1. AMINO ACID METABOLISM

2. Metabolic relationship of amino acids
BODY PROTEINS
Proteosynthesis
Degradation
AMINO ACIDS
DIETARY
PROTEINS
GLYCOLYSIS
KREBS CYCLE
Digestion
Transamination
NONPROTEIN
DERIVATIVES
Porphyrins
Purines
Pyrimidines
Neurotransmitters
Hormones
Complex lipids
Aminosugars
UREA
NH3
(Carbon skeleton)
Conversion
250 – 300
g/day
ACETYL CoA
GLUCOSE
CO2
KETONBODIES

3. Enzymes cleaving the peptide bond
Endopeptidases – hydrolyse the peptide bond inside a chain: pepsin, trypsin, chymotrypsin
Exopeptidases – split the peptide bond at the end of a protein molecule: aminopeptidase, carboxypeptidases
Dipeptidases
pepsin (pH 1.5 – 2.5) – peptide bond derived from Tyr, Phe,
bonds between Leu and Glu
trypsin (pH 7.5 – 8.5) – bonds between Lys a Arg
chymotrypsin (pH 7.5 – 8.5) – bonds between Phe a Tyr

4. Gamma-glutamyl cycle

5. Essential amino acids in humans
Lysine
Methionine
Threonine
Phenylalanine
Tryptophan
Arginine*
Histidine*
Isoleucine
Leucine
Valine
*Required to some degree in young growing period and/or sometimes during illness.

6. Non-essential and nonessential amino acids in humans
Can be formed from a-keto acids by transamination and subsequent reactions.
Alanine
Asparagine
Aspartate
Glutamate
Glutamine
Glycine
Proline
Serine
Cysteine (from Met*)
Tyrosine (from Phe*)
* Essential amino acids

7. General reactions of amino acid catabolism +
COO- +
NH4+
deamination
transamination C O R
COO- CH
NH2 R
COO- CH
NH2 R
COO-

8. The fate of the amino group during amino acid catabolism

9. Transamination reaction
The first step in the catabolism of most amino acids is removal of a-amino groups by enzymes transaminases or aminotransferases
All aminotransferases have the same prostethic group and the same reaction mechanism.
The prostethic group is pyridoxal phosphate (PPL), the coenzyme form of pyridoxine (vitamin B6)

10. Biosynthesis of amino acid: transamination reactions
amino acid1 +a-keto acid amino acid2 +a-keto acid1
Keto-acid +
Glutamate
Pyridoxal phosphate (PLP)-
dependent aminotransferase +
Amino acid
a-Ketoglutarate

11. Active metabolic form of vitamin B6

12. Mechanism of transamination reaction: PPL complex with enzyme accept an amino group to form pyridoxamine phosphate, which can donate its amino group to an a-keto acid.

13. Transaminases are differ in their specificity for L-amino acids.
All amino acids except threonine, lysine, and proline can be transaminated
Transaminases are differ in their specificity for L-amino acids.
The enzymes are named for the amino group donor.

14. Clinicaly important transaminases
Alanine-a-ketoglutarate transferase ALT
(also called glutamate-pyruvate transaminase – GPT)
Aspartate-a-ketoglutarate transferase AST
(also called glutamate-oxalacetate transferase – GOT)
Important in the diagnosis of heart and liver damage caused by heart attack, drug toxicity, or infection.
ALT

15. Glucose-alanine cycle
Alanine plays a special role in transporting amino groups to liver.
Ala is the carrier of ammonia and of the carbon skeleton of pyruvate from muscle to liver.
The ammonia is excreted and the pyruvate is used to produce glucose, which is returned to the muscle.
According to D. L. Nelson, M. M. Cox :LEHNINGER. PRINCIPLES OF BIOCHEMISTRY Fifth edition

16. Glutamate releases its amino group as ammonia in the liver
The amino groups from many of the a-amino acids are collected in the liver in the form of the amino group of L-glutamate molecules.
Glutamate undergoes oxidative deamination catalyzed by L-glutamate dehydrogenase.
Enzyme is present in mitochondrial matrix.
It is the only enzyme that can use either NAD+ or NADP+ as the acceptor of reducing equivalents.
Combine action of an aminotransferase and glutamate dehydrogenase referred to as transdeamination.

17. Ammonia transport in the form of glutamine
Excess ammonia is added to glutamate to form glutamine.
Glutamine synthetase
Glutamine enters the liver and NH4+ is liberated in mitochondria by the enzyme glutaminase.
Ammonia is remove by urea synthesis.

18. Relationship between glutamate, glutamine and a-ketoglutarate
NH3
NH3 E E
a-ketoglutarate
glutamate
glutamine
NH3
NH3 E
A. Glutamate dehydrogenase E
glutamate + +
NAD+
H2O
a-ketoglutarate +
NH3 +
NADH
From transamination
reactions
To urea cycle
B. Glutamine synthetase (liver)
ATP
ADP E +
glutamine
glutamate
NH3
C. Glutaminase (kidney) E + +
glutamine
H2O
glutamate
NH3

19. E Oxidative deamination Amino acids FMN H2O + a-keto acids FMNH2 NH3
L-amino acid oxidase
A. Oxidative deamination
H2O2 O2
catalse
B. Nonoxidative deamination
serine
pyruvate
threonine
a-ketobutirate
Serin-threonin dehydratase
L-amino acid oxidase produces ammonia and a-keto acid directly, using FMN as cofactor.
The reduced form of flavin must be regenerated by O2 molecule.
This reaction produces H2O2 molecule which is decompensated by catalase.
Is possible only for hydroxy amino acids

25. UREA CARBAMOYL PHOSPHATE + Ornithine Citrulline Arginine UREA + CYCLE
Aspartate
Fumarate
Argininosuccinate

26. Urea Cycle

27. REGULATION OF THE UREA CYCLE Acute: N-acetylglutamate, allosteric
effector, up regulates CPS I

28. N-Acetylglutamate is synthesized from glutamate and acetyl-CoA by a mitochondrial NAG synthase.

29. Metabolic Diseases of the Urea Cycle
Arginase Deficiency
Argininosuccinic
acidemia
Type II
Hyperammonemia:
Type I
Citrullinemia

30. Metabolic Diseases of the Urea Cycle
Disorders present in infants:
Symptoms: Lethargy, swelling of the brain
leads to mental retardation/brain damage
Diagnosis: Low blood urea nitrogen (BUN) levels
-high levels of ammonia in the blood
elevated circulating glutamine
-other metabolites that accumulate depend on
the specific enzyme defect
Treatment:
Long term, dietary restriction.
Low protein diet. Supplemented with Arginine

31. Excessive ammonia is toxic to the central nervous system.
Alternative pathway therapy:
Sodium benzoate to produce hippuric acid
Sodium phenylacetate or phenyl-butyrate to produce phenylacetylglutamine

35. Amino acid metabolism and central metabolic pathways
20 amino acids are converted to 7 products:
pyruvate
acetyl-CoA
acetoacetate
a-ketoglutarate
succynyl-CoA
oxalacetate
fumarate

36. Glucogenic Amino Acids
formed: a-ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-CoA
Aspartate
Asparagine
Arginine
Phenylalanine
Tyrosine
Isoleucine
Methionine
Valine
Glutamine
Glutamate
Proline
Histidine
Alanine
Serine
Cysteine
Glycine
Threonine
Tryptophan

37. formed acetyl CoA or acetoacetate
Ketogenic Amino Acids
formed acetyl CoA or acetoacetate
Lysine
Leucine

38. Both glucogenic and ketogenic amino acids
formed: a-ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-CoA in addition to acetyl CoA or acetoacetate
Isoleucine
Threonine
Tryptophan
Phenylalanine
Tyrosine

39. Glycogenic and Ketogenic Amino Acids
1.Glucogenic: converted to glucose via pyruvate
2.Ketogenic: converted to ketone bodies
3.Some are both
4.During fasting when FA are the major fuel FA cannot be converted to glucose therefore AA → glucose & ketone bodies (especially for brain)
-AA → pyruvate → liver → glucose
-keto AA + FA → ketone bodies (acetoacetate & 3 hydroxybutyrate)

40. Asparagine, Aspartate

41. The C4 family: aspartate and asparagine are converted into oxalacetate
Aspartic acid
Asparagine
Oxalacetate

42. Glutamine, and Glutamate

44. The C5 family: several amino acids are converted into a-ketoglutarate through glutamate
Glutamine
a-ketoglutarate
Proline
Arginine
Histidine

45. The C3 family: alanine, serine, cysteine and threonine are converted to pyruvate

46. Interconversion of amino acids and intermediates of carbohydrate metabolism and Krebs cycle

47. Metabolism of some selected amino acids

48. Tryptophan catabolism
Tryptophan has complex catabolic pathway:
the indol ring is ketogenic
the side chain forms the glucogenic products
Kynurenate and xanthurenate are excrete in the urine.

49. Glycine biosynthesis from serine
Reaction involves the transfer of the hydroxymethyl group from serine to the cofactor tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF.
Copy from:

50. Glycine oxidation to CO2
Glycine produced from serine or from the diet can also be oxidized by glycine decarboxylase (also referred to as the glycine cleavage complex, GCC) to yield a second equivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Copy from:

51. Cysteine and methionine are metabolically related
The sulfur for cysteine synthesis comes from the essential amino acid methionine.
SAM
Condensation of ATP and methionine yield S-adenosylmethionine (SAM)
SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the conversion of norepinephrine to epinenephrine).

52. Cysteine synthesis * Conversion of SAM to homocysteine.
Conversion of homocysteine back to Met. N5-methyl-THF is donor of methyl group.
*folate + vit B12
Conversion of SAM to homocysteine.
Condensation of homocysteine with serine to cystathione.
Cystathione is cleavaged to cysteine.
Copy from:

53. Homocystinuria Genetic defects for both the synthase and the lyase.
Missing or impaired cystathionine synthase leads to homocystinuria.
High concentration of homocysteine and methionine in the urine.
Homocysteine is highly reactive molecule.
Disease is often associated with mental retardation, multisystemic disorder of connective tissue, muscle, CNS, and cardiovascular system.

54. Cysteine catabolism

55. Biosynthesis of Tyrosine from Phenylalanine
Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced state by the NADH-dependent enzyme dihydropteridine reductase

56. Phenylketonuria
Hyperphenylalaninemia - complete deficiency of phenylalanine hydroxylase (plasma level of Phe raises from normal 0.5 to 2 mg/dL to more than 20 mg/dL).
The mental retardation is caused by the accumulation of phenylalanine, which becomes a major donor of amino groups in aminotransferase activity and depletes neural tissue of α-ketoglutarate.
Absence of α-ketoglutarate in the brain shuts down the TCA cycle and the associated production of aerobic energy, which is essential to normal brain development.
Newborns are routinelly tested for blood concentration of Phe.
The diet with low-phenylalanine diet.

57. Alternative pathways of phenylalanine
catabolism in phenylketonuria

58. Homogentisic Acid Formation
Transamination
Tyrosine
p-Hydroxyphenyl-
pyruvate
Deficient in
alkaptonuria O2
p-Hydroxyphenyl-
pyruvate
dioxygenase
(ascorbate-dep.)
Homogentisate
dioxygenase
Cleavage of
aromatic ring
CO2 O2
Fumarate + acetoacetate
Homogentisate

59. Alkaptonuria Homogentisate appears in urine
First defect to which inborn error of metabolism applied – Sir Archibald Garrod in early 1900’s
Homogentisate appears in urine
Deposited in cartilage and elsewhere  polymerization (black)
Deficiency of homogentisate dioxygenase
Urine turns dark on standing
Oxidation of homogentisic acid
Asymptomatic in childhood
Tendency toward arthritis in adulthood

61. Catabolism of branched amino acids
valine
isoleucine
leucine
a-ketoglutarate glutamate (transamination)
a-ketoisovalerate
a-keto-b-methylbutyrate
a-ketoisokaproate
NAD+
oxidative decarboxylation
Dehydrogenase of a-keto acids*
CO2
NADH + H+
isobutyryl CoA
a-methylbutyryl CoA
isovaleryl CoA
Dehydrogenation etc., similar to fatty acid b-oxidation
acetyl CoA
acetyl CoA
propionyl CoA + +
propionyl CoA
acetoacetate

62. Branched-chain aminoaciduria
Disease also called Maple Syrup Urine Disease (MSUD) (because of the characteristic odor of the urine in affected individuals).
Deficiency in an enzyme, branched-chain α-keto acid dehydrogenase leads to an accumulation of three branched-chain amino acids and their corresponding branched-chain α-keto acids which are excreted in the urine.
There is only one dehydrogenase enzyme for all three amino acids.
Mental retardation in these cases is extensive.

64. Essential & Non-essential AA
 Conditionally essential
(i) ARG:can be made, but not enough
(ii) HIS: controversial (essential for growth in children)
(iii) PHE essential, TYR can be made from PHE but when enzyme is missing (phenyl- ketonuria) then PHE > TYR; Therefore TYR is essential
(iv) MET CYS; Similarly, if MET > CYS then CYS essential
 Even with excess, important in excretion NH4+ therefore continue to be made

65. Serine biosynthesis from glycolytic intermediate 3-phosphoglycerate
Copy from:

66. Formation of alanine

67. Asparagine synthetase reaction

68. The glutamate dehydrogenase

69. Ammonia transport in the form of glutamine
Excess ammonia is added to glutamate to form glutamine.
Glutamine synthetase
Glutamine enters the liver and NH4+ is liberated in mitochondria by the enzyme glutaminase.
Ammonia is remove by urea synthesis.

70. Biosynthesis of proline from glutamate

71. Conversion of methionine to propionyl-CoA.

72. The phenylalanine hydroxylase reaction

73. Conversion of Amino Acids to Specialized Products

74. Catecholamine Biosynthesis
Tyr hydroxylase O2
Tyrosine
Dihydroxyphenylalanine
(DOPA)
DOPA
decarboxylase
Epinephrine
(Adrenaline)
CO2
Dopamine
hydroxylase
Methyl
transferase
S-Adenosyl-
homocysteine
Dopamine
SAM
Norepinephrine
DOPA, dopamine, norepinephrine,
and epinephrine are all neurotransmitters

75. Histidine Metabolism: Histamine Formation
decarboxylase
Histidine
CO2
Histamine
Histamine:
Synthesized in and released by mast cells
Mediator of allergic response: vasodilation, bronchoconstriction

76. Tryptophan Metabolism: Serotonin Formation
Indole ring
Trp
hydroxylase
Decarboxylase O2
5-Hydroxy-
tryptophan
Tryptophan
(Trp)
CO2
5-Hydroxy-
tryptamine (5-HT);
Serotonin

77. Serotonin Metabolism: Melatonin
2 Steps
Serotonin
Melatonin
Melatonin:
Formed principally in pineal gland
Synthesis controlled by light, among other factors
Induces skin lightening
Suppresses ovarian function
Possible use in sleep disorders

78. Creatine and Creatinine
Arginine-glycine
transamidinase
(Kidney)
Glycine Ornithine
Arginine
Guanidoacetate
SAM + ATP
S-Adenosyl-
homocysteine
+ ADP
Guanidoacetate
Methyltransferase
(Liver)
Creatinine
(Urine)
Non-enzymatic
(Muscle)
Creatine kinase
(Muscle)
Creatine
ADP
+ Pi
Phosphocreatine
ATP

79. Biosynthesis and metabolism of creatine and creatinine

80. β-Alanyl Dipeptides

81. Biosynthesis of hippurate

82. Polyamines: Conversion of spermidine to spermine

83. Nitric Oxide

84. glutathione

85. Carnitine
GABA

87. Formation of S-adenosylmethionine

88. Biosynthesis of epinephrine and norepinephrine