1. Chapter 25 Nitrogen Acquisition and Amino Acid Metabolism Biochemistryby
Reginald Garrett and Charles Grisham

2. Outline
Which Metabolic Pathways Allow Organisms to Live on Inorganic Forms of Nitrogen?
What Is The Metabolic Fate of Ammonium?
What Regulatory Mechanisms Act on Escherichia coli Glutamine Synthetase?
How Do Organisms Synthesize Amino Acids?
How Does Amino Acid Catabolism Lead into Pathways of Energy Production?

3. 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+)

4. Thus, Nitrogen acquisition must involve
The Reduction of the oxidized forms (N2 and NO3-) to NH4+
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.
(+3)
(-3)
(+5)
(-3)
(+5)
(+2)
(0)
Figure 25.1 The nitrogen cycle.
(+1)
(+3)

5. The Reduction of Nitrogen
Nitrogen assimilation and nitrogen fixation
Nitrate assimilation occurs in two steps:
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
Nitrogen fixation involves reduction of N2 in prokaryotes by nitrogenase

6. Nitrate Assimilation Nitrate assimilation
the reduction of nitrate to NH4+ in plants, various fungi, and certain bacteria
Two steps:
Nitrate reductase
NO H e- → NO H2O
Nitrite reductase
NO H e- → NH H2O
Electrons are transferred from NADH to nitrate

7. Nitrate reductase
Nitrate reductases are cytosolic kD dimeric protein, pathway involve
SH of enzyme
FAD
Cytochrome b557
Molybdenum cofactor
MoCo required both for reductase activity and for assembly of enzyme subunits to active dimer
NADH NO3-
[E-SH →FAD→cytochrome b557 →MoCo]
NAD NO2-

8. 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
siroheme
In higher plants

9. Figure 25.3 Domain organization within the enzymes of nitrate assimilation. The numbers denote residue number along the amino acid sequence of the proteins.

10. Nitrogen fixation 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:
Nitrogenase
A strong reductant (reduced ferredoxin)
ATP
O-free conditions

11. Nitrogenase Complex Two metalloprotein components:
Nitrogenase reductase
Nitrogenase

12. Nitrogenase reductase
Fe-protein
A 60 kD homodimer with a single 4Fe-4S cluster
Extremely O2-sensitive
Binds MgATP and hydrolyzes 2 ATPs per electron transferred
Because reduction of N2 to 2NH4+ + H2 requires 8 electrons, 16 ATP are consumed per N2 reduced

13. N2 reduction to ammonia is thermodynamically favorable
Figure 25.4 The triple bond in N2 must be broken during nitrogen fixation.
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

14. Nitrogenase MoFe-protein—a 220 kD a2b2 heterotetramer
An ab-dimer serve as the functional unit
Contains two types of metal centers
P-cluster (figure 25.5a)
8Fe-7S center
FeMo-cofactor (figure 25.5b)
7Fe-1-Mo-9S cluster
Oxygen labile
Nitrogenase is a rather 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

15. 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. (8Fe-7S)
(b) The FeMo-cofactor contains 1 Mo, 7Fe, and 9S atoms. Homocitrate provides two oxo ligands to the Mo atom.

16. The regulation of nitrogen Fixation
Two regulatory controls
ADP inhibits the activity of nitrogenase
NH4+ represses the expression of nif genes
In some organisms, the nitrogenase complex is regulated by covalent modification. ADP-ribosylation of nitrogenase reductase leads to its inactivation.
Figure Regulation of nitrogen fixation.

17. 25.2 – What Is The Metabolic Fate of Ammonium?
NH4+ enters organic linkage via three major reactions in all cells
Glutamate dehydrogenase (GDH)
Glutamine synthetase (GS)
Carbamoyl-phosphate synthetase I (CPS-I)
Asparagine synthetase (some microorganisms)

18. 1. Glutamate dehydrogenase (GDH)
Reductive amination of a-ketoglutarate to form glutamate
NH a-ketoglutarate + NADPH H+ →
glutamate + NADP H2O
Mammalian GDH plays a prominent role in amino acid catabolism (oxidative amination)

19. 2. Glutamine synthetase (GS)
ATP-dependent amidation of g-carboxyl of glutamate to glutamine
NH glutamate + ATP →
glutamine + ADP + Pi
Glutamine is a major N donor in the biosynthesis of many organic N compounds, therefore GS activity is tightly regulated
Glutamine is the most abundant amino acid in human

20. Figure 25.10 (a) The enzymatic reaction catalyzed by glutamine synthetase.
(b) The reaction proceeds by (a) activation of the g-carboxyl group of Glu by ATP, followed by (b) amidation by NH4+.

21. 3. Carbamoyl-phosphate synthetase I (CPS-I)
Ammonium is converted to carbamoyl-P
This reaction is an early step in the urea cycle
NH4+ + HCO ATP →
carbamoyl phosphate + 2 ADP + Pi H+
Two ATP required
one to activate bicarbonate
one to phosphorylate carbamate

22. The major pathways of Ammonium Assimilation lead to glutamin synthesis
Two principal pathways :
Principal route: GDH/GS in organisms rich in N
Secondary route: GS/GOGAT in organisms confronting N limitation
GOGAT is glutamate synthase or glutamate:oxo-glutarate amino transferase
GDH has a higher Km for NH4+ than does GS

23. The glutamate synthase (GOGAT)reaction, showing the reductants exploited by different organisms in this reductive amination reaction.

24. 25.3 – What Regulatory Mechanisms Act on Glutamine Synthetase
GS in E. coli is regulated in three ways:
Feedback inhibition (allosteric regulation)
Covalent modification (interconverts between inactive and active forms)
Regulation of gene expression and protein synthesis control the amount of GS in cells
But no such regulation occurs in eukaryotic versions of GS
E. coli GS is a 12-mer

25. 1. Allosteric Regulation of Glutamine Synthetase
9 different feedback inhibitors: Gly, Ala, Ser, His, Trp, CTP, AMP, carbamoyl-P, and glucosamine-6-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 for binding at the ATP substrate site
Gly, Ala, and Ser compete with Glu for binding at the active site
This effectively controls glutamine’s contributions to metabolism

26. Figure 25.15 The allosteric regulation of glutamine synthetase activity by feedback inhibition.

27. 2. Covalent Modification of Glutamine Synthetase
Each subunit can be adenylylated at Tyr-397
Adenylylation inactivates GS
ATP:GS:adenylyl transferase (AT) catalyzes both the adenylylation and deadenylylation
PII (regulatory protein) controls these
AT:PIIA catalyzes adenylylation
AT:PIID (PII-UMP) catalyzes deadenylylation
-Ketoglutarate and Gln also affect
-Ketoglutarate activates AT:PIID and inhibit AT:PIIA
Gln activates AT:PIIA and inhibit AT:PIID

28. Figure 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.

29. (Adenylylation)
(Deadenylylation)
Figure The cyclic cascade system regulating the covalent modification of GS.

30. 3. Gene Expression regulates GS
Gene GlnA is actively transcribed only if a 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
(kinase)
(phosphatase)

31. 25.4 – Amino Acid Biosynthesis
Organisms show substantial differences in their capacity to synthesize the 20 amino acids common to proteins
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
Amino acids are formed from a-keto acids by transamination
Amino acid1 + a-keto acid2 → a-keto acid1 + Amino acid2

32. Figure Glutamate-dependent transamination of a-keto acid carbon skeletons is a primary mechanism for amino acid synthesis.

33. The Mechanism of the Aminotransferase (Transamination) Reaction

34. Mammals can make only 10 of the 20 AAs
*Arginine and histidine are essential in the diets of juveniles, not adults
Mammals can make only 10 of the 20 AAs
The others are classed as "essential" amino acids and must be obtained in the diet

35. The pathways of amino acid biosynthesis can be organized into families
According to the intermediates that they are made from
a-ketoglutarate
Oxaloacetate
Pytuvate
3-phosphoglycerate
Phosphoenolpyruvate and erythrose-4-P (aromatic)

37. 1. The -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 -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

38. Figure 25. 20 The pathway of proline biosynthesis from glutamate
Figure The pathway of proline biosynthesis from glutamate. The enzymes are (1) g-glutamyl kinase, (2) glutamate-5-semialdehyde dehydrogenase, and (4) D1-pyrroline-5-carboxylate reductase; reaction (3) occurs nonenzymatically.

39. (1) N-acetylglutamate synthase
(2) N-acetylglutamate kinase
(3) N-acetylglutamate-5-semialdehyde dehydrogenase
(4) N-acetylornithine d-aminotransferase
(5) N-acetylornithine deacetylase

40. 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
d-NH3+ of ornithine is carbamoylated by onithine transcarbamoylase in urea cycle

41. Carbamoyl-phosphate synthetase I
Carbamoyl-phosphate synthetase I (CPS-I)
NH3-dependent mitochondrial CPS isozyme
HCO3- is activated via an ATP-dependent phosphorylation
Ammonia attacks the carbonyl carbon of carbonyl-P, displacing Pi to form carbamate
Carbamate is phosphorylated via a second ATP to give carbamoyl-P

42. Figure 25.22 The mechanism of action of CPS-I

43. CPS-I represents the committed step in urea cycle
Activated by N-acetylglutamate
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

44. Urea Cycle Ornithine transcarbamoylase (OTCase)
Argininosuccinate synthetase
Urea Cycle
Arginase
Argininosuccinase

45. The Urea Cycle
The carbon skeleton of arginine is derived from a-ketoglutarate (Ornithine)
N and C in the guanidino group of Arg come from NH4+, HCO3- (carbamoyl-P), and the -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

46. Lysine Biosynthesis Two pathways: Lysine derived from -ketoglutarate
-aminoadipate pathway
diaminopimelate pathway (Asp)
Lysine derived from -ketoglutarate
Reactions 1 through 4 are reminiscent of the first four reactions in the citric acid cycle
-ketooadipate
Transamination gives -aminoadipate
Adenylylation activates the -COOH for reduction
Reductive amination give saccharopine
Oxidative cleavage yields lysine

47. Figure 25.24 Lysine biosynthesis in certain fungi and Euglena: the a-aminoadipic acid pathway.

48. 2. 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
-Aspartyl semialdehyde and homoserine are branch points
Isoleucine, four of its six carbons derived from Asp (via Thr) and two come from pyruvate

49. Figure 25.25 Aspartate biosynthesis via transamination of oxaloacetate by glutamate.
Figure Asparagine biosynthesis from Asp, Gln, and ATP by asparagine synthetase.

50. Figure Biosynthesis of threonine, methionine, and lysine, members of the aspartate family of amino acids.
b-Aspartyl-semialdehyde is a common precursor to all three.
It is formed by aspartokinase (reaction 1) and b-aspartyl-semialdehyde dehydrogenase (reaction 2).

51. Figure Biosynthesis of threonine, methionine, and lysine, members of the aspartate family of amino acids.

52. Figure Biosynthesis of threonine, methionine, and lysine, members of the aspartate family of amino acids.

53. b-Aspartyl-semialdehyde
Figure Biosynthesis of threonine, methionine, and lysine, members of the aspartate family of amino acids.

54. In E. coli, The first reaction is an ATP-dependent phosphorylation catalyzed by aspartokinase
Three isozymes of aspartokinase (I, II, and III)
Uniquely controlled by one of the three end-products
Form I is feedback-inhibited by threonine
Form III is feedback-inhibited by lysine

56. Important role of methionine
in methylations via S-adenosylmethionine (SAM; S-AdoMet)
polyamine biosynthesis
Figure The synthesis of S-adenosylmethionine (SAM)

57. 3. 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 )
Threonine deaminase (also called threonine dehydratase or serine dehydratase) is sensitive to Ile
Ile and val pathway employ the same set of enzymes
Leu synthesis begins with an -keto isovalerate
Isopropylmalate synthase is sensitive to Leu

58. Figure 25.29 Biosynthesis of valine and isoleucine.
Threonine deaminase
Isopropylmalate synthase
HydroxyethylthiaminePP
Acetohydroxy acid synthase
Isopropylmalate dehydratase
Acetohydroxy acid isomeroreductase
Isopropylmalate dehydrogenase
Dihydroxy acid dehydratase
Leucine aminotransferase
Glutamate-dependent aminotransferase

59. 4. 3-Phosphoglycerate Family Ser, Gly, Cys
3-Phosphoglycerate dehydrogenase diverts 3-PG from glycolysis to amino acid synthesis pathways (3-phosphohydroxypyruvate)
Transamination by Glu gives 3-phosphoserine (3-phosphoserine aminotransferase)
Phosphoserine phosphatase yields serine

60. Serine hydroxymethylase (PLP-dependent) transfers the -carbon of Ser to THF to make glycine
Figure Biosynthesis of glycine from serine (a) via serine hydroxymethyltransferase and (b) via glycine oxidase.

61. A PLP-dependent enzyme makes Cys
Some bacteria
most microorganism and plants
O-acetylserine sulfhydrylase
serine acetyltransferase
Figure Cysteine biosynthesis. (a) Direct sulfhydrylation of serine by H2S. (b) H2S-dependent sulfhydrylation of O-acetylserine.

62. ATP sulfurylase
Adenosine-5'-phosphosulfate-3'-phosphokinase.
Figure Sulfate assimilation and the generation of sulfide for synthesis of organic S compounds.
Sulfite oxidase

63. 5. Aromatic Amino Acids Phe, Tyr, Trp, His
The aromatic amino acids, Phe, Tyr, and Trp, are derived from shikimate pathway yields chorismate, thence Phe, Tyr, Trp
Chorismate as a branch point in this pathway (Figs )
Chorismate is synthesized from PEP and erythrose-4-P
Via shikimate pathway
The side chain of chorismate is derived from a second PEP

64. Figure 25.35 Some of the aromatic compounds derived from chorismate.

65. 2-keto-3-deoxy-D-arabino-heptulosonate-7-P synthase
dehydroquinate synthase
5-dehydroquinate dehydratase
shikimate dehydrogenase
shikimate kinase
3-enolpyruvyl-shikimate-5-phosphate synthase
chorismate synthase.

66. The Biosynthesis of Phe, Tyr, and Trp
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-4-monooxygenase)
Figure The formation of tyrosine from phenylalanine.

67. Figure 25.37 The biosynthesis of phenylalanine, tyrosine, and tryptophan from chorismate.
chorismate mutase
prephenate dehydratase
phenylalanine aminotransferase
prephenate dehydrogenase
tyrosine aminotransferase
anthranilate synthase
anthranilate-phosphoribosyl transferase
N-(5'-phosphoribosyl)-anthranilate isomerase
indole-3-glycerol phosphate synthase
tryptophan synthase (a-subunit)
tryptophan synthase (b-subunit).

68. Histidine 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-4-carboxamide ribonucleotide (AICAR) is a purine precursor (replenish ATP; Ch 26)

69. Figure 25.40 The pathway of histidine biosynthesis.
ATP-phosphoribosyl transferase
pyrophosphohydrolase
phosphoribosyl-AMP cyclohydrolase
phosphoribosylformimino-5-aminoimidazole carboxamide ribonucleotide isomerase
glutamine amidotransferase
imidazole glycerol-P dehydratase
L-histidinol phosphate aminotransferase
histidinol phosphate phosphatase
histidinol dehydrogenase.

70. 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.

71. Amino acid synthesis inhibitors as herbicides
(inhibitor of 3-enolpyruvyl-shikimate-5-phosphate 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)

72. 25.5 – Degradation of Amino Acids
The 20 amino acids are degraded to produce (mostly) TCA intermediates
The primary physiological purpose of amino acids is to serve as building blocks for protein synthesis
Energy requirement
90% from oxidation of carbohydrates and fats
10% from oxidation of amino acids
The classifications of amino acids in Fig
Glucogenic and ketogenic

73. Figure 25. 41 Metabolic degradation of the common amino acids
Figure 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
a-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.

74. The 20 amino acids are degraded by 20 different pathways that converge to just 7 metabolic intermediates

75. C-4 family (oxaloaceate & fumarate):
C-3 family (pyruvate):
Ala, Ser, Cys, Gly, Thr, Trp
C-4 family (oxaloaceate & fumarate):
Oxaloaceate: Asp, Asn
Fumarate: Asp, Phe, Tyr
C-5 family (a-ketoglutarate):
Glu, Gln, Arg, Pro, His
Succinyl-CoA:
Ile, Met, Val
Acetyl-CoA & acetoacetate
Ile, Leu, Thr, Trp
Leu, Lys, Phe, Tyr

76. C-3 family: Ala, Ser, Cys, Gly, Thr, Trp
Figure Formation of pyruvate from alanine, serine, cysteine, glycine, tryptophan, or threonine.

77. Figure The degradation of the C-5 family of amino acids leads to a-ketoglutarate via glutamate. The histidine carbons, numbered 1 through 5, become carbons 1 through 5 of glutamate, as indicated.

78. Figure 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 a-carboxyl group as CO2.
Methionine first becomes S-adenosylmethionine, then homocysteine (see Figure 25.28).
The terminal two carbons of isoleucine become acetyl-CoA.

79. Leucine is Degraded to Acetyl-CoA and Acetoacetate
Figure Leucine is one of only two purely ketogenic amino acids; the other is lysine.
Deamination of leucine via a transamination reaction yields α-ketoisocaproate, 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.

80. Hereditary defects in BCKDH leads to maple sugar urine disease
Unlike the other 17 amino acids, which are broken down in the liver, Val, Ile, and Leu are also degraded in adipose tissue.

81. The Predominant Pathway of Lysine Degradation is the Saccharopine Pathway
Figure Lysine is degraded through saccharopine and α-aminoadipate to α-ketoadipate. Oxidative decarboxylation yields glutaryl-CoA, which can be transformed into acetoacetyl-CoA and then acetoacetate.

82. 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 p-hydroxyphenylpyruvate
A vitamin C-dependent dioxygenase then produces homogentisate
Ring opening and isomerization gives 4-fumaryl-acetoacetate, which is hydrolyzed to acetoacetate and fumarate

83. Figure 25.48 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.

84. Tryptophan is a crucial precusor for synthesis of a variety of important substances
Serotonin (5-hydroxytryptophan) is a neurotransmitter
Melatonin (N-acetyl-5-methoxytrptophan) is a hormone

85. Hereditary defects Maple syrup urine disease Phenylketonuria
After the initial step (deamination) to produce a-keto 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)

86. Nitrogen excretion Ammonotelic: Ureotelic: Uricotelic: Ammonia
Aquatic animals
Ureotelic:
Urea
Terrestrial vetebrates
Uricotelic:
Uric acid
Birds and reptiles