Amino acid metabolism

1. METABOLISM OF AMINO ACIDS AND
PROTEINS
NARESH PANIGRAHI
ASST.PROFESSOR
PHARMACEUTICAL CHEMISTRY
GITAM INSTITUTE OF PHARMACY
GITAM UNIVERSITY

12. Proteins function in the organism.
 All enzymes are proteins.
 Storing amino acids as nutrients and as building
blocks for the growing organism.
 Transport function (proteins transport fatty acids,
bilirubin, ions, hormones, some drugs etc.).
 Proteins are essential elements in contractile and
motile systems (actin, myosin).
 Protective or defensive function (fibrinogen,
antibodies).
 Some hormones are proteins (insulin, somatotropin).
 Structural function (collagen, elastin).

13. Amino acids
Proteins of food
Metabolites of
glycolysis and
Krebs cycle
Anabolic ways Catabolic ways
Synthesis of
cell and
extracell
proteins
Synthesis of
peptide
physiologi-
cally active
substances
Trans-
ami-
nation
Deami-
nation
Decar-
boxila-
tion
Urea, CO2, H2O
Amines
Proteins and peptides
of the organism
GENERAL PATHWAYS OF AMINO ACIDS METABOLISM

14.  Nitrogen balance is a comparison between
Nitrogen intake (in the form of dietary
protein) and Nitrogen loss (as undigested
protein in feces, NPN as urea, ammonia,
creatinine & uric acid in urine, sweat &
saliva & losses by hair, nail, skin).
 NB is important in defining
1.overall protein metabolism of an individual
2.nutritional nitrogen requirement.
Nitrogen Balance (NB):

15. Nitrogenous balance
It may be positive, negative and neutral (zero).
Positive nitrogenous balance – the amount of nitrogen entered the
organism is more than amount of nitrogen removed from the
organism. It occurs in young growing organism, during
recovering after severe diseases, at the using of anabolic
medicines pregnancy, lactation and convulascence
Negative nitrogenous balance – the amount of nitrogen removed
from the organism is more than amount of nitrogen entered the
organism. It occurs in senile age, destroying of malignant tumor,
vast combustions, poisoning by some toxins. High loss of tissue
proteins in wasting diseases like burns, hemorrhage & kidney
diseases with albuminurea (High breakdown of tissue proteins )
in hyperthyroidism, fever, infection
Zero nitrogenous balance – the amount of nitrogen removed from the
organism is equal to the amount of nitrogen entered the organism. It
occurs in healthy adult people
Normal adult: will be in nitrogen equilibrium, Losses = Intake

16. A deficiency of
even one amino
acid results in a
negative nitrogen
balance.
In this state, more
protein is
degraded than
synthesized.

17. The normal daily requirement of protein for
adults is 0.8 g/Kg body wt. day-1.
• That requirement is increased in healthy
conditions:
during the periods of rapid growth, pregnancy,
lactation and adolescence.
• Protein requirement is increased in disease
states:
illness, major trauma and surgery.
• RDA for protein should be reduced in:
hepatic failure and renal failure
Protein Requirement for humans
in Healthy and Disease Conditions

18. BV is : a measure for the ability of dietary
protein to provide the essential amino acids
required for tissue protein maintenance.
• Proteins of animal sources (meat, milk,
eggs) have high BV because they contain all
the essential amino acids.
• Proteins from plant sources (wheat, corn,
beans) have low BV thus combination of
more than one plant protein is required (a
vegetarian diet) to increase its BV.
Biological Value for Protein (BV)

19. Chemical composition of digestive juices.
Gastric juice contains water, enzymes, hydrochloric
acid, mineral salts and other compounds. About 2.5 l of
gastric juice is secreted per day.
The role of hydrochloric acid in digestion.
 Denaturate proteins (denaturated proteins easier
undergo digestion by pepsin than native proteins).
 Stimulates the activity of pepsin.
 Hydrochloric acid has bactericidial properties.
 Stimulates the peristalsis.
 Regulate the enzymatic function of pancreas.
Protein digestion

20. Mechanism of amino acid absorbtion.
This explanation is called the sodium cotransport
theory for amino acid transport; it is also called secondary
active transport of amino acid.
Absorption of amino acids through the intestine mucosa can occur
far more rapidly than protein can be digested in the lumen of the
intestine.
Since most protein digestion occurs in the upper small intestine
most protein absorption occurs in the duodenum and jejunum.

21. Most proteins are completely digested to free amino acids
Amino acids and sometimes short oligopeptides are absorbed by the
secondary active transport
Amino acids are transported via the blood to the cells of the body.

22. The sources of amino acids:
1) absorption in the intestine;
2) formation during the protein decomposition;
3) synthesis from the carbohydrates and lipids.
Using of amino acids:
1) for protein synthesis;
2) for synthesis of nitrogen containing compounds (creatine, purines, choline,
pyrimidine);
3) as the source of energy (oxidation – deamination, transamination,
decarboxilation);
4) for the gluconeogenesis;
5) for the formation of biologically active compounds.

23. PROTEIN TURNOVER
How can a cell distinguish proteins that are meant
for degradation?
Protein turnover — the degradation and resynthesis
of proteins
Half-lives of proteins – from several minutes to many years
Structural proteins – usually stable (lens protein crystallin lives
during the whole life of the organism)
Regulatory proteins - short lived (altering the amounts of these
proteins can rapidly change the rate of metabolic processes)

24. overview of amino acid metabolism
• The amino acids undergo certain common reactions like
transamination followed by deamination for the liberation of
ammonia.
• The amino group of the amino acids is utilized for the formation of
urea which is an excretory end product of protein metabolism.
• The carbon skeleton of the amino acids is first converted to keto
acids (by transamination) which meet one or more of the following
fates.
• 1. Utilized to generate energy.
• 2. Used for the synthesis of glucose.
• 3. Diverted for the formation of fat or ketone bodies.
• 4. lnvolved in the production of non-essential amino acids.

25. GENERAL WAYS OF AMINO
ACIDS METABOLISM
The fates of amino acids:
1) for protein synthesis;
2) for synthesis of other nitrogen containing compounds
(creatine, purines, choline, pyrimidine);
3) as the source of energy;
4) for the gluconeogenesis.

26. The general ways of amino acids degradation:
 Deamination
 Transamination
 Decarboxilation
The major site of amino acid degradation - the liver.
Deamination of amino acids
Deamination - elimination of amino group from amino acid with
ammonia formation.
Four types of deamination:
- oxidative (the most important for higher animals),
- reduction,
- hydrolytic, and
- intramolecular

27. Oxidative deamination
L-Glutamate dehydrogenase plays a central role in amino acid
deamination
In most organisms glutamate is the only amino acid that has
active dehydrogenase
Present in both the cytosol and mitochondria of the liver

28. Reduction deamination:
R-CH(NH2)-COOH + 2H+  R-CH2-COOH + NH3
amino acid fatty acid
Hydrolytic deamination:
R-CH(NH2)-COOH + H2O  R-CH(OH)-COOH + NH3
amino acid hydroxyacid
Intramolecular deamination:
R-CH-CH(NH2)-COOH  R-CH=CH-COOH + NH3
amino acid unsaturated fatty acid

29. 29
General scheme of oxydative
transamination
CH2CH2COOH
O
CHOOC+R CH
NH2
COOH
aminokyselina 2-oxoglutarát
HOOC CH CH2CH2COOH
NH2
+R C
O
COOH
glutamát2-oxokyselina
aminotransferasa
pyridoxalfosfát
amino acid
2-oxo acid
2-oxoglutarate
glutamate
aminotransferase
(pyridoxal phosphate)

30. 30
Glutamate dehydrogenase (GMD, GD, GDH)
• requires pyridine cofactor NAD(P)+
• GMD reaction is reversible: dehydrogenation with NAD+,
hydrogenation with NADPH+H+
• two steps:
• dehydrogenation of CH-NH2 to imino group C=NH
• hydrolysis of imino group to oxo group and ammonia

31. 31
In transaminations, nitrogen of most
AA is concentrated in glutamate
Glutamate then undergoes
dehydrogenation + deamination
and releases free ammonia NH3

32. Transamination of amino acids
Transamination - transfer of an amino group from an -
amino acid to an -keto acid (usually to -ketoglutarate)
Enzymes: aminotransferases (transaminases).
-amino acid
-keto acid
-keto acid -amino acid

33. There are different transaminases
The most common:
alanine aminotransferase alanine + -ketoglutarate  pyruvate +
glutamate
aspartate aminotransferase
aspartate + -ketoglutarate  oxaloacetate + glutamate
Aminotransferases funnel -amino groups from a variety of
amino acids to -ketoglutarate with glutamate formation
Glutamate can be deaminated with NH4
+ release

34. Mechanism of transamination
All aminotransferases require the
prosthetic group pyridoxal
phosphate (PLP), which is derived
from pyridoxine (vitamin B6).
First step: the amino group of
amino acid is transferred to
pyridoxal phosphate, forming
pyridoxamine phosphate and
releasing ketoacid.
Second step: -ketoglutarate
reacts with pyridoxamine
phosphate forming glutamate
Ping-pong kinetic mechanism

35. Ping-pong kinetic mechanism of aspartate transaminase
aspartate + -ketoglutarate  oxaloacetate + glutamate

36. Decarboxylation – removal of carbon dioxide from amino
acid with formation of amines.
Usually amines have high physiological activity
(hormones, neurotransmitters etc).
amine
Enzyme: decarboxylases
Coenzyme – pyrydoxalphosphate
Decarboxylation of amino acids

37. DECARBOXYLATION OF AMINO
ACIDS
α-decarboxilation
ω-decarboxilation
Decarboxilation with transamination
Decarboxilation with conjugation of two molecules

38. Significance of amino acid decarboxylation
1. Formation of physiologically active compounds
glutamate gamma-aminobutyric acid
(GABA)
histaminehistidine

39. 1) A lot of histamine is formed in inflamatory place;
It has vasodilator action;
Mediator of inflamation, mediator of pain;
Responsible for the allergy development;
Stimulate HCI secretion in stomach. -CO2
2) Tryptophan  Serotonin
Vasokonstrictor
Takes part in regulation of arterial pressure, body
temperature, respiration, kidney filtration, mediator of
nervous system
3) Tyrosine  Dopamine
It is precursor of epinephrine and norepinephrine.
mediator of central nervous system
4) Glutamate  -aminobutyrate (GABA)
Is is ingibitory mediator of central nervous system. In
medicine we use with anticonvulsion purpose (action).

40. 2. Catabolism of amino acids during the decomposition
of proteins
ornithine putrescine
lysine cadaverine
Enzymes of microorganisms (in colon; dead organisms)
decarboxylate amino acids with the formation of diamines.

41. DEAMINATION
• The removal of amino group from the amino
acids as NH3 is deamination.
• Transamination involves only the shuffling of
amino groups among the amino acids.
• On the other hand, deamination results in the
liberation of ammonia for urea synthesis .
• Simultaneously, the carbon skeleton of amino
acids is converted to keto acids.
• Deamination may be either oxidative or non-
oxidative.

42. Oxidative deamination
• Oxidative deamination is the liberation of free
ammonia from the amino group of amino acids
coupled with oxidation. This takes place mostly in liver
and kidney.
• The purpose of oxidative deamination is to provide
NH3 for urea synthesis and α-keto acids for a variety of
reactions, including energy generation.

43. NON-OXIDATIVE DEAMINATION
• Some of the amino acids can be deaminated
to liberate NH3 without undergoing oxidation

45. METABOLISM OF AMMONIA

47. Steps in urea cycle
• 1. Synthesis of carbamoyl phosphate
• 2. Formation of citrulline
• 3. Synthesis of arginosuccinate
• 4. Cleavage of arginosuccinate
• 5. Formation of urea.

48. Overall reaction and energy
• The ureacycle is irreversible and consumes 4
ATP.
• Two ATP are utilized for the synthesis of
carbamoyl phosphate.
• One ATP is converted to AMP and PPi to
produce arginosuccinate which equals to 2
ATP.
• Hence 4 ATP are actually consumed.

49. The first reaction catalysed by carbamoyl phosphate
synthase t (CPS l) is rate limiting reaction of
committed step in urea synthesis.
CPS I is allosterically activated b y N-
acetylglutamate(NAG). lt is synthesized from
glutamate and acetyl CoA by synthase and degraded
by a hydrolase

52. Metabolic disorders of urea cycle

53. • Metabolic defects associated with each of the
• five enzymes of urea cycle have been reported
• All the disorders invariably lead to a build-up in
blood ammonia (hyperammonemia), leading to
toxicity.
• Other metabolites of urea cycle also accumulate
which, however, depends on the specific enzyme
defect.
• The clinical symptoms associated with defect in
urea cycle enzymes include vomiting, lethargy,
Irritability, ataxia and mental retardation.

55. SPECIFIC WAYS OF AMINO ACID
CATABOLISM
The carbon skeletons of 20 fundamental amino acids
are funneled into seven molecules:
 pyruvate,
 acetyl CoA,
 acetoacetyl CoA,
 -ketoglutarate,
 succinyl CoA,
 fumarate,
 oxaloacetate.
After removing of amino group the carbon skeletons of amino
acids are transformed into metabolic intermediates that can be
converted into glucose, fatty acids, ketone bodies or oxidized
by the citric acid cycle.

56. METABOLISM OF INDIVIDUAL AMINOACIDS

57. Fates of carbon
skeleton of
amino acids

58. METABOLISM OF Glycine
• Glycine (Cly, C) is a non-essential, optically
inactive and glycogenic (precursor for glucose)
amino acid.
• Glycine is actively involved in the synthesis of
many specialized products (heme, purines,
creatine etc.) in the body, besides its
incorporation into proteins, synthesis of serine
and glucose and participation in one-carbon
metabolism.
• Glycine is the most abundant amino acid
normally excreted into urine (0.5-l .0 g,/g
creatinine).

61. Synthesis of glycine
• Glycine is synthesized from serine by the
• enzyme serine hydroxymethyl transferase which
• is dependent on tetrahydrofolate(THF).
• Glycine can also be obtained from
1) Threonine, catalysed by threonine aldolase.
2) Glycine synthase can convert a one-carbon unit
(N5, N1o-methylene THF), CO2 and NH3 to
glycine.

63. Synthesis of specialized products

69. Metabolism of Phenylalanine and Tyrosine
• Phenylalanine (Phe,F ) and tyrosine( Tyr ,Y) are
structurally related aromatic amino acids.
• Phenylalanine is an essential amino acid while
tyrosine is non-essential.
• Besides its incorporation into proteins the only
function of Phenylalanine is its conversion to
tyrosine. For this reason, ingestion of tyrosine can
reduce the dietary requirement of phenylalanine.
This phenomenon is referred to as 'sparing
action' of tyrosine on phenylalanine.

71. Conversion of phenyalanine to tyrosine

74. Synthesis of melanin

89. Metabolism of Tryptophan

92. Kynurenine (kynurenine-anthraniIate) pathway
• This pathway mostly occurs in liver leading to oxidation of
tryptophan and the synthesis of NAD+ and NADP+.
• Tryptophan pyrrolase or oxygenase cleaves the five-
membered ring of the indole nucleus to produce formyl
kynurenine.
• Tryptophan pyrrolase is a metallo protein containing an
iron porphyrin ring.
• Formamidase hydrolyses formyl kynurenine and Iiberates
formate which enters the one carbon pool.
• Kynurenine formed in this reaction is a branch point with
different fates.
• In the prominent pathway, kynurenine undergoes NADPH-
dependent hydroxylation to give 3-hydroxykynurenin

93. • Kynureninase, a pyridoxal phosphate( PLP)-
dependent enzyme acts on the 3-hydroxy
kynurenine and splits off alanine.
• Tryptophan is glucogenic, since alanine is a good
precursor for glucose.
• The enzyme kynureninase is very sensitive to
vitamin B6 deficiency.
• Due to the lack of PLP, kynureninase reaction is
blocked and 3-hydroxykynurenine is diverted to
form xanthurenate.
• EIevated excretion of xanthurenate serves as an
indication of B6 deficiency.
• Administration of isoniazid, an antituberculosis
drug-induces B6 deficiency and results in
xanthurenate excretion in urine.

94. • Defects in the activity of kynureninase (in B6
deficiency) cause reduced synthesis of NAD+ and
NADP+ from tryptophan. The symptoms of pellagra-
observed in B6 deficiency-are explained on this basis.

95. SEROTONIN PATHWAY

98. The sulfur containing amino acid

103. Degradation of cysteine

106. BRANCHED CHAIN AMINO ACIDS
• Valine, Ieucine and isoleucine are the
branched chain and essential amino acids.
• These three amino acids initially undergo a
common pathway and then diverge to result
in different end products.
• Based on the products obtained from the
carbon skeleton, the branched chain amino
acids are either glycogenic or ketogenic.

107. • These amino acids serve as an alternate
source of fuel for the brain especially under
conditions of starvation.
• The first three metabolic reactions are
common to the branched chain amino acids.
1. Transamination
2. Oxidative decarboxylation
3. Dehydrogenation

108. 1) Transamination:
• The three amino acids undergo a reversible
transamination to form their respective keto
acids.
2) Oxidative decarboxylation:
• α-Keto acid dehydrogenase is a complex
mitochondrial enzyme.

109. • It is comparable in function to PDH (pyruvate
dehydrogenase complex ) complex &
requires 5 coenzymes – TPP (thiamine
pyrophosphate) , lipoamide, FAD (Flavin
adenine dinucleotide), coenzyme A & NAD+.
• α-Keto acid dehydrogenase catalyses
oxidative decarboxylation of the keto acids
to the corresponding acyl CoA thioesters.
• This is a regulatory enzyme.

110. 3) Dehydrogenation:
• The dehydrogenation is similar to that in
fatty acid oxidation.
• FAD is the coenzyme & there is an
incorporation of a double bond.
• There are two enzymes responsible for
dehydrogenation.
• The branched chain amino acids diverges &
takes independent routes.

111. • Valine is converted to propionyl CoA, a
precursor for glucose.
• Leucine produces acetyl CoA & acetoacetate,
the substrates for fatty acid synthesis.
• Isoleucine is degraded to propionyl CoA &
acetyl CoA.
• Valine is glycogenic & leucine is ketogenic.
• Isoleucine is both glycogenic & ketogenic.

115. Metabolic defects of branched chain
amino acids
• Maple syrup urine disease:
• The urine of the affected individuals smells
like maple syrup or burnt sugar.
• Enzyme defect: Branched chain α-keto acid
dehydrogenase.

116. • This causes a blockade in the conversion of α-
keto acids to the respective acyl CoA
thioesters.
• The plasma & urine concentrations of
branched amino acids & their keto acids are
highly elevated.
• This disease is also known as branched chain
ketonuria.

117. Biochemical complications & symptoms
• Accumulation of branched chain amino acids
causes an impairment in transport & function
of other amino acids.
• Protein biosynthesis is reduced.
• Branched chain amino acids competitively
inhibit glutamate dehydrogenase.
• The disease results in acidosis, lethargy,
convulsions, mental retardation, coma &
death within one year after birth.

118. • Diagnosis:
• Urine contains branched chain keto acids,
valine, leucine & isoleucine.
• Rothera's test is positive.
• Diagnosis depends on enzyme analysis.
• Diagnosis should be done prior to 1 week
after birth.
• Treatment:
• Diet low in branched chain amino acids.

119. lntermittent branched chain ketonuria
• This is a less severe variant form of maple
syrup urine disease.
• Enzyme defect:α-keto acid dehydrogenase.
• There is an impairment & no total blockade
in the conversion of α-keto acids to their
respective acyl CoA thioesters.
• Careful diet planning is adequate.

120. lsovaleric acidemia
• This is a specific inborn error of leucine
metabolism.
• Enzyme defect: Enzyme isovaleryl CoA
dehydrogenase.
• The conversion of isovaleryl CoA to
methylcrotonyl CoA is impaired.

121. • The excretion of isovalerate is high in urine.
• The affected individuals exhibit a 'cheesy'
odor in the breath & body fluids.
• The symptoms include acidosis & mild mental
retardation.

122. Hypervalinemia
• This inborn error is characterized by
increased plasma concentration of valine
while leucine & isoleucine levels remain
normal.
• The transamination of valine alone is
selectively impaired.

123. HISTIDINE, PROLINE AND ARGININE
• The metabolism of histidine, proline and
• arginine is considered together, as all the
three are converted to glutamate.

125. HISTIDINE
• The metabolism of histidine is important for the
generation of one-carbon unit, namely formimino
group.
• The enzyme histidase acts on histidine to split off
ammonia. Urocanate formed in this reaction is acted
upon by urocanase to produce 4-imidazole-5-
propionate.
• Imidazole ring of the product is cleaved by a hydrolase
to give N-formiminoglutamate (FIGLU).
Tetrahydrofolate (THF) takes up the formimino group to
form Ns-formimino THF, and glutamate is liberated.
• Deficiency of folate blocks this reaction and causes
elevated excretion of FIGLU in urine. Histidine loading
test is commonly employed to assess folate deficiency.

126. • Histidine, on decarboxylation, gives the corresponding
amine-histamine. Histamine regulates HCI secretion by
gastric mucosa. Excessive production of histamine
causes asthma and allergic reactions.
• Histidinemia:
• The frequencyo f histidinemiais I in 20,000. lt is
due to a defect in the enzyme histidase.
• Histidinemia is characterized by elevated plasma
histidine levels and increased excretion of
imidazole pyruvate and histidine in urine.
• Most of the patients of histidinemia are mentally
retarded
• and have defect in speech. No treatment will
• improve the condition of the patients.

127. Lysine
• Lysine is an essential amino acid.
• Cereal proteins are deficient in lysine.
• lt does not Participate in transamination
reactions. Some of the lysine residues in protein
structure are present as hydroxylysine,
methyllysine or Acetyllysine.
• These derivatives can be hydrolysed to liberate
free lysine.
• Lysine is a ketogenic amino acid. The
• summary of lysine metabolism is below.

130. Glutamate and glutamine
• glutamate and glutamine are non-essential
• glycogenic amino acids. Both of them play a
• Predominant role in the amino acid metabol
ism, and are directly involved in the final
transfer of amino group for urea synthesis.

134. Aspartate and aspargine

137. Serine

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