Amino Acid Degradation
April 14, 2003
Bryant Miles
The carbon skeletons of amino acids are broken down into metabolites that can either be oxidized into
CO2 and H2O to generate ATP, or can be used for gluconeogenesis. The catabolism of amino acids
accounts for 10 to 15% of the human body’s energy production. Each of the 20 amino acids has a
separate catabolic pathway, yet all 20 pathways converge into 5 intermediates, all of which can enter the
citric acid cycle. From the citric acid cycle the carbon skeletons can be completely oxidized into CO2 or
diverted into gluconeogensis or ketogenesis.
Glucogenic amino acids are broken down into one of the following metabolites: pyruvate, αketoglutarate, succinyl CoA, fumarate or oxaloacetate. Ketogenic amino acids are broken down into
acetoacetate or acetyl-CoA. Larger amino acids, tryptophan, phenylalanine, tyrosine, isoleucine and
threonine are both glucogenic and ketogenic. Only 2 amino acids are purely ketogenic they are lysine and
leucine. If 2 of the amino acids are purely ketogenic and 5 amino acids are both ketogenic and glucogenic,
than that leaves 13 amino acids that are purely glucogenic: Arg, Glu, Gln, His, Pro, Val, Met, Asp, Asn,
Ala, Ser, Cys, and Gly.
I. Amino Acids that are Catabolized into Pyruvate.
Pyruvate is the entry point for amino acids that contain 3 carbons, alanine, serine and cysteine.
Alanine transaminase reversibly transfers the amino group from alanine to α-ketoglutarate to form
pyruvate and glutamate. Note that enzyme requires a pyridoxal phosphate cofactor. The α-ketoglutarate
is regenerated by glutamate dehydrogenase.
O
O
C
C
H3 N
O
O
O
+
CH
O
C
CH2
O
H3 N
O
CH
+
C
CH2
CH2
CH2
CH3
O
C
O
C
CH3
C
C
O
O
O
O
Alanine Transaminase
O
O
NAD(P)+
H3N
NAD(P)H
C
O
C
O
CH
O
C
CH2
CH2
Glutamate Dehydrogenase
CH2
C
CH2
C
O
O
O
O
Serine dehyratase is another enzyme that requires a pyridoxal phosphate cofactor. This enzyme catalyzes
the β-elimination of the hydroxyl group of serine to form an amino acrylate intermediate which
tautomerizes into the imine which is then hydrolyzed to produce ammonia and pyruvate.
H3N
CH
C
CH3
CH2
O
H3N
O
C
Serine
Dehydratase
CH2
OH
C
O-
H2N
C
O
C
H2O
-
O
O
NH4+
H3C
C
C
O
O
O-
Glycine is converted into pyruvate via conversion of glycine to serine by serine hydroxymethyl
transferase which is an incredibly interesting enzyme. It contains a pyridoxal phosphate cofactor and a
N5,N10-methylene-tetrahydrofolate which is a cofactor we have not encountered yet. The N5,N10methylene-tetrahydrofolate is produced by the glycine cleavage system which transfers a methylene group
from glycine to THF. The THF cofactor is a one carbon acceptor and donor. We will discuss this
cofactor further when we get to amino acid biosynthesis.
O
O
H3N
CH
C
+ THF
O
H3N
CH
C
O
+ N5,N10-Methylene-THF
H
H
Serine hydroxymethyl
Transferase
NAD+
Glycine Cleavage System
THF
NADH
O
+
NH4
5, 10
+ CO2 + N N -Methylene-THF
H3N
CH
CH2
OH
C
O
They are several pathways by which cysteine is converted into pyruvate. The three alkyl carbons of
trypophan are converted into alanine which is then converted by alanine transaminase into pyruvate.
Threonine is both glucogenic and ketogenic. There are a couple of routes for the degradation of
threonine. The major route is shown below. Threonine is converted into acetyl CoA and glycine.
Glycine is then converted into serine by serine hydroxymethyl transferase, and serine is then converted
into pyruvate by serine dehydratase.
O
O
C
H3 N
NADH + H+
NAD+
C
O
O
C
CH
H3 N
CoA
O
H3 N
CH
CH
H
Threonine Dehydrogenase
CH
OH
C
CH3
O
+
O
O
CH3
CoA
α-amino-β-ketobutyrate
S
C
CH3
α-amino-β-ketobutyrate lyase
O
C
H3 N
O
H3C
CH
O
O
C
C
O-
H
II. Amino Acids Degradated to Oxaloacetate
Aspartate and asparagines are both degraded into oxaloacetate. Asparagine is hydrolyzed into aspartate
and ammonia by asparaginase. Aspartate is converted into oxaloacetate by aspartate amino transferase
which is a PLP enzyme that transfers an amino group from aspartate to α−ketoglutarate to form glutamate
and oxaloacetate.
O
C
H3N
O
H2O
+
O
CH
Asparaginase
O
C
O
CH
CH2
C
NH2
O
O
O
O
C
C
O
+
CH
O
CH2
C
CH2
O
C
O
O
O
C
O
O
C
CH2
O
C
H3N
CH2
H3N
NH4
Aspartate
Aminotransferase
O
+
C
O
H3N
O
CH
CH2
CH2
C
C
O
O
CH2
C
O
O
III. Amino Acids Degraded to α-Ketoglutarate.
Glutamine, proline, arginine and histidine are
converted into glutamate which is then deaminated
by a transaminase to form α-ketoglutarate.
Glutamine is converted into glutamate by glutaminase. Proline is oxidized by proline oxidase to form
pyrroline 5-carboxylate which spontaneously hydrolyzes to from glutamate γ-semialdehyde. From the
urea cycle we know that arginase converts arginine into ornithine and urea. Ornithine δ-aminotransferase
transfers the δ-amino group of ornithine to α-ketoglutarate to form glutamate γ-semialdehyde and
glutamate.
Glutamate γ-semialdehyde is oxidized to form glutamate by glutamate -5-semialdehyde dehydrogenase.
Histidine is deaminated by histidine ammonia lyase which forms urocanate. Urocanate hydratase adds
water to form 4-Imidazolone-5-propionate which is hydrolyzed by imidazalone propionase to form Mformiminoglutamate. Glutamate formiminotransferase transfers the formimino group to tetrahydrofolate
to generate glutamate and N5-formimino-THF.
IV. Amino Acids that are Broken Down in Succinyl-CoA.
Methionine, valanine and isoleucine are broken down into propoinyl CoA. By studying β-oxidation of
odd chain fatty acids we know that propionyl CoA is converted into D-methylmalonyl CoA by propionyl
CoA carboxylase. D-methylmalonyl CoA is racemized into L-methylmalonyl CoA by methylmalonyl
CoA racemase. Methylmalonyl mutase produces succinyl CoA.
The degradation of methionine requires 9 steps. One of which involves the synthesis of Sadenoylmethionine (SAM). The methyl group of SAM is highly reactive making it an important
methylating reagent. SAM is a common methyl-group donor in the cell. The degradation of methionine
is shown on the next page.
The first step is catalyzed by methionine adenosyl transferase which tranfers the adenosyl group of ATP
to the sulfer of methionine to form SAM. Sam methylase transfers the activated methyl group to an
acceptor to form S-adenosylhomocysteine which is hydrolyzed by adenosylhomocysteinase to form
homocysteine. Cystathionine β-synthase is a PLP dependent enzyme that catalyzes the condensation of a
serine residue with homocysteine to form cystathionine.
Cystathioniine γ-lyase cleaves cystathionine into cysteine and α-ketobutyrate. α−ketobutyrate is
converted into propionyl CoA by α-ketobutyrate dehydrogenase which catalyzes a reaction that is
analogous to pyruvate dehydrogenase and α-ketoglutarate dehydrogenase.
Branched Chain Amino Acids
The degradation of branched chain amino acids uses some of the enzymes we have already encountered in
the citric acid cycle or β-oxidation.
O
O
C
S
C
CoA
S
CoA
CH CH3
CH CH3
CH2
CH3
CH3
O
O
H3N
CH C
O
H3N
FAD
CH C
O
CH CH3
CH CH3
FADH2
FADH2
CH3
CH2
O
O
CH3
α-KG
α-KG
C
S
CoA
C
CH3
CH3
Glu
C
S
C
CH3
CoA
CH2
CH
Branched Chain Amino Acid
aminotransferase
H2O
Tiglyl-CoA
H 2O
Glu
O O
O O
C
C
C
O
O
C
O
O
CH CH3
CH CH3
CH2
CH3
C
HO
CoA-SH +
Branched Chain α-ketoacid
Dehydrogenase
NAD+
NADH + CO2
CoA
C
S
HO
CH2
CH3
S
CoA
CH CH3
CH2
O
NAD+
CH
NADH
H2O + NAD+
NADH
NADH
C
CoA
CH CH3
C
CH CH3
CH
O
CH CH3
CH3
CoA
CoA
NAD+
O
O
S
S
O
S
CH3
CoA-SH + NAD+
C
C
CH CH3
CH3
NADH + CO2
FAD
Acyl CoA Dehydrogenase
CoA
CoASH
S
S
CoA
O
CH CH3
O
O
CH3
H3 C
C
H3C
S
CoA
C
C
CH2
C
O
CO2
S
CH CH3
O
C
O-
CoA
Most amino acid catabolism occurs in the liver. The branched chain amino acids are not catabolized in the
liver. Branched chain amino acids are catabolized mainly in the muscle, adipose, kidney and the brain.
The liver does not contain the branched amino acid aminotransferase enzyme which these other tissues
contain.
Branched chain α-ketoacid dehydrogenase is a huge multienzyme complex homologous to pyruvate
dehydrogenase and α-ketoglutarate dehydrogenase. This enzyme contains a thiamine pyrophosphate
cofactor, a lipoamide cofactor, a FAD prosthetic group. The chemistry, mechanism and structure of these
enzymes is very similar.
Branched chain α-ketoacid dehydrogenase is phosphorylated by a kinase which inactivates the enzyme in
a similar manner that pyruvate dehydrogenase is phosphorylated and inactivated. The intake of dietary
branched amino acids activates a phosphatase which activates this enzyme.
A genetic deficiency in the branched chain α-ketoacid dehydrogenase enzyme is called maple syrup urine
disease. The deficiency causes an excessive buildup of branched α-ketoacids in the blood and the urine.
The urine of these patients has the odor of maple syrup and hence the name of the disease. Maple syrup
disease usually leads to mental retardation unless the patient is placed on diet that is low in valine,
isoleucine and leucine early in life.
V. Amino Acids that are degraded into Acetyl CoA and Acetoacetate.
There are only two amino acids that are purely ketogenic, lysine and leucine. Leucine catabolism is
similar to the branched amino acids valine and isoleucine.
First leucine is transaminatedby branched amino acid aminotransferase to form α-ketoisocaproate which
is then oxidatively decarboxylated to form isovaleryl CoA by the branched chain α-ketoacid
dehydrogenase complex we just discussed. In the next step isovaleryl CoA is dehydrogenated to form βmethylcrotonyl CoA. The enzyme that catalyzes this dehydrogenation is isovaleryl CoA dehydrogenase.
β-methylcrotonyl CoA is then carboxylated by a biotin containing enzyme called methylcrotonyl CoA
carboxylase to form β-methylglutaconyl CoA.
ATP
+
HCO32O
CH3
FAD
O
FADH2
CH3
O
CoA
CoA
S
CH3
Isovaleryl CoA
S
β-Methylcrotonyl CoA
ADP
+
Pi
CH3
CH3
CO-2
CoA
S
C
H2
β-Methylglutaconyl CoA
β-methylglutaconyl CoA is then hydrated by β-methylglutaconyl CoA. hydratase to form β-hydroxy-βmethylglutaryl CoA which is then cleaved into acetyl CoA and acetoacetate. The enzyme that catalyzes
the last step is HMG-CoA lyase, a familiar enzyme from ketogenesis.
If you are curious the pathway for the catabolism of lysine is shown in the text on page 630. The carbons
of lysine end as acetyl CoA and acetoacetate.
VI. Catabolism of Aromatic Amino acids.
•
•
•
•
•
•
•
The degradation of aromatic amino acids requires molecular
oxygen to break down the aromatic rings.
The degradation of phenylalanine begins with a
monooxygenase, phenylalanine hydroxylase which adds a
hydroxyl group to phenylalanine to from tyrosine.
Tyrosine aminotransferase deaminates tyrosine to form phydroxyphenylpyruvate.
p-hydroxyphenylpyruvate dioxygenase catalyzes the
formation of homogentisate.
Homogentisate 1,2-dioxygenase catalyzes the formation of
maleylacetoacetate.
Maleylacetoacetate isomerase produces fumarylacetoacetate.
Fumarylacetoacetase produces fumarate and acetoacetate.
Many genetic defects of phenylalanine catabolism in humans has
been identified.
A deficiency in phenylalanine hydroxylase is responsible for the
disease phenylkotonuria (PKU) which is caused by elevated
concentrations of phenylalanine.
Individuals with a deficiency in phenylalanine hydroxylase rely on a
secondary catabolic pathway which in normal individuals is not used.
In this pathway phenylalanine is converted into phenylpyruvate by a
transaminase which transfers the amino group to pyruvate to form
alanine. Phenylalanine and phenylpyruvate accumulate in the blood
and excreted in the urine hence the term phenylketonuria.. Some of
the phenylpyruvate is decarboxylated to form phenylacetate. Some
of the phenylpyruvate is reduced to phenyllactate. Phenyllactate
gives the urine a distinctive odor used for diagnosis.
The high concentration of phenylalanine in the blood
limits the transport of amino acids across the blood-brain
barrier resulting in impairment of normal brain
development, causing severe mental retardation. This can
be avoided by early detection and rigid dietary control.
The diet must only provide enough phenylalanine and
tyrosine to meet the needs of protein synthesis.
Alkapotonuria is another inheritable disease of phenylalanine
catabolism. In this case the defective enzyme is homogentisate
dioxygenase. This disease is less severe than PKU. Large amounts of
homogentisate are excreted in the urine. The oxidation of
homogentisate turns the urine black. Individuals who have this disease
develop arthritis at an early age.
Tryptophan catabolism is shown below:
Like phenylalanine catabolism, dioxygenases are required to catabolize the aromatic rings.