Course: Nutrition and Metabolism

Part (3): Amino Acids & Protein Metabolism

Lecture (3): Carbon Skeleton Catabolism

Dr. Nuha AminMobile: +249910050800

Amino Acid Catabolism

NH3Carbon Skeleton

Urea

Urea

Cycle

H2N C

O

NH2

Amino acid H3+N C

H

R

C

O

O-

energy

Fate of carbon skeleton of amino acids

Carbon skeletons are used for energy.

Amino Acid Catabolism

NH3Carbon Skeleton

Urea

Urea

Cycle

H2N C

O

NH2

Amino acid H3+N C

H

R

C

O

O-

GlucoseKetone bodies

Carbon skeleton fate

• All twenty common amino acids are

converted to only seven compounds: 1. Pyruvate

2. Acetyl CoA

3. Acetoacetyl CoA

4. alpha-Ketoglutarate

5. Succinyl CoA

6. Fumarate

7. Oxaloacetate

Products of Amino Acid Catabolism

• Glucose– Pyruvate

– a-Ketoglutarate

– Succinyl-CoA

– Fumarate

– Oxaloacetate

• Ketone bodies– Acetyl-CoA

– Acetoacetyl-CoA

Amino Acid Classification

Amino acids are grouped into :

1. Glucogenic amino acids produce glucose

2. Ketogenic amino acids produce acetyl-

CoA or acetoacetyl-CoA.

3. Mixed amino acids produce both glucose

and acetyl-CoA or acetoacetyl-CoA

GlucogenicArginine HistidineGlutamateGlutamineProlineAspartateAsparagineSerineGlycineAlanineValineMethionineThreonineCysteine

KetogenicLeucine

MixedPhenylalanineTyrosineTryptophanIsoleucineLysine

Amino Acid Catabolism.

–The fates of ketogenic (red) amino acid carbon

skeletons and glucogenic (yellow) amino acids.

Pyruvate

Acetyl-CoA

a-Ketoglutarate

Succinyl-CoA

Fumarate

Oxaloacetate

Acetoacetate

Glucose

Ile, Met, Val

Gln, Glu, His, Arg, Pro

Leu, Lys, Ile, Phe,

Trp, Tyr

Ala, Ser, Gly ,Trp, Thr, Cys

Asn, Asp

Phe, Tyr Glucose

Glucose

Glucose

Glucose

Five Amino acids give a-Ketoglutarate

• Glutamine

• Glutamate

• Histidine

• Arginine

• Proline

1.Glutamine is

converted to

a-ketoglutarate

H2O

N+H4

glutaminase

NAD(P)+

+ H2O

NAD(P)H + H+

+ N+H4

glutamatedehydrogenase

CH2

CH2

C C H COO-

N H3+

O

H3+N

glutamine

-OOC

H2

CH2

C C H COO-

N H 3+

glutamate

-OOC

H2

CH2

C C

O

COO-

a-ketoglutarate

2.Glutamate is

converted to

a-ketoglutarate

NAD(P)+

+ H2O

NAD(P)H + H+

+ N+H4

glutamatedehydrogenase

-OOC

H2

CH2

C C H COO-

N H 3+

glutamate

-OOC

H2

CH2

C C

O

COO-

a-ketoglutarate

3.Histidine

4.Arginine

5.

Proline

Six Amino acids give Pyruvate

• Alanine

• Serine

• Glycine

• Cysteine

• Tryptophan

• Threonine

1.Alanine is

converted to

pyruvate.H3C C H

N+H3

COO-

alanine

H3C C

O

COO-

pyruvate

a-ketoglutarate

glutamate(NH3)

alanine

transaminase

2.Glycine

3. Serine catabolism

2.Serine is deaminated to pyruvate by serine

Dehydratase.

HO CH2

HC COO

−

NH3+

C COO−

OH2O NH4+

C COO−

NH3+

H2C H3C

H2O

serine aminoacrylate pyruvate

Serine Dehydratase

Serine

Pyruvate

GlycineSerine

hydroxymethyl-

transferase

Serine

dehydratase

4. Cysteine

5. Threonine

Pyruvate

6. Tryptophan gives pyruvate

and acetoacetyl-CoA

Two Amino acids give Oxaloacetate

• Asparagine

• Aspartate

L-asparginase

CH2

C CH COO-

NH3+

O

H2N

CH2

C CH COO-

NH3+

O

-O

H2O

N+H4

asparagine

aspartate

a-ketoglutarate

glutamate(NH3)

aspartateaminotransferase

-OOC

H2

C C COO-

Ooxaloacetate

Breakdown of asparagine

to intermediate

oxaloacetate.

L-asparginase

CH2

C CH COO-

NH3+

O

H2N

CH2

C CH COO-

NH3+

O

-O

H2O

N+H4

asparagine

aspartate

a-ketoglutarate

glutamate(NH3)

aspartateaminotransferase

-OOC

H2

C C COO-

Ooxaloacetate

Breakdown of aspartate

intermediate

oxaloacetate.

COO-

CH3+N

CHH3C

CH3

H

COO-

CH3+N

CHH3C

CH2

H

CH3

COO-

CH3+N

CH2

CH

H

CH3

H3C

COO-

C

CHH3C

CH3

O

COO-

C

CHH3C

CH2

O

CH3

COO-

C

CH2

CH

O

CH3

H3C

C

CHH3C

CH3

O SCoA

C

CHH3C

CH2

CH3

O SCoA

C

CH2

CH

CH3

H3C

O SCoA

CoASH CO2

a-keto acid

dehydrogenase

complexbranched-

chain

amino

transferase

valine

isoleucine

leucine

Acyl-CoA

derivatives

Breakdown of branched amino acids

Leucine

• Is the only purely ketogenic amino acid

• Gives only acetyl-CoA on breakdown

Three Amino acids give Succinyl-CoA

• Isoleucine

• Valine

• Methionine

Methionine, isoleucine and valine

are converted to succinyl CoA

Two Amino acids give Fumarate

and Acetoacetate

•Phenylalanine

•Tyrosine

Phenylalanine and tyrosine

Phenylalanine and tyrosine

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