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?

Pag

e 99

5

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.

Pag

e 10

02

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

Pag

e 10

04Figure 26-21The degradation of the

branched-chain amino acids (A) isoleucine, (B) valine, and (C)

leucine.

Pag

e 10

04

Pag

e 10

04

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

Pag

e 10

04

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

Pag

e 10

04

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.

Pag

e 10

06

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.

Pag

e 10

07

Pag

e 10

07

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.

Pa

ge

100

8

Pag

e 10

07

6. Amino carboxymuiconate semialdehyde decarboxylase

7. Aminomuconate semialdehyde dehydrogenase

8. Hydratase, 9. Dehydrogense 10-16. Reactions 5-11 in lysine degradation.

Pag

e 10

06

• -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

Pag

e 10

09

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.

Pag

e 10

09

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.

Pag

e 10

10

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)

Pag

e 10

09

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