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Chapter 25
Nitrogen Acquisition and
Amino Acid Metabolism
Biochemistry
by
Reginald Garrett and Charles Grisham
25.1 – Which Metabolic Pathways
Allow Organisms to Live on
Inorganic Forms of Nitrogen?
Nitrogen is cycled between organisms and
inanimate enviroment
• The principal inorganic forms of N are in an
oxidized state
– As N2 in the atmosphere
– As nitrate (NO3-) in the soils and ocean
• All biological compounds contain N in a
reduced form (NH4+)
Outline
1. Which Metabolic Pathways Allow Organisms
to Live on Inorganic Forms of Nitrogen?
2. What Is The Metabolic Fate of Ammonium?
3. What Regulatory Mechanisms Act on
Escherichia coli Glutamine Synthetase?
4. How Do Organisms Synthesize Amino
Acids?
5. How Does Amino Acid Catabolism Lead into
Pathways of Energy Production?
• Thus, Nitrogen acquisition must involve
1. The Reduction of the oxidized forms (N2 and
NO3-) to NH4+
2. The incorporation of NH4+ into organic linkage
as amino or amido groups
• The reduction occurs in microorganisms and
green plants. But animals gain N through
diet.
The reduction of Nitrogen
(+3)
Nitrogen assimilation and nitrogen fixation
1. Nitrate assimilation occurs in two steps:
(+5)
– 2e- reduction of nitrate to nitrite
– 6e- reduction of nitrite to ammonium (fig 25.1)
• Nitrate assimilation accounts for 99% of N
acquisition by the biosphere
(-3)
(+2)
(+1) (0)
Figure 25.1 The nitrogen cycle. Organic nitrogenous
compounds are formed by the incorporation of NH4+ into
carbon skeletons. Note that denitrification and nitrogen
fixation are anaerobic processes.
Nitrate Assimilation
•
•
Nitrate assimilation – the reduction of
nitrate to NH4+ in plants, various fungi, and
certain bacteria
Two steps:
1. Nitrate reductase
NO3- + 2 H+ + 2 e- → NO2- + H2O
2. Nitrite reductase
NO2- + 8 H+ + 6 e- → NH4+ + 2 H2O
•
Electrons are transferred from NADH to
nitrate
2. Nitrogen fixation involves reduction of N2 in
prokaryotes by nitrogenase
Nitrate reductase
•
•
•
Pathway involves -- SH of enzyme, FAD,
cytochrome b557 and Molybdenum cofactor
-- all protein-bound
Nitrate reductases are cytosolic 210-270 kD
dimeric protein
MoCo required both for reductase activity
and for assembly of enzyme subunits to
active dimer
NADH
NO3[-SH →FAD→cytochrome b557 →MoCo]
NADH+
NO2-
Nitrite Reductase
Light drives reduction of ferredoxins and
electrons flow to 4Fe-4S and siroheme and
then to nitrite
• Nitrite is reduced to ammonium while still
bound to siroheme
• In higher plants, nitrite reductase is in
chloroplasts, but nitrate reductase is cytosolic
Figure 25.2
The novel prosthetic groups of nitrate reductase and nitrite reductase. (a) The molybdenum
cofactor of nitrate reductase. The molybdenum-free version of this compound is a pterin
derivative called molybdopterin. (b) Siroheme, a uroporphyrin derivative, is a member of the
isobacteriochlorin class of hemes, a group of porphyrins in which adjacent pyrrole rings are
reduced. Siroheme is novel in having eight carboxylate-containing side chains. These
carboxylate groups may act as H+ donors during the reduction of NO2- to NH4+.
Nitrogen fixation
•
•
•
•
Figure 25.3 Domain organization within the enzymes of nitrate
assimilation. The numbers denote residue number along the amino acid
sequence of the proteins.
N2 + 10 H+ + 8 e- → 2 NH4+ + H2
Only occurs in certain prokaryotes
Rhizobia fix nitrogen in symbiotic association
with leguminous plants
Rhizobia fix N for the plant and plant provides
Rhizobia with carbon substrates
Fundamental requirements:
1.
2.
3.
4.
Nitrogenase
A strong reductant (reduced ferredoxin)
ATP
O-free conditions
Nitrogenase Complex
Figure 25.4
The triple bond in
N2 must be broken
during nitrogen
fixation.
Two metalloprotein components:
nitrogenase reductase and nitrogenase
• Nitrogenase reductase
– Fe-protein
– is a 60 kD homodimer with a single 4Fe-4S cluster
• N2 reduction to ammonia is thermodynamically favorable
• However, the activation barrier for breaking the N-N triple
bond is enormous
• 16 ATP provide the needed activation energy
• Very oxygen-sensitive
• Binds MgATP and hydrolyzes 2 ATPs per
electron transferred
• Reduction of N2 to 2NH3 + H2 requires 4 pairs
of electrons, so 16 ATP are consumed per N2
Nitrogenase
• MoFe-protein
• A 220 kD α2β2 heterotetramer
• Each molecule of enzyme contains 2 Mo, 32
Fe, 30 equivalents of acid-labile sulfide (FeS
clusters, etc)
• Four 4Fe-4S clusters plus two FeMoCo, an
iron-molybdenum cofactor
• Nitrogenase is slow enzyme
– 12 e- pairs per second, i.e., only three molecules of
N2 per second
– As much as 5% of cellular protein may be
nitrogenase
Figure 25.5 Structures of the
two types of metal clusters
found in nitrogenase.
(a) The P-cluster consists of
two Fe4S3 clusters that
share an S atom.
(b) The FeMo-cofactor
contains 1 Mo, 7Fe, and
9S atoms. Homocitrate
provides two oxo ligands
to the Mo atom.
The regulation of nitrogen Fixation
•
1.
2.
•
Two regulatory controls
ADP inhibits the activity of nitrogease
NH4+ represses the expression of nif genes
Some organism, ADP-ribosylation of
nitrogenase reductase
Figure 25.6 The nitrogenase reaction. Depending on the bacterium,
electrons for N2 reduction may come from light, NADH, hydrogen
gas, or pyruvate. The primary e- donor for the nitrogenase system
is reduced ferredoxin.
Figure 25.8 Regulation of
nitrogen fixation.
(a) ADP inhibits nitrogenase
activity.
(b) NH4+ represses nif gene
expression.
(c) In some organisms, the
nitrogenase complex is
regulated by covalent
modification. ADPribosylation of nitrogenase
reductase leads to its
inactivation.
Nitrogenase reductase is a
distant relative of the
signal-transducing Gprotein superfamily.
25.2 – What Is The Metabolic Fate of
Ammonium?
Ammonium enters organic linkage via
three major reactions in all cells
1.
2.
3.
•
Carbamoyl-phosphate synthetase (CPS)
Glutamate dehydrogenase (GDH)
Glutamine synthetase (GS)
Asparagine synthetase (some
microorganisms)
Carbamoyl-phosphate synthetase I (CPS-I)
Glutamate dehydrogenase
Ammonium is converted to carbamoyl-P
NH4+ + α-ketoglutarate + NADPH + 2 H+ →
glutamate + NADP+ + H2O
NH4+ + HCO3- + 2 ATP →
carbamoyl phosphate + 2 ADP + Pi + 2 H+
(H2N-COO-PO32-)
•
•
Reductive amination of α-ketoglutarate to
form glutamate
Two ATP required
– one to activate bicarbonate
– one to phosphorylate carbamate
•
This reaction is an early step in the urea
cycle
Glutamine synthetase
NH4+ + glutamate + ATP →
glutamine + ADP + Pi
• ATP-dependent amidation of γ-carboxyl of
glutamate to glutamine
• Glutamine is a major N donor in the
biosynthesis of many organic N compounds,
therefore GS activity is tightly regulated
Figure 25.10
(a) The enzymatic
reaction catalyzed by
glutamine
synthetase.
(b) The reaction
proceeds by (a)
activation of the γcarboxyl group of Glu
by ATP, followed by (b)
amidation by NH4+.
The major pathways of Ammonium
Assimilation
Two principal pathways
1. Principal route: GDH/GS in organisms
rich in N
–
See Figures 25.12 and 25.13
2. Secondary route: GS/GOGAT in
organisms confronting N limitation
–
–
GOGAT is glutamate synthase or
glutamate:oxo-glutarate amino transferase
See Figures 25.12 and 25.13
Figure 25.11 The GDH/GS pathway of ammonium assimilation.
The sum of these reactions is the conversion of 1 αketoglutarate to 1 glutamine at the expense of 2 NH4+, 1 ATP,
and 1 NADPH.
Figure 25.11 The GDH/GS pathway of ammonium assimilation.
Figure 25.12 The glutamate synthase (GOGAT)reaction,
showing the reductants exploited by different organisms in this
reductive amination reaction.
Figure 25.13 The GS/GOGAT pathway of ammonium assimilation. The sum
of these reactions results in the conversion of 1 α-ketoglutarate to 1 glutamine at the
expense of 2 ATP and 1 NADPH.
25.3 – What Regulatory Mechanisms
Act on Glutamine Synthetase
•
GS in E. coli is regulated in three ways:
1. Feedback inhibition
2. Covalent modification (interconverts between
inactive and active forms)
3. Regulation of gene expression and protein
synthesis control the amount of GS in cells
•
But no such regulation occurs in eukaryotic
versions of GS
Allosteric Regulation of
Glutamine Synthetase
• Nine different feedback inhibitors: Gly, Ala, Ser,
His, Trp, CTP, AMP, carbamoyl-P and glucosamine6-P
• Gly, Ala, Ser are indicators of amino acid
metabolism in cells
• Other six are end products of a biochemical pathway
• AMP competes with ATP of binding at the ATP
substrate site
• This effectively controls glutamine’s contributions to
metabolism
Figure 25.14
The subunit organization
of bacterial glutamine
synthetase.
(a) Diagram showing its
dodecameric structure as
a stack of two hexagons.
(b) Molecular structure of
glutamine synthetase
from Salmonella
typhimurium (a close
relative of E.coli), as
revealed by X-ray
crystallographic analysis.
(From Almassy, R. J.,
Janson, C. A., Hamlin, R.,
Xuong, N.-H., and Eisenberg,
D., 1986. Novel subunitsubunit interactions in the
structure of glutamine
synthetase.Nature 323:304.
Photos courtesy of S.-H.
Liaw and D. Eisenberg.)
Figure 25.15
The allosteric regulation of
glutamine synthetase activity
by feedback inhibition.
Covalent Modification of
Glutamine Synthetase
• Each subunit is adenylylated at Tyr-397
• Adenylylation inactivates GS by adenylyl
transferase
• Adenylyl transferase catalyzes both the
adenylylation and deadenylylation
– PII (regulatory protein) controls these
• AT:PIIA catalyzes adenylylation
• AT:PIID (PII-UMP) catalyzes deadenylylation
• α-Ketoglutarate and Gln also affect
Figure 25.16
Covalent modification of GS: Adenylylation of Tyr397 in the glutamine synthetase polypeptide
via an ATP-dependent reaction catalyzed by the converter enzyme adenylyl transferase
(AT). From 1 through 12 GS monomers in the GS holoenzyme can be modified, with
progressive inactivation as the ratio of [modified]/[unmodified] GS subunits increases.
Figure 25.17
The cyclic cascade system regulating the covalent modification of GS.
Gene Expression regulates GS
• Gene GlnA is actively transcribed only if
transcriptional enhancer NRI is in its
phosphorylated form, NRI-P
• NRI is phosphorylated by NRII, a protein
kinase
• If NRII is complexed with PIIA it acts as a
phosphatase, not a kinase
Figure 25.18
Transcriptional regulation of GlnA expression through the reversible phosphorylation of NRI,
as controlled by NRII and its association with PIIA.
(kinase)
25.4 – Amino Acid Biosynthesis
• Plants and microorganisms can make all 20
amino acids and all other needed N metabolites
• In these organisms, glutamate is the source of N,
via transamination (aminotransferase) reactions
• Mammals can make only 10 of the 20 AAs
• The others are classed as "essential" amino
acids and must be obtained in the diet
(phosphatase)
Amino acids are formed from αketo acids by transamination
Amino acid1 + α-keto acid2 → α-keto acid1 + Amino acid2
• Transamination (aminotransferase) reactions
• Named according their amino acid substrate
– Glutamate-asparate aminotransferase
Figure 25.19
Glutamate-dependent transamination of α-keto acid carbon skeletons is a primary
mechanism for amino acid synthesis.
Amino Acid Biosynthesis can be
organized into families
• According to the intermediates that they are made from
The Mechanism of the Aminotransferase (Transamination) Reaction
The α-Ketoglutarate Family
Glu, Gln, Pro, Arg, and sometimes Lys
• The routes for Glu and Gln synthesis were
described when we considered pathways of
ammonia assimilation
– Transamination of α-Ketoglutarate gives glutamate
– Amidation of glutamate gives glutamine
• Proline is derived from glutamate
• Ornithine is also derived from glutamate
– the similarity to the proline pathway
• Arginine are part of the urea cycle
(1) N-acetylglutamate synthase
(2) N-acetylglutamate kinase
(3) N-acetylglutamate-5semialdehyde dehydrogenase
(4) N-acetylornithine δ-aminotransferase
Figure 25.20
The pathway of proline biosynthesis from glutamate. The enzymes are (1)
γ-glutamyl kinase, (2) glutamate-5-semialdehyde dehydrogenase, and
(4) Δ1-pyrroline-5-carboxylate reductase; reaction (3) occurs
nonenzymatically.
(5) N-acetylornithine deacetylase
Carbamoyl-phosphate synthetase I
•
Ornithine has three metabolic roles
–
–
–
•
To serve as precursor to arginine
To function as an intermediate in the urea cycle
To act as an intermediate in arginine degradation
δ-NH3+ of ornithine is carbamoylated by
onithine transcarbamoylase in urea cycle
•
Carbamoyl-phosphate synthetase I (CPS-I)
–
NH3-dependent mitochondrial CPS isozyme
1. HCO3- is activated via an ATP-dependent
phosphorylation
2. Ammonia attacks the carbonyl carbon of
carbonyl-P, displacing Pi to form carbamate
3. Carbamate is phosphorylated via a second ATP
to give carbamoyl-P
•
CPS-I represents the committed step in urea
cycle
•
Activated by N-acetylglutamate
–
Figure 25.22
The mechanism of action of CPS-I,
the NH3-dependent mitochondrial
CPS isozyme. (1) HCO3- is
activated via an ATP-dependent
phosphorylation. (2) Ammonia
attacks the carbonyl carbon of
carbonyl-P, displacing Pi to form
carbamate. (3) Carbamate is
phosphorylated via a second ATP
to give carbamoyl-P.
The Urea Cycle
• The carbon skeleton of arginine is derived
from α-ketoglutarate
• N and C in the guanidino group of Arg
come from NH4+, HCO3- (carbamoyl-P), and
the α-NH2 of Glu and Asp
• Breakdown of Arg in the urea cycle releases
two N and one C as urea
• Important N excretion mechanism in livers
of terrestrial vertebrates
• Urea cycle is linked to TCA by fumarate
1.
2.
3.
4.
Because N-acetylglutamate is a precursor to
orinithine synthesis and essential to the
operation of the urea cycle
→
amino acid catabolism ↑
→
glutamate level (N-acetylglutamate) ↑
→
Stimulate CPS-I
→
Raise overall Urea cycle activity
Ornithine transcarbamoylase (OTCase)
Argininosuccinate synthetase
Argininosuccinase
Arginase
The Urea Cycle
Lysine Biosynthesis
•
Two pathways:
1. α-aminoadipate pathway
2. diaminopimelate pathway (Asp)
•
Lysine derived from α-ketoglutarate
–
–
•
•
•
•
Reactions 1 through 4 are reminiscent of the first four
reactions in the citric acid cycle
α-ketooadipate
Transamination gives α-aminoadipate
Adenylylation activates the δ-COOH for reduction
Reductive amination give saccharopine
Oxidative cleavage yields lysine
Figure 25.24
Lysine biosynthesis in certain fungi
and Euglena: the α-aminoadipic acid
pathway. Reactions 1 through 4 are
reminiscent of the first four reactions
in the citric acid cycle, except that the
product α-ketoadipate has an
additional ⎯CH2⎯ unit. Reaction 5 is
catalyzed by a glutamate-dependent
aminotransferase; reaction 6 is the
adenylylation of the δ-carboxyl of αaminoadipate to give the 6-adenylyl
derivative. Reductive deadenylylation
by an NADPH-dependent
dehydrogenase in reaction 7 gives αaminoadipic-6-semialdehyde, which in
reaction 8 is coupled with glutamate
via its amino group by a second
NADPH-dependent dehydrogenase.
Oxidative removal of the αketoglutarate moiety by NAD+dependent saccharopine
dehydrogenase in reaction 9 leaves
this amino group as the ε-NH3+ of
lysine.
The Aspartate Family
Asp, Asn, Lys, Met, Thr, Ile
• Transamination of Oxaloaceate gives Aspartate
(aspartate aminotransferase)
• Amidation of Asp gives Asparagine ( asparagine
synthetase)
• Met, Thr and Lys are made from Aspartate
• β-Aspartyl semialdehyde and homoserine are
branch points
• Isoleucine, four of its six carbons derived from
Asp (via Thr) and two come from pyruvate
Figure 25.25 Aspartate biosynthesis via transamination of
oxaloacetate by glutamate.
Figure 25.27 Biosynthesis
of threonine, methionine, and
lysine, members of the
aspartate family of amino
acids.
β-Aspartyl-semialdehyde is a
common precursor to all
three.
Figure 25.26 Asparagine biosynthesis from Asp, Gln, and ATP
by asparagine synthetase. β-Aspartyladenylate is an enzymebond intermediate.
It is formed by aspartokinase
(reaction 1) and β-aspartylsemialdehyde dehydrogenase
(reaction 2).
Figure 25.27 Biosynthesis of threonine,
methionine, and lysine, members of the
aspartate family of amino acids.
Figure 25.27 Biosynthesis of threonine, methionine,
and lysine, members of the aspartate family of amino
acids.
β-Aspartyl-semialdehyde
Figure 25.27
Biosynthesis of
threonine, methionine,
and lysine, members
of the aspartate family
of amino acids.
• In E. coli
– Three isozymes of aspartokinase
– Uniquely controlled by one of the three endproducts
• Role of methionine
– in methylations via S-adenosylmethionine (SAM;
S-AdoMet)
– polyamine biosynthesis
The Pyruvate Family
Ala, Val, Leu, and Ile
• Transamination of pyruvate gives Alanine
• Valine is derived from pyruvate
• Ile synthesis from Thr mimics Val synthesis
from pyruvate (Fig. 25.29)
– Threonine deaminase (also called threonine
dehydratase or serine dehydratase) is sensitive to Ile
– Ile and val pathway employ the same set of enzymes
Figure 25.28
The synthesis of Sadenosylmethionine (SAM)
• Leu synthesis begins with an α-keto isovalerate
– Isopropylmalate synthase is sensitive to Leu
Figure 25.29 Biosynthesis of
valine and isoleucine.
Threonine
deaminase
isopropylmalate
synthase
isopropylmalate
dehydratase
Acetohydroxy acid synthase
Acetohydroxy acid
isomeroreductase
isopropylmalate
dehydrogenase
Dihydroxy acid
dehydratase
Glutamate-dependent
aminotransferase
3-Phosphoglycerate Family
Ser, Gly, Cys
leucine
aminotransferase
Figure 25.30 Biosynthesis of leucine.
•
Serine hydroxymethylase (PLP-dependent) transfers
the β-carbon of Ser to THF to make glycine
1. 3-Phosphoglycerate dehydrogenase
diverts 3-PG from glycolysis to
amino acid synthesis pathways (3phosphohydroxypyruvate)
2. Transamination by Glu gives 3phosphoserine (3-phosphoserine
aminotransferase)
3. Phosphoserine phosphatase yields
serine
Figure 25.32 Biosynthesis of
glycine from serine (a) via serine
hydroxymethyltransferase and (b)
via glycine oxidase.
•
A PLP-dependent enzyme makes Cys
ATP sulfurylase
Some bacteria
Adenosine-5'-phosphosulfate-3'-phosphokinase.
most microorganism and plants
O-acetylserine sulfhydrylase
serine acetyltransferase
Figure 25.33 Cysteine biosynthesis. (a) Direct sulfhydrylation of serine by H2S.
(b) H2S-dependent sulfhydrylation of O-acetylserine.
Figure 25.34 Sulfate assimilation and
the generation of sulfide for synthesis
of organic S compounds.
Aromatic Amino Acids
Phe, Tyr, Trp, His
• Shikimate pathway yields chorismate,
thence Phe, Tyr, Trp
• Chorismate as a branch point in this pathway
(Figs. 25.35)
• Chorismate is synthesized from PEP and
erythrose-4-P
– Via shikimate pathway
– The side chain of chorismate is derived from a
second PEP
Figure 25.35
Some of the aromatic
compounds derived from
chorismate.
(1) 2-keto-3-deoxy-D-arabino-heptulosonate-7-P synthase
(2) dehydroquinate synthase
(3) 5-dehydroquinate dehydratase
(4) shikimate dehydrogenase
The biosynthesis of phenylalanine,
tyrosine, and tryptophan
• At chorismate, the pathway separates into
three branches, each leading to one of the
aromatic amino acids
• Mammals can synthesize tyrosine from
phenylalanine by phenylalanine
hydroxylase (Phenylalanine-4monooxygenase)
(5) shikimate kinase
(6) 3-enolpyruvyl-shikimate-5-phosphate synthase
(7) chorismate synthase.
Figure 25.37 The biosynthesis of
phenylalanine, tyrosine, and
tryptophan from chorismate.
(1) chorismate mutase
(2) prephenate dehydratase
(3) phenylalanine aminotransferase
(4) prephenate dehydrogenase
(5) tyrosine aminotransferase
(6) anthranilate synthase
(7) anthranilate-phosphoribosyl
transferase
(8) N-(5'-phosphoribosyl)anthranilate isomerase
(9) indole-3-glycerol phosphate
synthase
(10) tryptophan synthase (a-subunit)
(11) tryptophan synthase (bsubunit).
Figure 25.38 The formation of tyrosine from phenylalanine.
Hitidine biosynthesis
• His synthesis, like that of Trp, shares
metabolic intermediates (PRPP) with purine
biosynthetic pathway
• His operon
• Begin from PRPP and ATP
• The intermediate 5-aminoimidazole-4carboxamide ribonucleotide (AICAR) is a
purine precursor (replenish ATP; Ch 26)
Figure 25.40 The pathway of
histidine biosynthesis.
(1) ATP-phosphoribosyl
transferase
(2) pyrophosphohydrolase
(3) phosphoribosyl-AMP
cyclohydrolase
(4) phosphoribosylformimino-5aminoimidazole carboxamide
ribonucleotide isomerase
(5) glutamine amidotransferase
(6) imidazole glycerol-P
dehydratase
(7) L-histidinol phosphate
aminotransferase
(8) histidinol phosphate
phosphatase
(9) histidinol dehydrogenase.
Amino Acid Biosynthesis Inhibitors as
Herbicides
• A variety of herbicides have been developed as
inhibitors of plant enzymes that synthesize
“essential” amino acids
• These substances show no effect on animals
• For example, glyphosate, sold as RoundUp, is a
PEP analog that acts as an uncompetitive
inhibitor of 3-enolpyruvylshikimate-5-P
synthase.
Amino acid synthesis inhibitors as herbicides
(inhibitor of 3-enolpyruvyl-shikimate-5phosphate synthase)
(fig 25.36)
(inhibitor of acetohydroxy acid synthase in
biosynthesis of valine and isoleucine) (fig 25.29)
(inhibitor of imidazol glycerol-P dehydrtase
in biosynthesis of histidine) (fig 25.40)
(inhibitor of glutamine synthetase)
25.5 – Degradation of Amino Acids
The 20 amino acids are degraded to produce
(mostly) TCA intermediates
• Energy requirement
– 90% from oxidation of carbohydrates and fats
– 10% from oxidation of amino acids
• The primary physiological purpose of amino
acids is to serve as building blocks for protein
synthesis
• The classifications of amino acids in Fig. 25.41
• Glucogenic and ketogenic
C-3 family (pyruvate):
Ala, Ser, Cys, Gly, Thr, Trp
C-4 family (oxaloaceate & fumarate):
Oxaloaceate: Asp, Asn
Fumarate: Asp, Phe, Tyr
Figure 25.41 Metabolic degradation of the
common amino acids. Glucogenic amino
acids are shown in pink, ketogenic in blue.
Those that give rise to
precursors for glucose synthesis,
such as
α-ketoglutarate,
succinyl-CoA,
fumarate,
oxaloacetate, and
pyruvate, are termed
glucogenic (shown in pink).
Those degraded to acetyl-CoA
or acetoacetate are called
ketogenic (shown in blue)
because they can be converted to
fatty acids or ketone bodies.
Some amino acids are both
glucogenic and ketogenic.
C-3 family:
Ala, Ser, Cys,
Gly, Thr,
Trp
C-5 family (α-ketoglutarate):
Glu, Gln, Arg, Pro, His
Succinyl-CoA:
Ile, Met, Val
Acetyl-CoA & acetoacetate
Ile, Leu, Thr, Trp
Leu, Lys, Phe, Tyr
Figure 25.42
Formation of pyruvate from
alanine, serine, cysteine, glycine,
tryptophan, or threonine.
Figure 25.44
Valine, isoleucine, and
methionine are converted via
propionyl-CoA to succinyl-CoA
for entry into the citric acid cycle.
The shaded carbon atoms of the
three amino acids give rise to
propionyl-CoA.
All three amino acids lose their
α-carboxyl group as CO2.
Figure 25.43 The degradation of
the C-5 family of amino acids leads
to α-ketoglutarate via glutamate.
The histidine carbons, numbered 1
through 5, become carbons 1
through 5 of glutamate, as
indicated.
Leucine is Degraded to Acetyl-CoA and
Acetoacetate
Figure 25.45 Leucine is one of only two purely ketogenic amino acids; the other
is lysine. Deamination of leucine via a transamination reaction yields αketoisocaproate, which is oxidatively decarboxylated to isovaleryl-CoA.
Subsequent reactions give β-hydroxy-β-methylglutaryl-CoA, which is then
cleaved to yield acetyl-CoA and acetoacetate, a ketone body.
Methionine first becomes Sadenosylmethionine, then
homocysteine (see Figure 25.28).
The terminal two carbons of
isoleucine become acetyl-CoA.
The Predominant Pathway of Lysine Degradation
is the Saccharopine Pathway
Figure 25.46 Lysine is degraded through saccharopine and α-aminoadipate to αketoadipate. Oxidative decarboxylation yields glutaryl-CoA, which can be
transformed into acetoacetyl-CoA and then acetoacetate.
Phenylalanine and Tyrosine Are Degraded to
Acetoacetate and Fumarate
• The first reaction in phenylalanine degradation is the
hydroxylation reaction of tyrosine biosynthesis
• Both these amino acids thus share a common
degradative pathway
• Transamination of tyrosine yields phydroxyphenylpyruvate
• A vitamin C-dependent dioxygenase then produces
homogentisate
• Ring opening and isomerization gives 4-fumarylacetoacetate, which is hydrolyzed to acetoacetate and
fumarate
Hereditary defects
Maple syrup urine disease
– After the initial step (deamination) to produce αketo acids
– The defect in oxidative decarboxylation of Ile,
Leu, and Val (25.44)
Phenylketonuria
– The defect in phenylalanine hydoxylase (25.38)
– Accumulation of phenylpyruvate
Alkaptouria
– Homogentisate dioxygenase (25.47)
Figure 25.47 Phenylalanine and tyrosine degradation.
(1) Transamination of Tyr gives p-hydroxyphenylpyruvate
(2) p-hydroxy-phenylpyruvate dioxygenase (vitamin C-dependent)
(3) homogentisate dioxygenase
(4) 4-Maleylacetoacetate isomerase
(5) is hydrolyzed by fumarylacetoacetase.
Nitrogen excretion
Ammonotelic:
– Ammonia
– Aquatic animals
Ureotelic:
– Urea
– Terrestrial vetebrates
Uricotelic:
– Uric acid
– Birds and reptiles