Module No.
BIOCHEMISTRY
CATABOLISM OF THE CARBON SKELETONS OF THE AMINO ACIDS
8.2.1
Doc. Marion G. Rivera
Feb. 22, 2017
LEGEND: From PPT Recording (English) Recording (Taglish)
*Why is i s g luta lutama mate te the end-produ end-product ct of transamination? - Because glutamate is the only amino acid capable of undergoing oxidative deamination.
Amino Acid Catabolism
2nd step: OXIDATIVE DEAMINATION
There will be a removal of an amino group. Then it goes back as αketoglutarate to accept other amino groups once more.
* Ammonia – potentially toxic Excessive ammonia ENCEPHALOPATHY
Recall: Major function of amino acid is for synthesis of proteins.
3rd step: UREA CYCLE
- Amino acids degraded into body proteins. *turnoverconstant degradation of proteins
synthesis
=>
Ammonia is converted into a less toxic form which is UREA.
and
- Body proteins will be hydrolyzed back to amino acids and recycled again for synthesis of new proteins.
Amino Acid Catabolism - For most amino acids the first step would be TRANSAMINATION.
1st step: Removal of α-amino group which would then be converted into urea.
Removal of alpha-amino nitrogen by transamination is the first catabolic reaction, EXCEPT for: o
PROLINE
o
HYDROXYPROLINE
o
THREONINE
o
LYSINE
nd
The 2 step: remaining structure will be the carbon skeleton which will be converted to glucose or fats which will be used as energy.
Catabolism of Amino Acid Nitrogen
Catabolism of Carbon Skeletons of Amino Acids
Convert them to citric acid cycle intermediates or their precursors so that they can be metabolized to CO2 & H2O or used in gluconeogenesis.
Account for 10-15% generated by animals.
of
metabolic
energy
Recall: Major function is to synthesize proteins. Only 10-15% will be used for the generation or production of energy.
Classification of Amino Acids 1. GLUCOGENIC AMINO ACIDS
uv ate, - Carbon skeletons are degraded to pyr uvate, --ketoglutarate, ketoglutarate, succinyl CoA, fumarate or oxaloacetate and are therefore glucose α
1st step: TRANSAM T RANSAMINATION INATION
Removal of amino group from an amino acid will then become a keto-acid. αketoglutarate will receive the removed amino group to form L-Glutamate which would then undergo oxidative deamination.
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precursors.
2. KETOGENIC AMINO ACIDS - Carbon skeletons are broken down into acetylCoA or acetoacetate and can thus be converted to fatty acid or ketone bodies.
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
Classification of Amino Acids Amino acids converted to:
PYRUVATE
OXALOACETATE
o
Alanine
o
Cysteine
o
Glycine
o
Serine
o
Threonine
o
Tryptophan
o
Aspartate
o
Asparagine
o
Arginine
o
Glutamate
o
Glutamine
o
Histidine
o
Proline
o
Aspartate
o
Tyrosine
o
Phenylalanine
o
Isoleucine
o
Methionine
o
Valine
o
Isoleucine
o
Leucine
o
Threonine
o
Lysine
o
Phenylalanine
o
Tryptophan
o
Tyrosine
- ALL of the intermediates of the citric acid cycle are potentially glucogenic.
Recall: α-KETOGLUTARATE
- Some amino acids are converted to acetyl-CoA.
FUMARATE
- Acetyl-CoA is the precursor for synthesis of fatty acids and ketone bodies. SUCCINYL CoA
- Any amino acid that will be converted to acetylCoA or acetoacetyl-CoA is ketogenic.
Classification of Amino Acids ACETYL-CoA &/or ACETOACETATE
*LEUCINE – PURELY KETOGENIC (Harper’s)
Amino Acid & Keto-acid Pairs *memorize Transer: shann.au
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
ALANINE – PYRUVATE
2. GLYCINE
ASPARTATE – OXALOACETATE
GLUTAMATE – α-KETOGLUTARATE
- Is acted upon by an enzyme complex called “GLYCINE-CLEAVAGE COMPLEX” of the liver mitochondria.
- Pag dinagdag natin yung amino group ng alanine magiging pyruvate siya OR yung pyruvate pag nilagyan ng amino group magiging alanine siya same with others…
- It splits glycine to CO 2 and NH 4+ and forms N5, N10-methylene tetrahydrofolate.
Amino Acids converted to Pyruvate 1. ALANINE - Transamination forms pyruvate which can then be decarboxylated to acetyl-CoA by pyruvate dehydrogenase.
- It consists of three enzymes and an “H protein” that has a covalently attached dihydrolipoyl moiety. “GLYCINE-CLEAVAGE COMPLEX”
- Yung amino group ng ALA NI NE pwede nating itransfer sa α - KETOGLUTARATE. So, yung α KETOGLUTARATE pag kinuha niya yung amino group ng A LA NI NE it will become GLUTAMATE or kung OXALOACETATE naman ang kumuha ng amino group ng ALANINE, OXALOACETATE will become ASPARTATE. So, only one of these will have to be present. Di kailangang sabay. - So, yung ALA NI NE :
-
When it TRANSAMINATION
undergoes α with it would form
KETOGLUTARATE PYRUVATE and GL UTAMATE. -
When it TRANSAMINATION
undergoes with would form
OXALOACETATE it PYRUVATE and ASPAR TATE.
*It is called “TRANSAMINATION” because we’re just transferring the amino group.
*What is the cofactor/coenzyme trans amination? - Vitamin B 6 /Pyridoxal Phosphate
1. First reaction is decarboxylation. Release of CO2. “Naiwan na structure” will attach to the sulfur atom of dihydrolipoate. 2. Next reaction is the transfer of C to tetrahydrofolate. There will then be a release of amino group as ammonia. 3. When dihydrolipoate undergoes a reaction it will be reduced.
for - Can undergo transamination to glyoxylate (ketoacid part of glycine) with Glu or Ala.
- Glyoxylate is then converted to oxalate. - Failure to catabolize glyoxylate will lead to “hyperoxaluria”.
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
3. SERINE
Module 8, Lecture 2.1
CYSTEINE
- With the removal of an amino group it is converted to pyruvate with the enzyme serine hydratase.
- catabolized via 2 pathways:
a. DIRECT OXIDATIVE PATHWAY
- Serine and glycine are interconvertible but the important coenzyme that has to be present is TETRAHYDROFOLATE .
- First is oxidation then followed by transamination.
- Degraded to glycine and N 5, N10-methylene tetrahydrofolate.
- (cysteine) HS naging (sulfinate) O 2S then will undergo transamination to form pyruvate.
b. TRANSAMINATION PATHWAY - First is transamination then followed by oxidation.
Recall: Active form of folic acid is tetrahydrofolate. - Tetrahydrofolate is important because it serves as a carrier of carbon units . And, the carbon units carried by tetrahydrofolate (in this case it is methylene tetrahydrofolate) -most of it is derived from serine.
- Cysteine will first undergo transamination to form 3mercatopyruvate and then it can undergo oxidation to form 3mercaptolactate OR it will just remove the SH 2 and then form pyruvate.
- Pag na-convert yung tetrahydrofolate into N5, N10methylene tetrahydrofolate it can be converted to other forms of tetrahydrofolate. Basta importante na ma-convert muna s iya as methylene tetrahydrofolate so it can be converted to other forms.
- Kapag walang serine, walang i-carry na carbon unit yung tetrahydrofolate.
4. CYSTINE AND CYSTEINE CYSTINE - converted to cysteine - Two cysteine residues joined by a di-sulfide bond.
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
5. THREONINE Cleaved to acetaldehyde and glycine; acetaldehyde is then oxidized to acetate which is then converted to acetyl-CoA.
- First reaction to occur is cleavage. Threonine is cleaved into glycine and acetaldehyde. The enzyme that cleaves it is called aldolase. - Acetaldehyde is then converted to acetate by the enzyme aldehyde dehydrogenase.
*Recall: Ethanol metabolism also forms acetaldehyde. - Coenzyme A together with acetate will form acetylCoA.
- First reaction is dehydrogenation. Removal of 2 hydrogen atoms to form a double bond. - Followed by cleavage of the ring to form γhydroxy-L-glutamate-γ-semialdehyde. - Will once again undergo another dehydrogenation. This time, the aldehyde will become a carboxyl. Thus, now called erythro- γ-hydroxy-L-glutamate.
6. HYDROXYPROLINE - Catabolized differently from proline - Catabolized differently because proline is included under amino acids that are converted to glutamine. Hydroxyproline on the other hand, is converted to pyruvate.
Transer: shann.au
- Erythro-γ-hydroxy-L-glutamate will then undergo transamination and then eventually it will be cleaved by an aldolase to release glyoxylate and pyruvate.
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
Amino Acids converted to Oxaloacetate 1. ASPARAGINE - Is an amide of aspartate
Recall: Amide – removal of an amino group from its carboxyl group - Converts into aspartate through the removal of an amino group through the enzyme asparaginase.
2. ASPARTATE - Can undergo transamination to form oxaloacetate.
Amino Acids converted to
α-Ketoglutarate
1. GLUTAMINE - Is an amide of glutamate (will remove the amino group and then it becomes glutamate)
2. GLUTAMATE - Formed through transamination of α-ketoglutarate.
- First step is dehydrogenation. - Next is hydrolysis semialdehyde.
to
form
glutamate- γ-
- Then the aldehyde from glutamate- γ-semialdehyde will be converted into a carboxyl, yielding glutamate. - Then, glutamate will be transaminated to αketoglutarate.
4. ARGININE 3. PROLINE - Oxidized to dehydroproline which adds water forming glutamate- γ-semialdehyde.
- Converted to ornithine through the enzyme arginase which then undergoes transamination to glutamate-y-semialdehyde.
- Glutamate-γ-semialdehyde is then oxidized to glutamate and transaminated to α-ketoglutarate.
*Recall: Urea Cycle
- Catabolism of proline is the reverse of the reaction of its synthesis.
- Hydrolyzed to form Urea
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
- Ornithine can be transaminated to form glutamateγ-semialdehyde.
- The aldehyde from glutamate- γ-semialdehyde will then be converted into a carboxyl yielding glutamate. (same reaction with proline for this part) - Then, glutamate will be transaminated to αketoglutarate. (same reaction with proline for this part)
5. HISTIDINE - Non-oxidatively deaminated then hydrated and its imidazole ring cleaved to form Nformiminoglutamate (FIGLU) ; formimino group is then transferred to TH 4 to form N-formimino TH 4 and glutamate.
- First step is deamination. There will be a removal of amino group. Although, it was specified as nonoxidative deamination. Basta tinanggal lang yung amino group. Now, histidine becomes urocanate. - Urocanate is hydrolyzed to form 4-imidazole-5propionate. - This is then followed by the hydrolysis of the imidazole ring. The ring is now open forming Nformiminoglutamate (FIGLU). - Tetrahydrofolate is needed to convert Nformiminoglutamate (FIGLU) into glutamate. - Folic acid deficiency => tataas ang FIGLU
SUMMARY:
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Amino Acids converted to Fumarate 1. PHENYLALANINE - First reaction in its degradation is its hydroxylation to tyrosine. - * We can synthesize tyrosine (non-essential) if we have phenylalanine (essential).
Module 8, Lecture 2.1
- Tyrosine once formed will undergo transamination into p-hydroxypheyl-pyruvate followed by concerted ring hydroxylation and side chain migration to form homogentisate with ascorbate as reductant; aromatic ring opens and is hydrolyzed to fumarate and acetoacetate. - First step is transamination hydroxyphenylpyruvate.
yielding
p-
- Next step is another hydroxylation. Phydroxyphenylpyruvate magiging homogentisate with another hydroxylation. Yung hydroxylation, yung OH ay i-aattach dito sa carbon 4 kaso may naka-attach na diyan. So, what will happen is yung side chain na yan will be moved to the adjacent carbon. This then is what you call concerted ring hydroxylation and side chain migration which would eventually yield your homogentisate. - Movement of carbon chains at maleylacetoacetate and it would become this fumarylacetoacetate. - Fumarylacetoacetate will then be cleaved forming fumarate and acetoacetate. - Then acetoacetate can eventually be converted into acetyl-CoA and actate. Phenylalanine hydroxylase (I) converts phenylalanine into tyrosine. For this reaction to occur the cofactor needed is tetrahydrobiopterin. - Pag ginamit natin yung tetrahydrobiopterin it becomes dihydrobiopterin. - Dihydrobiopterin has to be converted back to tetrahydrobiopterin. Dihydrobiopterin reductase (II) converts dihydrobiopterin back to tetrahydrobiopterin.
3. ASPARTATE 2. TYROSINE Transer: shann.au
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
- Yung SAM serves as a methyl donor. Pag dinonate na niya yung kanyang methyl group it becomes S-adenosyl-homocysteine. Ngayun, pag tinanggal naman natin yung adenosine from Sadenosyl-homocysteine- what would be left is homocysteine.
- Citrulline here combines with aspartate to form argininosuccinate and argininosuccinate will then be cleaved into arginine and fumarate. - The amino group of aspartate mapupunta sa arginine and maiiwan yung carbon skeleton ng aspartate. Yung carbon skeleton ay ma-rerelease ulit but this time in the form of fumarate. - Fumarate can be converted into malate then into oxaloacetate which is part of the citric acid cycle. - Oxaloacetate is then transaminated and becomes aspartate. Aspartate will then be used again in the urea cycle.
SUMMARY:
Amino Acids converted to Succinyl CoA 1. METHIONINE - Precursor for synthesis of cysteine we need methionine and serine. - Will donate the sulfur atom of cysteine. - Reactions that are involved in the catabolism of methionine is part of the reaction that synthesizes cysteine. - Condenses with ATP to form S-adenosylmethionine (SAM, an important methyl donor ) *adenosyl comes from ATP
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
of methyl group forms Removal S-adenosyl-homocysteine which is hydrolyzed to adenosine and homocysteine. - Homocysteine then combines to serine to yield cystathionine which subsequently forms cysteine and α-ketobutyrate. - Yung serine may OH group siya tapos yung homocysteine may SH group naman siya. Magcondense sila to form cystathionine. Pag cleave sa kanya yung sulfur sasama sa serine and then it becomes cysteine. Kaya saan galing yung carbon skeleton ng cysteine? Galing sa serine. Saan galing yung sulfur niya? Galing sa methionine. Kase yung homocysteine galing sa methionine.
Module 8, Lecture 2.1
Amino Acids converted Acetyl-CoA and/or Acetoacetate 1. LYSINE - Has several pathways for degradation but the pathway that proceeds via formation of saccharopine predominates in mammalian liver. - Pathway involves transamination, oxidative decarboxylation and reactions similar to fatty acyl CoA oxidation.
Recall: Lysine is a basic amino acid because it has an extra amino group.
- α-ketobutyrate is degraded and undergoes decarboxylation to propionyl CoA and then to succinyl CoA.
2. ISOLEUCINE AND VALINE
ISOLEUCINE - converted to propionyl CoA
VALINE - converted to methylmalonyl Co
SUMMARY:
- In this reaction lysine will condense with αketoglutarate and the product is saccharopine. - Saccharopine will be cleaved. Pag cleave sa kaniya yung amino group ng lysine sasama duon sa α -ketoglutarate. Then what happens next is yung α ketoglutarate naging glutamate the remaining structure is aminoadipate-semialdehyde. - In the next reaction the aldehyde will be oxidized to form a carboxylic group. It is now called aminoadipate. - In the next reaction etong amino group from aminoadipate will be removed and will then become α -ketoadipate. - α -ketoadipate will then undergo oxidative decarboxylation. During oxidative decarboxylation matatanggal yung carboxyl group and will release CO 2 and mag-aattach ng coenzyme A to form glutaryl-CoA. - Next reaction will be similar to fatty acid oxidation.
Recall: Fatty acid oxidation is reduction-hydrationreduction.
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
- So glutaryl CoA will be reduced using FAD to form crotonyl CoA. ( *reaction that is similar to fatty acid oxidation)
Module 8, Lecture 2.1
- First reaction is catalyzed by tryptophan oxygenase. And this involves cleavage of indole ring and thus will form N-L-formylkynurenine. - Eh di nag-open na yung ring, etong carbon na toh will be removed as formate and what would be left would be L-kynurenine. - L-kynurenine will be hydroxylate, mag-add ng OH and so it becomes 3-L-hydroxykynurenine. - 3-L-hydroxykynurenine is then converted to 3hydroxyyanthranilate.
- And then crotonyl CoA will undergo hydration to form B-hydroxybutyryl CoA. ( *reaction that is similar to fatty acid oxidation )
- 3-hydroxyyanthranilate can be converted into these series of reaction and form α-ketoadipate. Recall: α -ketoadipate is also an intermediate in lysine. Pareho silang nag-form ng α -ketoadipate.
- And then another reduction to form acetoacetylCoA. (*reaction that is similar to fatty acid oxidation )
- Eventually, it also forms acetoacetate and undergoes the same reaction with lysine.
- Acetoacetyl-CoA will then be converted to HMG CoA. And then eventually to acetoacetate. ( *these steps are similar to ketogenesis )
- Yung sa pathway ng tryptophan may mga side reactions. So etong 3-L-hydroxykynurenine can be converted to xanthurenate and etong 3hydroxyanthranilate pwede ding mag-form ng niacin, ng NAD.
2. TRYPTOPHAN
- Yung NAD, other derivatives ng niacin, pwedeng mag-feedback inhibition dun sa tryptophan oxygenase. So yung tryptophan oxygenase can be feedback-inhibited by nicotinic acid derivatives, NADH and NADPH. Kase nga in this pathway, pwede kang maka-form ng NAD. So etong 3hydroxyanthranilate pwedeng mag-proceed sa ibang pathway to form NAD. And, NAD since it’s a product of the pathway pwede niyang ma-feedbackinhibition yung tryptophan oxygenase.
Recall: Tryptophan is a source of the vitamin called niacin. - Carbon atoms of side chain and aromatic ring completely degraded via kynurenine-anthranilate pathway. - Initial reaction involves cleavage of indole ring with incorporation of 2 atoms of molecular oxygen by tryptophan oxygenase.
ACETOACETATE
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
*Why is this enzyme complex being emphasized?
- Glucocorticoid can induce tryptophan oxygenase. So, in the presence of glucocorticoid there will be an increase in the catabolism of tryptophan.
- This complex is functionally similar to PDH complex. It is made up of several enzymes. E1 is attached to thiamine pyrophosphate so it’s a decarboxylase. E2 is attached to lipoamide. It’s a dihydrolipoyl transacylase. E3 is dihydrolipoamide dehydrogenase.
TRYPTOPHAN OXYGENASE
An iron porphyrin metalloprotein.
Inducible in the liver by adrenal corticosteroid and by tryptophan.
Feedback inhibited by nicotinic acid derivatives including NADH and NADPH.
- Tryptophan itself and glucocorticoid can induce tryptophan oxygenase but it can be suppressed or feedback inhibited by NADH and other derivatives of nicotinic acid.
3. LEUCINE
Module 8, Lecture 2.1
- Recall:
PDH/ pyruvate dehydrogenase complex → converts pyruvate into acetyl-CoA.
α-ketoglutarate
dehydrogenase complex → converts α-ketoglutarate to succinyl CoA.
- α-ketoacid dehydrogenase complex, PDH complex and α-ketoglutarate dehydrogenase complex are all functionally similar.
Branched Chain Amino Acids
- Purely ketogenic amino acid - Degraded to HMG CoA which is converted to acetoacetate and acetyl-CoA
Catabolism of Branched Chain Amino Acids Branched Chain Amino Acids
o
LEUCINE
o
VALINE
o
ISOLEUCINE
Branched Chain Amino Acids
Shares the same first 3 reactions that employs common enzymes.
1. LEUCINE
Resulting products are then catabolized by distinct pathways.
- It will be converted to HMG CoA. And, HMG CoA will be cleaved into acetoacetate and acetyl-CoA. Thus, purely ketogenic.
- So, after the first 3 reactions mag-separate na sila ng pathways.
Same first 3 reactions shared by branched chain amino acids: 1. Transamination ketoacids.
to
corresponding
2. Oxidative decarboxylation corresponding acyl CoA.
α-
to
3. Dehydrogenation by FAD to form a double bond.
- Yung 2nd reaction, which is oxidative decarboxylation, it is catalyzed by branched chain
-ketoaci d dehydrog enas e complex.
α
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
2. VALINE - It will be converted to succinyl CoA. Thus, purely glucogenic.
AMINO ACIDOPATHIES
Almost always associated with amino acid catabolism rather than amino acid biosynthesis
Sufficient amounts of all amino acids- whether essential or non-essential – are present in a well-balanced diet.
Failure to catabolize amino acids will result in accumulation of the amino acid and its metabolites to the point that they become toxic.
AMINO ACIDOPATHIES
PHENYLKETONURIA - Results from the inability to convert phenylalanine to tyrosine.
3. ISOLEUCINE - Will eventually be converted to propionyl CoA and acetyl-CoA. Both glucogenic and ketogenic.
- Defect may be in the enzymes phenylalanine hydroxylase (classic PKU), tetrahydrobiopterine synthase or dihydrobiopterine reductase . - defect doesn’t have to be phenylalanine hydroxylase only. It can also be from the synthesis or recycling of tetrahydrobiopterine. - Major consequence is mental retardation. - Treatment is a diet low in phenylalanine. - Alternative pathways of phenylalanine catabolism in phenylketonurics. Ferric chloride test - detect the presence of phenylpyruvate; not reliable.
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Biochemistry
Catabolism of the Carbon Skeleton of Amino Acids
Module 8, Lecture 2.1
KEY POINTS!!!
ALKAPTONURIA - Defect in the catabolism of tyrosine. - Deficiency of enzyme homogentisate oxidase.
-Most striking manifestation is the darkening of urine that stands in air due to the presence of homogentisate. - Later develops arthritis and connective tissue pigmentation.
HARTNUP DISEASE - Defect in the intestinal and renal transport of neutral amino acids including tryptophan. - Manifest with pellagra-like signs and symptoms because of limited conversion of tryptophan to niacin.
MAPLE SYRUP URINE DISEASE - Defect in the absence of branched αketoacid dehydrogenase complex (resembles pyruvate dehydrogenase and α ketoglutarate dehydrogenase complex) - Odor of urine resembles maple syrup or burnt sugar. - Brain damage develops unless promptly treated with diet low in BCAA.
Transer: shann.au
Glucogenic amino acids are degraded to pyruvate, α-ketoglutarate, succinyl CoA, fumarate or oxaloacetate are therefore glucose precursors.
Ketogenic amino acid are broken down to acetyl-CoA or acetoacetate and can thus be converted to fatty acids or ketone bodies.
Asparagine and aspartate are converted to oxaloacetate.
Glutamine, glutamate, proline, arginine and histidine are converted to α-ketoglutarate.
Alanine, cysteine, glycine, serine, threonine and tryptophan are converted to pyruvate.
Phenylalanine, tyrosine and aspartate are converted to fumarate.
Isoleucine, methionine and converted to succinyl CoA.
The initial reactions are the same for the catabolism of branched chain amino acids.
Leucine is a purely ketogenic amino acid.
Phenylketonuria results from a defect in the conversion of phenylalanine to tyrosine.
Maple syrup urine disease is a condition resulting from the catabolism of branched amino acids.
valine
Sources: Doc Rivera’s ppt Recording Harper’s - images
Checked by: billygin
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