Biochemistry I
Lecture 7 2008 (J.S.)
Amino acid metabolism II
Metabolism of individual amino acids
2
The degradation of amino acids usually begins with deamination.
However, transamination or oxidative deamination is not the first
reaction in catabolism of eight amino acids:
Serine and threonine are deaminated by dehydration, and
histidine undergoes deamination by desaturation
(both reactions were mentioned previously).
The five remaining amino acids are deaminated later on, after partial
transformation:
Arginine – deamination occurs after transfomation to ornithin,
lysine – transamination follows the transformation to α-aminoadipate,
methionine – deamination of homoserine,
proline – deamination after conversion to glutamate,
tryptophan – after its transformation to kynurenine, alanine is
released.
3
Each carbon skeleton of deaminated amino acids follows
a unique metabolic pathway to compounds , which can be
completely oxidized by way of the citrate cycle to CO2 and water.
In spite of this common fate, amino acids are classified as
glucogenic and ketogenic according to the type of their
intermediate metabolites.
The ketogenic amino acids give rise to acetoacetate or
acetyl-CoA (from which acetoacetate can be synthesized)
that cannot be transformed to glucose.
The glucogenic amino acids give rise to pyruvate or some of
the intermediate of the citrate cycle, which can serve as
substrates for gluconeogenesis.
4
Glucogenic and ketogenic amino acids
5
Irreversible conversions in the metabolism of amino acids
show which proteinogenic amino acids are essential:
6
Essential amino acids:
Threonine
Methionine
Lysine
Valine
Leucine
Isoleucine
Histidine
Phenylanine
Tryptophan
Nonessential amino acids
Glycine
Alanine
Serine
Cysteine
Aspartate
Asparagine
Glutamate
Glutamine
Proline
Arginine
Tyrosine
7
The metabolism of amino acids will be described
in the following sequence:
1 The most simple AA that give pyruvate – Ala, Ser, Gly, Thr
2 Amino acids containing sulfur – Met, Cys
3 Sources of one-carbon units and use of those units in syntheses
4 Aspartic acid
5 Glutamic acid and its relation to Arg, Pro, His
6 Branched-chain amino acids – Val, Ile, Leu
7 Lysine
8 Aromatic amino acids – Phe, Tyr, and Trp
8
Alanine - by transamination.
Serine - by deamination catalyzed of dehydratase (hydrolyase).
Glycine - by accepting one-carbon group gives serine.
Threonine - by splitting gives glycine that may give serine.
1 Amino acids that are converted to pyruvate:
Cysteine also gives pyruvate by deamination and desulfuration
(see "Amino acids containing sulfur"), as well as
tryptophan that after transformation to kynurenin releases alanine
(see "Aromatic amino acids").
9
Alanine is nonessential and glucogenic;
it undergoes transamination to pyruvate readily:
H3C–CH–COOH
NH2
H3C–CH–COOH
NH2
alanine aminotransferase (ALT)
pyruvateGlu
2-oxoglutarate
H3C–C–COOH
O
alanine
pyruvate
glucose glucose
pyruvate
alanine
LiverMuscle
Blood
gluconeogenesisglycolysis
Concentrations of alanine in blood plasma are 300 – 400 μmol/l (the
second highest next to glutamine). Alanine is released from muscle
tissue and serves both as the vehicle for NH3 transport from muscle
to liver and a substrate for liver gluconeogenesis. This bidirectional
transport is called the alanine cycle (or glucose-alanine cycle).
10
Serine
CH2–CH–COOH
OH NH2 is nonessential and glucogenic;
Serine does not take part in transamination,
but it is directly deaminated by dehydration:
– nonessential – synthesis of the carbon skeleton from 3-phosphoglycerate
– glucogenic – direct deamination by serine dehydratase to pyruvate
pyruvate
CH2–CH–COOH
OH NH2
serine
enamine
CH2=C–COOH
NH2
imine
CH3–C–COOH
NH
=H2O
H2O
NH3
CH3–C–COOH
O
=
11
Decarboxylation of serine results in ethanolamine
(a constituent of phospholipids) that gives choline
by methylation.
ethanolamine
HO–CH2–CH2–NH2
CH3
choline
HO–CH2–CH2–N–
CH3
CH3
+
Serine is a substantial source of one-carbon groups:
its -CH2-OH group is readily transferred to tetrahydrofolate
(coenzyme of C1-group transferase), the product is glycine that
is able to yield the second C1-group.
The reaction is reversible, but the synthesis of serine from glycine
and a C1-group is not an advantage.
glycine
CH2–CH–COOH
OH NH2
serine
+ H4folate + CH2OH–H4folate
hydroxymethyl-H4folate
CH2–COOH
NH2
12
Demands for serine in the body are great – both one-carbon
groups and substrates for the synthesis of complex lipids have to
be supplied.
Therefore, the synthesis of carbon skeleton from glucose is of
great significance:
glucose serine
3-P-hydroxypyruvate3-phosphoglycerate
CH–OH
COOH
CH2–O–P=O
O–
O–
COOH
C=O
CH2–O–P=O
O–
O–
COOH
CH2–O–P=O
O–
O–
CH–NH2
3-phosphoserine
NAD+
Glu
2-oxoglutarate
13
Utilization of serine:
NH3
serineglycine
pyruvate
CH2-OH–H4folate
glucose
3-phosphoglycerate
phosphohydroxypyruvate
phosphoserine
Glu
palmitoyl-CoA
phosphatidyloserines
ethanolamine
choline
acetylcholine
phosphatidylethanolamines
phosphatidylcholines
sphingosine sphingomyelins
14
Glycine
CH2–COOH
NH2
is nonessential and glucogenic;
– nonessential – originates from serine or from CO2, NH3, and C1-group
– glycogenic (weakly) – may accept C1-group and give serine
Reversible reaction
glycine + CH2OH–H4folate serine + H4folate
(described as an important source of C1-groups) is not a useful
way of glycine catabolism, because it consumpts one C1-group.
Transamination of glycine with pyruvate
glycine + pyruvate glyoxylate + alanine
as well as oxidative deamination of glycine
glycine + FAD glyoxylate + FADH2
are possible, although limited; the enzymes catalyzing those reactions
have sufficient activity only in peroxisomes. It is worth mentioning that
glyoxylate formed in those minor pathways gives small amounts of
unwanted oxalate. High production of oxalate is dangerous.
15
The major pathway of glycine catabolism is
oxidative cleavage of glycine in mitochondria:
CH2–COOH
NH2
+ H4folate CO2 + NH3 + N5
,N10
-methylene-H4folate
glycine
The reaction is reversible and catalyzed by glycine synthase and
controlled by respiration and energetic charge of the cell. For
the synthesis of glycine, 3 molecules of ATP are lost.
Molecule of glycine is the substrate required for the syntheses
of several very important compounds, e.g.
purine bases of nucleic acids, porphyrins of haemoproteins,
phosphocreatine of skeletal muscles (phosphagen), and
tripeptide glutathione (intracellular antioxidant).
16
glycineserine
hydroxymethyl-H4folate
methylene-H4folate
CO2 NH3
Gly oxidative splitting / Gly synthesis
(mitochondria)
pyruvate
Ala
glyoxylate
(oxaluria)
Synthesis of
creatine
purine
porphyrin
glutathione
glycine conjugates
of bile acids,
of aromatic acids
(hippuric acids)
Utilization of glycine:
17
Threonine is essential and both glucogenic and ketogenic
CH3–CH2–CH–COOH
OH NH2
It does not undergo transamination
Splitting of threonine to glycine
– glucogenic – gives glycine by splitting
or succinyl-CoA (by dehydratation and
and oxid. decarboxylation to propionyl-CoA)
– ketogenic – by splitting to glycine gives acetyl-CoA
CH2–COOH
NH2
glycine
CH3–CH=O
acetaldehyde
serine
CH3–CH2–CH–COOH
OH NH2
CH3-CO–S-CoA
acetyl-CoA
pyruvate
Ser CH2OH transferase
CH2OH-H4folate
Thr dehydrogenase
2-amino-3-oxobutyrate
specific lyase
oxid.
oxid.
18
An alternative pathway is
the direct deamination of threonine by dehydration:
(slow) oxidative
decarboxylation
CH3-CH2-CO–S-CoA
propionyl-CoA
HOOC-CH2-CH2-CO–S-CoA
sukcinyl-CoA
carboxylation to methylmalonyl-CoA
rearrangement (B12 coenzyme)
[ enamine imine ]
CH3–CH2–CH–COOH
OH NH2
2-oxobutyrate
O
=
CH3–CH2–C–COOH
H2O
H2O NH3
19
2 Sulfur containing amino acids
Methionine CH2–CH2–CH–COOH
CH3–S NH2
ATP
PPi + Pi
is essential and glucogenic
Methionine is a common methyl donor in the cell:
– glucogenic (it yields succinyl-CoA)
CH2–S–CH3
CH2
CH-NH2
COOH
homocysteine
CH2
CH-NH2
COOH
CH2
–
SH
S-adenosylmethionine
CH2
CH-NH2
COOH
CH3
Rib-AdeCH2–S
S-adenosylhomocysteine
CH2
CH-NH2
COOH
CH2–S
Rib-Ade
H4folate
CH3-H4folate
Ado
methionine
remethylation
substrates
methylated substrates
(B12 coenzyme)
20
S-adenosylmethionine (S-AM)
HOOC-CH-CH2-CH2–S
NH2
CH3
CH2
O
OHOH
N
N
N
N
NH2
+
is the methyl donor. The methyl
group may be transferred from
a sulfonium ion to various acceptors.
Activated methionine
Examples:
synthesis of choline from phosphatidylethanolamine,
synthesis of creatine (by methylation of guanidinoacetate),
methylation of noradrenaline to adrenaline,
inactivation of catecholamines by catechol-O-methyl transferase,
methylation of histones, etc.
21
Catabolism of methionine
propionyl-CoA
– demethylation to homocysteine
– transsulfuration with serine to homoserine and cysteine
– conversion to 2-oxobutyrate (homoserine deaminase),
propionyl-CoA, and succinyl-CoA.
methionine
S-AM
–CH3 + Ado
– H2O
2-oxobutyratehomoserine
homocysteine
cystathionine
cysteine
+ H2O
NH3
serine
CoA-SH CO2
NAD+
NADH + H+
succinyl-CoA
methylmalonyl-CoA
CO2 (biotin)
(coenz. B12)
22
At present, high concentration of homocysteine in blood plasma is
included among other biochemical markers of cardiovascular diseases –
as a risk factor for atherosclerosis that is quite independent on the
concentration of cholesterol.
Homocysteine is an important intermediate in metabolism of methionine;
it is readily transformed, either remethylated to methionine
(the reaction requires tetrahydrofolate and cobalamin)
or decomposed to homoserine by transsulfuration with serine,
(vitamin B6 dependent).
If those mechanisms are not sufficient and the concentration of homocysteine in
biological fluids increases, injury of endothelial cells by homocysteine (e.g., high
production of reactive oxygen species, lipoperoxidation) and decreased vitality
of blood platelets may appear.
23
Cysteine
CH2–CH–COOH
SH NH2
is nonessential and glucogenic
– nonessential – synthesis from serine
(methionine supplies the sulfur atom)
– glucogenic – cysteine is converted into pyruvate
(sulfur atom is released as SO3
2–
, HS–
, or SCN–
)
cysteine
CH-NH2
COOH
CH2–SH
cysteine sulfinate
CH2
CH–NH2
COOH
S-O H+
O
ll
pyruvate
COOH
CH3
C=O
oxidation of SH-group
(mitochondrial dioxygenase)
O2 + NADPH + H+
The major catabolic pathway is the direct oxidation of SH-group:
transamination
2-OG Glu
3-sulfinylpyruvate
CH2
C=O
COOH
S-O H+
O
ll
H+
O -S
O
ll
O H+
H+
O -S=O
O
ll
O H+
SO4
2–
sulfate
SO3
2–
sulfite
sulfite oxidase
24
Oxidation of S–II
to SIV
or SVI
(sulfinate, sulfite, sulfate) is
a proton-producing process, nonvolatile acids are formed from
non-ionized groups. The catabolism of sulfur-containing amino acids
slightly acidifies the body.
desulfurationC=O
COOH
CH2–SH2-oxoglutarate
Glu
transamination
cysteine
CH-NH2
COOH
CH2–SH
HS–
H+
An alternative catabolic pathway of cysteine is transamination :
3-sulfanylpyruvate pyruvate
COOH
CH3
C=O
Hydrogen sulfide HS–
ion is mostly oxidized to sulfite SO3
2–
or, if cyanide ion CN–
is present (e.g. tobacco smokers), hydrogen sulfide gives thiocyanate SCN–
.
Sulfite anion is oxidized to sulfate anion, which is either excreted into
the urine (approx. 20 – 30 mmol/d) or utilized for sulfations after activation:
(molybdopterin, cyt b5)
mitochondrial sulfite oxidase
sulfate
SO4
2–
sulfite
SO3
2–
excretion
(ATP)
3´-phosphoadenosyl
5´-phosphosulfate, PAPS
25
Examples of sulfations by means of PAPS:
synthesis of proteoglycans (sulfation of glycosaminoglycans),
sulfation of saccharidic components in glycolipids and glycoproteins,
formation of sulfate esters in inactivation of steroid hormones, catecholamines,
and in the phase II of biotransformation of phenols.
is the mixed anhydride of sulfuric and
phosphoric acid called "active sulfate";
it serves as the sulfate donor in forming
of sulfate esters (or N-sulfates).
3‘-Phosphoadenosyl-5‘-phosphosulfate (PAPS)
O
O–P=O
O
3'
5'
CH2
O
OH
N
N
N
N
NH2
O
O
O
O
O–S–O–P–O–
26
Utilization of methionine and cysteine
2-aminoethanethiol
(cysteamine, constituent of coenzyme A)
methionine
taurinehypotaurine
conjugation with
bile acids
decarbox.
cysteine
3-cysteine
sulfinate
pyruvate
serine
methylations
SAM homoserine
succinyl-CoA
SO3
2–
SO4
2–
PAPS
sulfate esters
of sugars and phenols
cystine
decarbox.
glutathione
(γ-glutamyl-cysteinyl-glycine)
mercapturic acids
(N-acetyl-S-arylcysteines)
– 2H
27
Glutathione (GSH, γ-glutamyl-cysteinyl-glycine)
CO–NH–CH–CO–NH–CH2–COOH
CH2 CH2–SH
CH–NH2
COOH
CH2
α
γ
(reduced form)
1 Reduced G-SH confronts oxidative stress,
it reduces peroxides (lipid hydroperoxides and hydrogen peroxide) in the reaction
catalyzed by a selenoprotein glutathione peroxidase, and (non-enzymatically)
methaemoglobin (FeIII
, hemiglobin) to haemoglobin (FeII
) and disulfides to thiols:
L-OOH + 2 G-SH L-OH + G-S–S-G + H2O
R-S–S-R + 2 G-SH 2 R-SH + G-S–S-G
reduced G-SH can be regenerated by glutathione reductase and NADPH + H+
.
2 Conjugation to lipophilic compounds (detoxification of reactive electrophiles).
3 Transport of amino acids into cells with concomitant attachment of γ-glutamyl
(group translocation, γ-glutamyl cycle).
2 G-SH G-S–S-G
– 2H
+ 2H
Functions:
is a tripeptide with a free sulfanyl group,
required to maintain the normal reduced
state in the cell:
28
Sites for bonding of one-carbon units
–CO–NH-CH-CH2-CH2-COOH
COOH
105N
N
N
N
H2N
OH
CH2-NH–
H
H
tetrahydropteroic acid
glutamate
(1 – 5 residues)
One-carbon groups are transferred by tetrahydrofolate (H4folate, FH4,
tetrahydropteroylglutamate).
Mammals can synthesize the pteridine ring, but they are unable to
conjugate it to the other two units. They obtain folate from diets or from
microorganisms in their intestinal tracts.
3 Sources of one-carbon groups and
utilization of those groups in syntheses
29
(The fully oxidized one-carbon group is CO2, but CO2 is transferred by biotin,
not by H4folate.)
The one-carbon groups transferred by H4folate exist in three oxidation states:
Example:
N5
,N10
-methylene FH4
–CO–NH-CH-CH2-CH2-COOH
COOH
105N
N
N
N
H2N
OH
CH2–N–
H
CH2
30
histidine tryptophan
glycine, serine
methionine
N5
,N10
-methylene FH4
N5
-methyl FH4
N5
-formimino FH4 N5
,N10
-methenyl FH4 N10
-formyl FH4
H2ONH3
NADH
NADPH
purine
nucleotides
thymine
nucleotides
S-AMhomocysteine
methylations
31
4 Aspartic acid and asparagine
Aspartate
HOOC–CH2–CH–COOH
NH2 Asparagine
H2N-CO–CH2–CH–COOH
NH2
is nonessential and glucogenic
– it gives oxaloacetate by transamination
oxaloacetate
AST
glutamate2-oxoglutarate
aspartate
asparagine
NH3
H2O
asparaginase
glutamate glutamine
+ AMP + 2 Pi + ATP
Asn synthetase
is the amide of aspartate
32
Utilization of aspartate and asparagine
asparagine
aspartateoxaloacetate
AST
Glu
incorporated into
the skeleton of
pyrimidine bases
CO2
β-alanine
Glu
NH3 transport in CNS ?
NH3
NH3 for the synthesis of
urea
AMP from IMP
purine
malate fumarate
33
5 Glutamic acid, glutamine, and the relationship
to proline, arginine, and histidine
Glutamate
Glutamine
HOOC–CH2–CH2–CH–COOH
NH2
H2N-CO–CH2–CH2–CH–COOH
NH2
2-oxoglutarate
ALT
alaninepyruvate
glutamate
glutamine
NH3
H2O
glutaminase
ADP + Pi NH4
+
+ ATP
Gln synthetase
is nonessential and glucogenic
– it gives oxaloacetate readily by oxidative
deamination or transamination
is an amide of glutamate
34
Direct oxidative deamination of glutamate by dehydrogenation
glutamate
NAD(P)+
NAD(P)H + H+
NH2
HOOC-CH-CH2-CH2-COOH
2-IminoglutarateNH
HOOC–C–CH2-CH2-COOH
HOOC–C–CH2-CH2-COOH
O
H2O
NH3
The reaction is catalysed by the mitochondrial enzyme
glutamate dehydrogenase (GLD). It requires either NAD+
or NADP+
as coenzyme,
and its activity in mitochondria is high.
The equilibrium favours glutamate
synthesis, but it is pulled in the direction
od deamination by the continuous
removal of NH3/NH4
+
.
2-oxoglutarate
35
Decarboxylation of glutamate (very active in brain)
glutamate
γ-aminobutyric acid (GABA)
an inhibitory neurotransmitter in CNS
γ
– CO2
Reversible reduction of glutamate
– intermediate in the synthesis
and degradation of proline and arginine
NADH+H+
2 H+
NAD+
H2O
glutamate glutamate 5-semialdehyde
ornithine
proline
36
Utilization of glutamate and glutamine
CO2
γ-aminobutyrate
(inhibitory neurotransmitter)
transport of NH3 to the liver and kidney
donor of NH3 for the syntheses of
carbamoyl phosphate
purine
amino sugars
glutamine
glutamate2-oxoglutarate
transaminases
AA 2-oxoA
histidine
prolineornithine
glutamate semialdehyde
NH3NH3
glutamate
(excitatory neurotransmitter)
folate
glutathione
pteroate
cysteine
glycine
37
Glutamate is widely used as a food additive to enhance flavour of
dishes, particularly in Chinese cookery in high amounts.
Excess in the diet (1 – 5 g of glutamate in one dose, e.g. in the form
of "Von-Ton“ soup) can cause unpleasant feelings in sensitive
persons – the Chinese restaurant syndrome.
38
Arginine
NH
C
H2N
NH
NH2
CH2–CH2–CH2–CH–COOH is nonessential and glucogenic
– nonessential in adult man (required in
the diet during the growth)
– degraded to 2-oxoglutarate
In the liver, arginine is hydrolyzed to ornithine and urea. Ornithine serves as
the substrate for ureosynthetic cycle:
urea
arginine ornithine
+ H2O
arginase
+
the ureosynthetic cycle
decarboxylation
CO2
putrescine (butan-1,4-diamine) for
synthesis of polyamines
39
Nitroxide (nitrogen monoxide, NO) originates from arginine:
arginine
nitroxide
(a radical)
•N=O
citrulline
+
Nω
-hydroxyarginine
O2. NADPHO2. NADPH
The reaction is a five-electron oxidation catalyzed by nitroxide synthase (NOS),
employing five redox cofactors (NADPH, FAD, FMN, cytochrome, H4biopterin).
There are three isoenzymes of NOS: endothelial NOS responsible for
vasodilation and inhibition of platelet aggregation, neuronal NOS modulation
events on synapses (both are Ca2+
-dependent), and NOS in phagocytes (NO
gives bactericidal peroxynitrite ONOO–
).
After hydrolysis of arginine to ornithine,
ornithine is degraded by transamination of the 5-amino group to
glutamate 5-semialdehyde that gives glutamate and 2-oxoglutarate.
40
Synthesis of creatine
Arginine is the donor of amidino group for the synthesis of creatine:
creatine
N1
-methylguanidinoacetate
(kidney) S-AM
(liver)
S-AdoHcy
slow non-enzymatic
dehydration creatinine
(excreted into the urine)
ATP
creatine kinase
ADP
Creatine in skeletal muscles:
phosphocreatine
(muscle phosphagen)
H2O
41
Proline (pyrrolidine-2-carboxylic acid)
COOHN
H
is nonessential and glucogenic
– nonessential – originates from glutamate
– glucogenic – it gives 2-oxoglutarate
1-pyrroline 5-carboxylate
(cyclic aldimine)
glutamate 5-semialdehyde
4-Hydroxyproline
occurs only in collagen, and is formed by posttranslational
hydroxylation of prolyl residues in procollagen polypeptide
chains. Similarly to proline, 4-hydroxyproline is degraded to
4-hydroxyglutamate, which is cleft to pyruvate and glyoxylate.
NAD+ NADH+H+
H2O
glutamate2-oxoglutarate
AA 2-oxoA
NAD+
+ H2O
NADH+H+
proline
42
Histidine
N-formimino-glutamate
(FIGLU)
N
N
H
CH2–CH–COOH
NH2
is nonessential and glucogenic
– nonessential for adults (essential for children)
– glucogenic - it gives glutamate and 2-oxoglutarate
Histidine mostly does not undergo transamination,
it is deaminated directly by elimination (desaturation):
NH3
NH3-lyase
histidine 4-imidazolone-5-propionate
urocanic acid
(urocanate)
H2O
H2O
glutamate2-oxoglutarate
AA 2-oxoA
FH4HN=CH–FH4
N5
-formimino tetrahydrofolate
43
Antihistaminics – drugs which antagonize the effects of histamine.
histidine histamine
CO2
Histamine is the product of histidine decarboxylation catalyzed
by specific histidine decarboxylase:
Histamine is a biogenic amine stored within granules of basophils and mast cells
(more than 90 % body stores) and within synaptosomes of certain CNS neurons.
When released, histamine induces complex physiological and pathological effects,
including immunological reactions (symptoms of allergic conditions of the skin
and airways), gastric acid secretion, smooth muscle contractions (e.g.
bronchoconstriction), and profound vasodilatation. Histamine exerts its action via
at least four distinct histamine receptor subtypes.
Released histamine is metabolized by oxidation (to imidazolylacetic acid) or
methylation (to tele-N-methylhistamine and tele-N-methylimidazolylacetic acid).
44
Amino acids metabolized to 2-oxoglutarate – relationships:
glutamine
glutamate 2-oxoglutarate
2-oxoA AA
glutamate semialdehyde N-formiminoglutamate
histidineproline
ornithine
arginine
urea
urocanate1-pyrroline-5-carboxylate
45
6 Branched-chain amino acids
are all essential, their
final metabolites are different:
valine is glucogenic,
leucine is ketogenic,
isoleucine both gluco- and ketogenic.
CH
CH3 CH3
CH–NH2
COOH
CH2
CH
CH3 CH3
CH–NH2
COOH
Valine
CH–NH2
COOH
CH3
CH
CH3
CH2
IsoleucineLeucine
These amino acids are taken up from the blood predominantly by skeletal muscles and
their catabolism (transamination) begins there.
The three initial catabolic reactions are common to all three
branched-chain amino acids:
– transamination to corresponding 2-oxoacids,
– oxidative decarboxylation catalyzed by 2-oxoacid dehydrogenase
producing corresponding acyl-CoA thioesters, and
– the second dehydrogenation between carbons α and β catalyzed
by flavin dehydrogenase resulting in corresponding 2-alkenoyl-CoA thioesters:
46
The resulting 2-alkenoyl-CoAs after three initial reactions:
3-hydroxy-3-methylglutaryl-CoA
(HMG-CoA)
leucine isoleucinevaline
The following reactions differ (expected addition of water, hydration, occurs
as the next reaction only in the case of valine and isoleucine).
Leucine is ketogenic. The alkenoyl-CoA is carboxylated (CO2 donor is
carboxy-biotin) and the product hydrated to HMG-CoA that splits to
free acetoacetate and acetyl CoA:.
acetyl-CoA
acetoacetate
CO2-biotin
biotin
carboxylation
H2O
hydration
47
Isoleucine
methylmalonyl-CoA
oxidationhydration splitting
acetyl-CoA
propionyl-CoA
CoA-SH
Valine
methylmalonyl-CoApropionyl-CoA
CoA-SH
oxidationhydration decarboxylation
CoA-SH
CO2
carboxylation
CO2
carboxylation
CO2
succinyl-CoA
(B12)
succinyl-CoA
(B12)
48
succinyl-CoA
propionyl-CoA
valine
acetyl-CoA
succinyl-CoA
propionyl-CoA
isoleucine
3-hydroxy-3-methylglutaryl-CoA
(HMG-CoA)
acetoacetate
acetyl-CoA
leucine
Branched-chain amino acids – summary:
glucogenic ketogenic
glucogenic
ketogenic
49
Lysine7
CH2–CH2–CH2–CH2–CH–COOH
NH2NH2
is essential and ketogenic
– it gives acetoacetyl-CoA
Lysine does not undergo transamination.
Primarily, ε-deamination occurs through the formation of saccharopine:
2-oxoglutarate
lysine
+ε
α
saccharopine 2-aminoadipate
NADPH+H+
H2O
reduction
allysine
glutamate
NAD+
H2O
oxidation
NAD+
Transamination of α-amino group in 2-aminoadipate follows:
50
2-aminoadipate 2-oxoadipate
glutaryl-CoA crotonoyl-CoA
acetoacetyl-CoA
2-oG Glu
CO2
NAD+
CoA-SH
dehydrogenation
hydratationH2O
NAD+
CO2
FAD
Lysyl side chains in collagen and elastin are
oxidatively ε-deaminated to allysyl side chains.
The aldehyde groups so formed react non-enzymatically
with each other, or with lysyl ε-NH2, and
form covalent crosslinks
(pyridinoline type in collagen, isodesmosine in elastin).
51
8 Aromatic amino acids
phenylalanine, tyrosine, and tryptophan
All three amino acids are essential (though tyrosine is also formed
by hydroxylation of phenylalanine),
and both glucogenic and ketogenic,
- phenylalanine and tyrosine give fumarate and acetoacetate,
- tryptophan gives alanine and acetoacetyl-CoA.
52
Phenylalanine and tyrosine
Hydroxylation of phenylalanine to tyrosine
is catalyzed by a monooxygenase – phenylalanine hydroxylase,
for which the reducing coenzyme is tetrahydrobiopterin (BH4):
Similarly, tyrosine is hydroxylated to DOPA by tyrosine 3-hydroxylase, and
tryptophan to 5-hydroxytryptophan by tryptophan 5-hydroxylase.
phenylalanine
tyrosine
5,6,7,8-tetrahydrobiopterin
q-7,8-dihydrobiopterin
NADH + H+
NAD+
O2
H2O
53
phenylalanine tyrosine
acetoacetate
fumarate maleinylacetoacetate
fumarylacetoacetate
4-hydroxy-
phenylpyruvate
H2O
hydroxylation
(O2, BH4)
transamination
2-oG Glu
1,2-dioxygenase
O2
dioxygenase
CO2
O2
(2,5-dihydroxy-
phenylacetate)
homogentisate
isomerization
54
Inborn metabolic disorders of phenylalanine catabolism
phenylalanine
(O2, BH4)
H2O
2-oG
Glu
tyrosine
fumarate + acetoacetate
maleinylacetoacetate
fumarylacetoacetate
4-hydroxyphenylpyruvate
O2
(2,5-dihydroxyphenylacetate)
homogentisate
CO2
O2
(O2, BH4)
DOPA
PHENYLKETONURIA,
hyperphenylalaninaemias
ALBINISM
HYPERTYROSINAEMIA II
ALKAPTONURIA
HYPERTYROSINAEMIA I
55
is a defect in phenylalanine hydroxylase,
the ability to convert Phe to tyrosine is
considerably impaired.
PKU have to be recognized through the
compulsory screening of newborn infants
and treated by a low-phenylalanine diet
till the age of 8 – 10 years.
The consequence of untreated PKU is
mental retardation (oligophrenia
phenylpyruvica). Besides high levels of
blood Phe, alternative catabolites are
produced and excreted in high amounts
(a "mousy" odour of the urine) :
Hyperphenylalaninaemia type I
(classic phenylketonuria, PKU)
transamination
phenylpyruvate
phenylacetate
o-hydroxyphenylacetate
phenylalanine
(tyrosine)
Phe hydroxylase
(O2, BH4)
Malignant hyperphenylalaninaemias type IV and V
BH4 (tetrahydrobiopterin) is lacking due to the defective dihydrobiopterin
biosynthesis from guanylate, or an ineffective reduction of BH2 to BH4.
56
Hypertyrosinaemias
occur in several forms. They may be caused by a deficit of enzymes which catalyze
either the transamination of tyrosine, or oxidation of p-hydroxyphenylpyruvate and
hydrolysis of fumarylacetoacetate. A low-tyrosine diet may be very useful.
Plasma levels of tyrosine are elevated, and large amounts of tyrosine,
p-hydroxyphenylpyruvate, –lactate, and –acetate are excreted into the urine
(tyrosyluria).
Alkaptonuria
is an inborn deficit of homogentisate
oxidase characterized by the excretion
of homogentisate in the urine. Except for
the darkening of the urine on the air,
there are no clinical manifestations in
youth until the second or third decade,
when deposits of pigments in the
connective tissue begins to appear
(ochronosis – bluish colouring of the
scleras, the ear and nasal cartilages,
etc.) which are the cause of deforming
arthritis.
homogentisate
homogentisate 1,2-dioxygenase
(homogentisate oxidase)
(maleinylacetoacetate)
oxidation to
benzoquinoneacetate
by polyphenol oxidase
or by the O2 in the air
and polymerization to blackbrown
pigments
57
tyrosine DOPA
(3,4-dihydroxyphenylalanine)
Biosynthesis of catecholamines
Inactivation of catecholamines occurs by means both oxidative deamination
(monoamine oxidase, MAO) to acidic metabolites and 3-O-methylation
(catechol-O-methyl transferase, COMT) to metanephrines.
(O2, BH4)
3-hydroxylation
- CO2
decarboxylation
dopamine noradrenaline
(norepinephrine)
adrenaline
(epinephrine)
N-methylation
(S-AM)
β-hydroxylation
(O2, L-ascorbate)
58
dopaquinone
Intermediates in the melanin biosynthesis
dopachrome
indole-5,6-quinone
black eumelanins
ochre pheomelanins
Pigments melanins occurs in the eye, skin, and hair. The initial steps are
a hydroxylation of tyrosine to DOPA and oxidation of DOPA to dopaquinone
– both reaction in the pigment-forming cells are catalyzed by the copper-containing
enzyme tyrosinase. The products of oxidation readily and spontaneously
undergo polymerization resulting in insoluble dark pigments.
tyrosine DOPA
59
Biosynthesis of the thyroid hormones
3,5-diiodotyrosyl residues
thyroxine (T4)
3,5,3´,5´-tetraiodothyronine
Within the thyroid cell, at the cell-colloid interface, iodide anions are oxidized
(to I+
, IO–
, or •I ?) by thyroperoxidase (TPO) and incorporated into tyrosyl
residues of thyroglobulin:
TPO
I–
, H2O2
tyrosyl residues
dehydroalanyl residue
proteolysis
iodinated thyronyl residue
TPO, H2O2
The coupling of iodotyrosyl residues
in thyroglobulin is also catalyzed
by thyroperoxidase. Proteolysis of
thyroglobulin follows in lysosomes
and thyroxine (or 3,3´,5´-T3) is secreted.
60
Tryptophan is essential and both glucogenic and ketogenic
– after opening of the indole pyrrole ring, it releases
alanine, the carbon atoms of aromatic ring give
acetoacetate.
Tryptophan mostly does not undergo transamination.
Catabolism of tryptophan is usually initiated by cleavage of the pyrrole ring
of indole by tryptophan dioxygenase (tryptophan pyrrolase):
hydroxylation
(O2, NADPH + H+
)
3-hydroxykynurenin3-hydroxyanthranilate
O2, NADPH + H+
Trp dioxygenase
kynurenineN-formylkynurenine
H2O
HCOO–
formate
N10
-formyl TH4
H2O
alaninepyruvate
(B6)
61
3-hydroxyanthranilate
(tryptophan)
2-aminomuconate
6-semialdehyde
O2
CO2
crotonoyl-CoA
2-oxoadipate glutaryl-CoA
CO2
NAD+
CoA-SH CO2
FAD
acetoacetyl-CoA
dehydrogenation
hydratationH2O
NAD+
hydrogenation
NH3
62
Nicotinate ring synthesis for NAD(P)+
:
Humans can provide nearly all of their nicotinamide requirement from tryptophan,
if there is a sufficient amount of tryptophan in the diet. Normally, about two-thirds
comes from this source:
3-hydroxyanthranilate
2-amino-3-carboxymuconate
6-semialdehyde
quinolinate
H2O
ribosyl 5´-phosphate
nicotinate mononucleotide
⊕
2 ATP
Gln
AMP
2 PPi
Glu
NAD+
PRPP
PPi
CO2
O2
Utilization of tryptophan
63
5-hydroxytryptophantryptophan serotonin
(5-hydroxytryptamine, 5-HT)
CO2
5-hydroxylation
BH4 BH2
O2 H2O
Secretion of melatonin from the pineal gland is increased in
darkness. Its physiologic roles remains to be elucidated, but
they involve chronobiologic rhythms.
(In frogs, melatonin is an antagonist of the melanocytestimulating
hormone, MSH.)
pineal gland
(N--acetylation
5-O-methylation)
melatonin
(N-acetyl-5-methoxytryptamine)
tryptamine
large intestine
decarboxylation
Serotonin is a neurotransmitter in CNS and a local hormone
of argentaffin cells of the intestinal mucosa. It is degraded
to 5-hydroxyindoleacetic acid (5-HIAA).
indole skatole
64
The fate of the carbon skeleton of amino acids – summary: