CoA synthase
11. HMG-CoA lyase
Trp is both glucogenic and ketogenic
• Trp is broken down into Ala (pyruvate) and acetoacetate.
• First 4 reactions lead to Ala and 3-hydroxyanthranilate.
• Reactions 5-9 convert 3-hydroxyanthranilate to a-ketoadipate.
• Reactions 10-16 are catalyzed by enzymes of reactions 5 - 11 in Lys degradation to yield acetoacetate.
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1. Tryptophan-2,3-dioxygenase, 2. Formamidase, 3. Kynurenine-3-monooxygense, 4. kynureninase (PLP dependent)
Kynureinase, another PLP mechanism
• Reaction 4: cleavage of 3-hydroxykynurenine to alanine and 3-hydroxyanthranilate is catalyzed by the PLP dependent enzyme kynureinase.
• This facilitates a C-C bond cleavage. (previous reactions catalyzed the C-H and C-C bond cleavage)
• Follows the same steps as transamination but does not hydrolyze the tautomerized Schiff base.
• Enzyme amino acid acts as a nucleophile tto attack the carbonyl carbon (Cof the tautomerized 3-hydroxykynurenine-PLP Schiff base.
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6. Amino carboxymuiconate semialdehyde decarboxylase
7. Aminomuconate semialdehyde dehydrogenase
8. Hydratase, 9. Dehydrogense 10-16. Reactions 5-11 in lysine degradation.
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• -keto acid dehydrogenase
• Glutaryl-CoA dehydrogenase
• Decarboxylase
• Enoyl-CoA hydratase
• -hydroxyacyl-CoA dehydrogenase
• HMG-CoA synthase
• HMG-CoA lyase
Phe and Tyr are degraded to fumarate and acetoacetate
• The first step in Phe degradation is conversion to Tyr so both amino acids are degraded by the same pathway.
• 6 reactions
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1. Phenylanalnine hydroxylase2. Aminotransferase3. p-hydroxyphenylpyruvate
dioxygenase4. Homogentisate dioxygenase5. Maleylacetoacetate isomerase6. Fumarylacetoacetase
Phenylalanine hydroxylase has biopterin cofactor
• 1st reaction is a hydroxylation reaction by phenylalanine hydroxylase (PAH), a non-heme-iron containing homotetrameric enzyme.
• Requires O2, FeII, and biopterin a pterin derivative.• Pterins have a pteridine ring (similar to flavins)• Folate derivatives (THF) also contain pterin rings.
Figure 26-27The pteridine ring, the
nucleus of biopterin and
folate.
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Active BH4 must be regenerated
• Active form in PAH is 5,6,7,8-tetrahydrobiopterin (BH4)• Produced from 7,8-dihydrobiopterin via dihydrofolate
reductase (NADPH dependent).• 5,6,7,8-tetrahydrobiopterin is hydroxylated to pterin-4a-
cabinolamine by phenylalanine hydroxylase.• pterin-4a-cabinolamine is converted to 7,8-
dihydrobiopterin (quinoid form) by pterin-4a-carbinoline dehydratase
• 7,8-dihydrobiopterin (quinoid form) is reduced by dihydropteridine reductase to regenerate the active cofactor.
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NIH shift
• A 3H that starts on C4 of Phe’s ring ends up on C3 of Tyr’s ring rather than being lost to solvent.
• Mechanism is called the NIH shift• 1st characterized by scientists at NIH
1 and 2: activation of the enzyme’s BH4 and Fe(II) cofactors to yield pterin-4a-carbinolamine and a reactive oxyferryl [Fe(IV)=O2-]
3: Fe(IV)=O2- reacts with Phe to form an epoxide across the 3,4 bond.
4: epoxide opening to form carbocation at C3
5: migration of hydride from C4 to C3 to form more stable carbocation.
6: ring aromatization to form Tyr
Phe and Tyr are degraded to fumarate and acetoacetate
• The first step in Phe degradation is conversion to Tyr so both amino acids are degraded by the same pathway.
• 6 reactions• Reaction 1 = 1st NIH shift• Reaction 3 is also an example of NIH shift (26-31)
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1. Phenylanalnine hydroxylase2. Aminotransferase3. p-hydroxyphenylpyruvate
dioxygenase4. Homogentisate dioxygenase5. Maleylacetoacetate isomerase6. Fumarylacetoacetase
Amino acids as precursors
• Amino acids are essential precursors to biomolecules:• Nucleotides• Nucleotide coenzymes• Heme• Hormones• Neurotransmitters• Glutathione