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?

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

catalyzed reaction and 

-ketobutyrate

.



-ketobutyrate

is oxidized and CO2 is released to yield in a PLP

propionyl-CoA.

Propionyl-CoA

proceeds thorugh to

succinyl-CoA.

5.

6.

7.

8.

1.

2.

3.

4.

9.

10.

11.

12.

Methionine adenosyltransferase Methyltransferase Adenosylhomocysteinase Methionine synthase (B12) Cystathionine  -synthase (PLP) Cystathionine  -synthase (PLP)  -ketoacid dehydrogenase Propionyl-CoA carboxylase (biotin) Methylmalonyl-CoA racemase Methylmalonyl-CoA mutase Glycine cleavage system or serine hydroxymethyltransferase

N 5 ,N 10

-methylene-tetrahydrofolate reductase (coenzyme B12 and FAD) NADH, H+

•

1.

2.

3.

Branched chain amino acid degradation

Degradation of Ile, Leu, and Val use common enzymes for the first three steps

Transamination to the corresponding



-keto acid Oxidative decarboxylation to the corresponding acyl-CoA Dehydrogenation by FAD to form a double bond.

First three enzymes 1.

2.

3.

Branched-chain amino acid aminotransferase Branched-chain



keto acid dehydrogenase (BCKDH) Acyl-CoA dehydrogeanse

Figure 26-21

The degradation of the branched-chain amino acids (A) isoleucine, (B) valine, and (C) leucine.

After the three steps, for Ile, the pathway continues similar to fatty acid oxidation (propionyl-CoA carboxylase, methylmalonyl-CoA racemase, methylmalonyl-CoA mutase).

4.

5.

6.

Enoyl-CoA hydratase - double bond hydration  -hydroxyacyl-CoA dehydrogenase dehydrognation by NAD+ Acetyl-CoA acetyltransferase thiolytic cleavage

For Valine: 7.

8.

9.

Enoyl-CoA hydratase - double bond hydration  -hydroxy-isobutyryl-CoA hydrolase -hydrolysis of CoA  hydroxyisobutyrate dehydrogenase - second dehydration 10. Methylmalonate semialdehyde dehydrogenase - oxidative carboxylation Last 3 steps similar to fatty acid oxidation

For Leucine: 11.  12.  -methylcronyl-CoA carboxylase carboxylation reaction (biotin) -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 26-23

The pathway of lysine degradation in mammalian liver.

1.

2.

3.

4.

5.

6.

Saccharopine dehydrogenase (NADP+, Lys forming) Saccharopine dehydrogenase (NAD+, Glu forming) Aminoadipate semialdehyde dehydrogenase Aminoadipate aminotransferase (PLP)  -keto acid dehydrogenase Glutaryl-CoA dehydrogenase 7.

8.

9.

Decarboxylase Enoyl-CoA hydratase  -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.

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 (

C

  of the tautomerized 3 hydroxykynurenine-PLP Schiff base.

6. Amino carboxymuiconate semialdehyde decarboxylase 7. Aminomuconate semialdehyde dehydrogenase 8. Hydratase, 9. Dehydrogense 10-16. Reactions 5-11 in lysine degradation.

• • • • • • •  -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

1.

2.

3.

4.

5.

6.

Phenylanalnine hydroxylase Aminotransferase p-hydroxyphenylpyruvate dioxygenase Homogentisate dioxygenase Maleylacetoacetate isomerase 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 26-27

The pteridine ring, the nucleus of biopterin and folate.

Active BH

4

must be regenerated

• • • • • Active form in PAH is

5,6,7,8-tetrahydrobiopterin (BH 4 )

Produced from

7,8-dihydrobiopterin

via

dihydrofolate reductase (NADPH dependent).

5,6,7,8-tetrahydrobiopterin cabinolamine

by is hydroxylated to

phenylalanine hydroxylase

.

pterin-4a 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.

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 BH 4 and Fe(II) cofactors to yield pterin-4a carbinolamine and a reactive oxyferryl [Fe(IV)=O 2 ]

3

: Fe(IV)=O 2 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)

1.

2.

3.

4.

5.

6.

Phenylanalnine hydroxylase Aminotransferase p-hydroxyphenylpyruvate dioxygenase Homogentisate dioxygenase Maleylacetoacetate isomerase Fumarylacetoacetase

Amino acids as precursors

• • • • • • • Amino acids are essential precursors to biomolecules: Nucleotides Nucleotide coenzymes Heme Hormones Neurotransmitters Glutathione