Chapter 7 Catabolism of Proteins

1
Chapter 7 Catabolism of Proteins

2
Nutritional Function of Proteins

• Functions
• Structural
• Catalytic,
• Transport action
• Signaling and hormonal functions
• Source of energy (16.7kJ/g)

3
Nutritional Requirement of Proteins

• Nitrogen Balance
• Proteins contain about 16 nitrogen
• Intake N losses N
• Intake N gt Losses N
• Intake N lt Losses N

4
Nutritional Quality of Proteins

• Essential Amino Acids
• cannot be synthesized by the body and must be
  obtained from diet
• Eight nutritional essential amino acids
• Tryptophan
• Phenylalanine
• Lysine
• Threonine
• Valine
• Leucine
• Isoleucine
• methionine

5
Nutritional Quality of Proteins

• Non-essential amino acids
• synthesized in the body
• synthesized by the transamination of
  a-keto acids
• Tyrosine and cysteine
• synthesized in the body by using essential
  amino acids
• from phenylalanine and methionine
  respectively
• semi-essential

6
Digestion of Dietary Proteins

• Dietary proteins are digested in the stomach and
  intestine

7
Digestion of Protein in the Stomach

The digestion of protein. Protein is broken down
into amino acids by the enzymes pepsin (secreted
by the stomach) and trypsin and peptidase (in the
small intestine).
8
Table 1. Phases of Digestion and Absorption of
Protein and its Degradative Products  

9
Gastric Parietal Cell
Lumen of the Stomach
Plasma
CO2
Production of gastric acid and its secretion
10
Dietary Protein
Phase 1- Gastric digestion
Figure 2. Gastric digestion of dietary protein.
Gastric Chief Cells
Pepsinogen
denaturation by stomach acid
hydrolysis by pepsin
autocatalysis
large peptide fragments free amino acids
aa
aa
Pyloric sphincter
aa
Duodenum

• Acid from parietal cells denatures protein to
  be more susceptible to pepsin cleavage.

• Pepsinogen activated to pepsin by
  autoactivation and autocatalysis by pepsin.

• Large peptide fragments/some amino acids pass
  through the pyloric sphincter to the duodenum

11
Phase 2- Digestion by pancreatic proteases
Duodenal Endocrine Cell
Duodenal Endocrine Cell
CCK-PZ
CCK-PZ
Trypsinogen
Blood- stream
(hydrolysis)
Pancreatic Acinar Cell
Mucosal Epithelial Cells
Figure 3. Secretion, activation and action of
pancreatic proteases and brush border
endopeptidases and aminopeptidases
12
Phase 2- Digestion by pancreatic proteases
Duodenal Endocrine Cell CCK-PZ
Duodenal Endocrine Cell CCK-PZ
free amino acids from gastric digestion
Trypsinogen
Secretin
Entero-peptidase (hydrolysis)
Blood- stream
autocatalysis
Trypsin
Pancreatic Acinar Cell
HCO3- neutralizes acid
Mucosal Epithelial Cells
Figure 3. Secretion, activation and action of
pancreatic proteases and brush border
endopeptidases and aminopeptidases
13
Phase 2- Digestion by pancreatic proteases
Duodenal Endocrine Cell CCK-PZ
Duodenal Endocrine Cell CCK-PZ Secretin
free amino acids from gastric digestion
Trypsinogen
Entero-peptidase (hydrolysis)
Blood- stream
autocatalysis
Trypsin
Pancreatic Acinar Cell
Chymotrypsinogen Proelastase Procarboxypeptidases
HCO3- neutralizes acid
Mucosal Epithelial Cells
Figure 3. Secretion, activation and action of
pancreatic proteases and brush border
endopeptidases and aminopeptidases
14
Figure 3. Secretion, activation and action of
pancreatic proteases and brush border
endopeptidases and aminopeptidases
brush border endo-/aminopeptidases hydrolyze
products amino acids, di-/tripeptides absorbed
by epithelial cells
15
. Summary of the gastric and pancreatic
digestive proteases
16

LUMEN OF INTESTINE
Na

Amino acids


Intestinal Epithelium

Phase 4 - Absorption
Brush border

Na
contraluminal membrane
Figure 4. Absorption of amino acids and di- and
tripeptides from the intestinal lumen
17
BRUSH BORDER TRANSPORT SYSTEMS
a) neutral amino acids (uncharged aliphatic and
aromatic) b) basic amino acids and cystine
(Cys-Cys) c) acidic amino acids (Asp, Glu) d)
imino acids (Pro) e) dipeptides and tripeptides
18

Na

Amino acids

LUMEN OF INTESTINE

Phase 4 - Absorption
Intestinal Epithelium


Dipeptides, tripeptides

Brush border
Phase 5

Amino acids
?
?
contraluminal membrane

?
Phase 5
?
?

capillaries
?
Figure 4. Absorption of amino acids and di- and
tripeptides from the intestinal lumen
19
Putrefaction

• Decomposition of amino acids and proteins by
  bacteria
• Most ingested proteins are absorbed from the
  small intestine
• 95 of total dietary proteins
• Undigested proteins
• pass into the large intestine
• Bacterial activity occurs

20
Putrefaction

• Bacteria putrefaction produces some nutritional
  benefits,
• Vitamin K, Vitamin B12, Folic acid
• Toxic for human
• Amines, phenol, indole, H2S

21

• Production of Amines
• Production of phenol
• Production indole
• Production of H2S
• Production of Ammonia
• Page 209

22
Degradation of Protein in Cells
23
The half-life of proteins is determined by rates
of synthesis and degradation
A given protein is synthesized at a constant rate
KS
A constant fraction of active molecules are
destroyed per unit time
KS is the rate constant for protein synthesis
will vary depending on the particular
protein
C is the amount of Protein at any time
KD is the first order rate constant of enzyme
degradation, i.e., the fraction destroyed
per unit time, also depends on the
particular protein
24
Steady-state is achieved when the amount of
protein synthesized per unit time equals the
amount being destroyed
0.693
KDC KS
t 1/2
KD
C
Protein concentration (enzyme activity)
Stop protein synthesis, measure rate of decay
Hours after stopping synthesis
25
Steps in Protein Degradation
Transformation to a
degradable form (Metal oxidized, Ubiquination,
N-terminal residues, PEST sequences)
Lysosomal Digestion
26S Proteasome digestion
7 ? type, 7 ? type subunits
Proteolysis to peptides
KFERQ
8 residue fragments
Ubiquination
N-end rule DRLKF 2-3 min AGMSV gt 20 hr
PEST Rapid degradation
26
Activation of Ubiquitin
Ubiquitin ligase
Ubiquination
Page 211
27
Amino Acid Catabolism

• Deamination of Amino Acids
• removal of the a-amino acids
• Oxidative Deamination
• Non-oxidative Deamination
• Transamination

28
Oxidative Deamination
Only a few amino acids can be deaminated
directly. Glutamate Dehydrogenase catalyzes a
major reaction that effects net removal of N from
the amino acid pool .  Glutamate Dehydrogenase
is one of the few enzymes that can utilize either
NAD or NADP as electron acceptor.  Oxidation at
the a-carbon is followed by hydrolysis, releasing
NH4.

29
At right is summarized the role of transaminases
in funneling amino N to glutamate, which is
deaminated via Glutamate Dehydrogenase, producing
NH4.

30
Non-oxidative Deamination

Serine Dehydratase catalyzes serine à pyruvate
NH4
31
Transamination

Transaminase enzymes (aminotransferases) catalyze
the reversible transfer of an amino group between
two a-keto acids.
32

33

• In another example shown at right, alanine
  becomes pyruvate as the amino group is
  transferred to a-ketoglutarate.

34

• Transaminases equilibrate amino groups among
  available a-keto acids. This permits synthesis of
  non-essential amino acids, using amino groups
  derived from other amino acids and carbon
  skeletons synthesized in the cell. Thus a balance
  of different amino acids is maintained, as
  proteins of varied amino acid contents are
  synthesized. 

35
Mechanism of Transamination

36

• In the "resting" state, the aldehyde group of
  pyridoxal phosphate is in a Schiff base linkage
  to the e-amino group of an enzyme lysine residue.

37

• The a-amino group of a substrate amino acid
  displaces the enzyme lysine, to form a Schiff
  base linkage to PLP.
• The active site lysine extracts a proton,
  promoting tautomerization (shift of the double
  bond), followed by reprotonation with
  hydrolysis. 

38

• What was an amino acid leaves as an a-keto acid.
  The amino group remains on what is now
  pyridoxamine Phosphate (PMP). 
• A different a-keto acid reacts with PMP, and the
  process reverses, to complete the reaction.

39
Purine Nucleotide Cycle

• The activity of L-glutamate dehydrogenase is low
  in the skeletal muscle and heart.
• In this tissues
• purine nucleotide cycle
• Figure 9-7 page 216

40
Metabolism of One Carbon Units

• One carbon units are one carbon containing groups
  produced in catabolism of some amino acids.
• Methyl (-CH3), methylene (CH2), formyl
  (OCH-) and formimino (HNCH-)

41
tetrahydrofolate (FH4)

• One carbon units are carried by tetrahydrofolate
  (FH4), a reduced form of folic acid.

42
tetrahydrofolate (FH4)

• FH4 is formed in reduction of folic acid
  catalyzed by dihydrofolate reductase. The four
  hydrogens are added to the four atoms of folic
  acid in positions 5 to 8. The N5 and N10 nitrogen
  atoms of FH4 participate in the transfer of one
  carbon groups

43
Production of One Carbon Units
Either glycine or serine can act as methylene
donor, giving N5,N10-methyleneTHF. This behaves
as "virtual formaldehyde" H2CO in reactions.
The oxidation level can be changed to methyl or
methenyl by reduction or oxidation methenylTHF
can be hydrolyzed to formylTHF.

44
Production of One Carbon Units from Histidine

• N5-formimino-tetrahydrofolate, produced in the
  pathway for degradation of histidine

45

• In the pathway of histidine degradation,
  conversion of N-formiminoglutamate to glutamate
  involves transfer of the formimino group to
  tetrahydrofolate (THF), yielding N5-formimino-THF.

46
Adenosylmethionine (SAM)

• S-adenosylmethionin (SAM) is the major donor of
  methyl group. FH4 can carry a methyl group on its
  N5 atom, but its transfer potential is too low
  for most biosynthetic methylation.
• The activated methyl donor is SAM, which is
  synthesized by the transfer of an adenosyl group
  from ATP to the sulfer atom of methionine. The
  S-adenosylhomocysteine is formed when the methyl
  group of SAM is transferred to an acceptor.

47
Conversion of One Carbon UnitsFigure 9-13
48
Metabolism of Methionine, Cysteine and Cystine

• Sulfur-containing amino acids
• Methionine is an essential amino acid

49
Methionine cycle and methylation

• In methionine cycle, the adenosyl group of ATP is
  transferred to a sulfur atom of methionine by
  methionine adenosyltransferase to form
  S-adenosylmethionine (Sam)

50
Methionine cycle and methylation

• All phosphates of ATP are lost in this reaction.
  The sulfonium ion of methionine is highly
  reactive and the methyl group of SAM is good
  leaving group. SAM then transfers the methyl
  group to some acceptors for their methylation by
  methyltransferase.

51
Methionine cycle and methylation

• The resulting S-adenosylhomocysteine is cleaved
  by adenosylhomocysteinase to produce homocysteine
  and adenosine.
• Homocysteine accepts a methyl group from
  N5-methyl-FH4 to regenerate methionine.

52
Methionine cycle and methylation

• This reaction is catalyzed by homocysteine
  methyltransferase, which requires vitamin B12 as
  a cofactor. This is the only reaction known that
  uses methyl-FH4 as a methyl group donor.
• The net result of the reaction is donation of a
  methyl group and regeneration of methionine to
  complete the methionine cycle.

53
Methionine cycle and methylation

• Person with elevated serum levels of homocysteine
  have a high risk for coronary heart disease and
  arteriosclerosis. The molecule basis of the
  action of homocysteine has not been clearly
  identified. It appears to damage cells of blood
  vessels and to increase the growth of vascular
  smooth muscle. Treatment with vitamin B12, folic
  acid and vitamin B6 is effective in reducing
  homocysteine level in some people.

54
Creatine and Creatine Phosphate

• Glycine, areginine and methionine participate in
  synthesis of creatine
• Transfer of guanidine group from arginine to
  glycine forms guanidoacetate catalyzed by
  transamidinase in kidney

55
Creatine and Creatine Phosphate

• Synthesis of creatine is completed by methylation
  f guanidoacetate in the liver. This reaction is
  catalyzed by guanidoacetate methyltransferase.
• SAM serves as a donor of a methyl group.
• Storage of high energy phosphate from ATP,
  creatine converts to creatine phosphate
  particularly in cardiac and skeletal muscle
  catalyzed by creatine kinase (CK)

56
Creatine and Creatine Phosphate

• This reaction is reversible and creatine
  phosphate can readily convert ADP to ATP in
  muscle to meet the energy requirement. The amount
  of creatine in the body is related to muscle
  mass.
• Creatinine is derived from dephosphorylation of
  creatine phosphate and also formed by hydrolysis
  of creatine nonenzymatically.

57

• Creatinine has no function and is excreted in
  urine. The amount of creatinine eliminated by an
  individual is constantly from day to day. When a
  24 hours urine sample is requested, the amount of
  creatinine in sample can be used as a gross
  determining test to know renal function.

58
Cysteine and Cystine

• Conversion of Cysteine To Cystine
• two molecules of cysteine are linked by
  a disulfide bond to form cystine. The major
  catabolic pathway of cystine is conversion of
  cysteine catalyzed by cystine reductase. The
  disulfide bond of cystine is important to
  maintain the conformation and function of
  proteins

59
Synthesis of Taurine

• Cysteine is the precusor of taurine. The major
  oxidative metabolite of cysteine is cysteine
  sulfinate, which is further decarboxylation to
  form taurine.
• Taurine is found rich in brain. It appears to
  play role in brain development, but its exact
  role is unknown
• Figure. Page 229

60
Formation 3-phosphoadenosine 5phosphosulfate
(PAPS)

• Sulfate is produced mostly from metabolism of
  cysteine. Catabolism of cysteine produces
  pyruvate, NH3 and H2S. Oxidation of H2S forms
  sulfate. Some sulfate group for addition to
  biomolecules, such as in biosynthesis of
  chondroitin sulfates and keratan sulfate.
• Figure. Page 229

61
Glutathione

• Glutathione is the tripeptide Gamma-glutamylcystei
  nylglycine containing a sulfhydryl group.
  Glutathione has several important role.
• serves as a transporter in the
  gamma-glutamyl cycle for amino acids across cell
  membranes
• protects erythrocytes from oxidative
  damage

62
Glutathione cycles (Meister cycle)figure.9-16

• The enzyme gamma-glutamyl transpeptidase, located
  on the cell membrane of kidneys and other tissue
  cells, catalyzes glutathion (GSH) to transfer its
  glutamyl group to amino acid, then the
  gamma-glutamyl-ammino acid is transported inside
  of the cell.

63
Glutathione cycles (Meister cycle)figure.9-16

• The gamma-glutamyl-amino acid releases amino acid
  and 5-oxiproline. This is the process for amino
  acid transportation into the cell.
• The 5-oxiproline converts to glutamate under the
  action of enzyme and uses ATP.

64
Glutathione cycles (Meister cycle)figure.9-16

• The 5-oxiproline converts to glutamate under the
  action of enzyme and uses ATP.
• Glutamate and the other parts of GSH, glycine and
  cysteine, are regenerated GSH in cytosol and 2
  ATPs are used. So 3 ATPs are required for the
  transportation of each amino acid.
• The key enzyme of the gamma-glutamyl cycle is
  gamma-glutamyl transpeptidase which is found in
  high levels in the kidneys

65
Glutathione cycles (Meister cycle)figure.9-16

• Glutathion cycles between a reduced form with a
  sulfhydryl group (GSH) and an oxidized form
  (GSSG), in which two GSHs are linked by a
  disulfide bond. GSH is reductant, its sulhydryl
  group can be used to reduce peroxides formed
  during oxygen transport.
• Glutathione plays a key role in detoxification by
  acting with hydrogen peroxide and organic
  peroxide.
• Glutathion peroxidase catalyzes this reaction, in
  which GSH converts to GSSG. Then GSSG is reduced
  to GSH by glutathione reductase, an enzyme
  containing NADPH as a cofactor.

66
Metabolism of Aromatic Amino Acids

• Formation of Tyrosine from phenylalanine
• First product in degradation of phenylalanine

67
Metabolism of Aromatic Amino Acids

• Formation of Tyrosine from phenylalanine
• first product in degradation of
  phenylalanine

• Phenylalanine hydroxylase

68
Phenylketonuria (PKU)

• Small amounts of phenylalanine can convert to
  phenylpyruvate by transamination to remove an
  amino group in a healthy person.
• If a genetic deficiency of phenylalanine
  hydroxylase occurs, phenylketonuria is caused

Phenylalanine hydroxylase
69
Phenylketonuria (PKU)

• PKU is the most common autosomal disease. Over
  170 mutations in the gene have been reported. The
  elevated phenylpyruvate, phenyllacetate
  (reduction product of phenylpyruvate) and
  phenylacetate (decarboxylation of phenlpyruvate)
  excreted in urine give urine its characteristic
  odor. The neurological symptoms and light color
  of skin and eyes are generally toxic effects of
  high levels of phenylpyruvate and low
  concentrations of tyrosine. The conventional
  treatment is to feed the effected infant a diet
  low in phenylalanine with dietary protein
  restrictions.
• Figure 9-17 Metabolism and major derivatives of
  phenylalanine and tyrosine

70
Metabolism of Tyrosine

• The first step in catabolism of tyrosine is
  transamination catalyzed by tyrosine transaminase
  to produce p-hydroxyphenylpyruvate, which
  converts to homogentisate by oxidase.
  Homogentisate is then cleaved to fumarate and
  acetoacetate. Fumarate is used in the TCA cycle
  for energy or for gluconeogenesis. Acetoacetate
  can convert to acetyl CoA for lipid synthesis or
  energy.

71
Production of Dopamine, Epinephrine and
Norepinephrine

• Some tyrosine is used as a precursor of
  catecholamines (term of dopamine, epinephrine and
  norepinephrine)
• The first step in the synthesis of catecholamines
  is catalyzed by tyrosine hydroxylase, which is an
  enzyme dependent on tetrahydrobiopterin.

72

• The product of this reaction is
  dihydroxyphenylalanin, known as Dopa. A product
  of decarboxylation of Dopa is dopamine, which is
  a neurotransmiter. Parkinsons disease is induced
  by decreasing production of dopamin.
• The adrenal medulla converts dopamine to
  norepinephrine by dopamine hydroxylase, which
  accepts a methyl group from S-adenosylmethionine
  to form epinephrine.

73
Synthesis of MelaninFigure 9-17

• Tyrosine is precursor of melanin. Dopa is the
  intermediate in the synthesis of both melanin and
  epinephrine.
• Different enzymes dydroxylate tyrosines in
  melanocytes and other cell type. In pigment
  cell, tyrosine is hydroxylated to form Dopa by
  tyrosinase, a copper-containing enzyme.
• Dopa forms dopamine then converts it to
  indo-5-6-quinone. Melanin is polymers of these
  tyrosine catabolites with proteins from the eyes
  and skin. There are various types of melanin,
  which are all aromatic quintines complexes giving
  color, colorless, yellow and dark to the skin.

74
Albinism

• Albinism results from a genetic lack of
  tyrosinase. Lack of pigment in the skin makes a
  patient sensitive to sunlight and increases the
  incidence of skin cancer in addition to burns.
  Lack of pigment in eyes may induce photophobia

75
Production of Thyroid Hormone

tetraiodothyronine, T4
triiodothyronine,T3.

• Tyrosine is the precursor of the thyroid hormone
  T4 and T3. The thyroid hormone has importance in
  regulating the general metabolism, development
  and tissue differentiation. Iodination of
  tyrosine residues in thyroglobulin forms T4 and T3

76
Metabolism of TryptophanFigure 9-18
77
Metabolism of TryptophanFigure 9-18

• Trytophan
• precursor of nicotinic acid, one of the B
  vitamins.
• b hydroxylation and decarboxylation forms
  5-hydroxytryptamine (5-HT, serotonin)
• Melatonin is a derivative of
  tryptophan, N-acetyl-5-methoxytryptamine. It is a
  sleep-inducing molecule and is synthesized in the
  pineal gland and retina mostly at night.
  Melatonin appears to function by inhibiting
  synthesis and secretion of other
  neurotransmitters, such as dopamine and GABA.

78
Degradation of Branched-Chain Amino Acids
(BCAAs)Figure 9-19

• Valine, isoleucine and leucine are branched-chain
  amino acids (BCCAs).
• BCAAs transaminases are present at a much higher
  level in muscle than that in liver

79

• Valine converts to succinyl CoA. So it is a
  glucogenic amino acid. Leucine converts to acetyl
  CoA and acetoacetate. Leucine is a ketogenic
  amino acid. Isoleucine produces acetyl CoA and
  succinyl CoA and is both glycogenic and ketogenic
  amino acid. All these intermediates of BCAAs
  degradation are oxidation in the TCA cycle to
  support energy in muscle.

80
Transport of Ammonia in Blood

• At physiological pH, 98.5 exists as ammonium ion
  (NH4)
• Only traces of NH3 are present
• Even trace of NH3 are toxic to the nervous system
• NH3 is rapidly removed

81
Glutamine synthetase fixes ammonia as glutamine

• Formation of glutamine is catalyzed by glutamine
  synthetase. Synthesis of the amide bond of
  glutamine is accomplished at the expense of
  hydrolysis of one mole of ATP to ADP and Pi.
  
• Glutamine Synthetase

82

• Hydrolysis of glutamine produces glutamate and
  NH3 in the liver and kidneys

83

• Glutamine supports an amide group for synthesis
  of asparagine from aspartate by asparagine
  synthetase. Since certain tumors such as leukemic
  cells seem to lose this ability and exhibit
  abnormally high requirements for asparagine and
  glutamine, hydrolysis of asparagine is catalyzed
  by asparaginase. So, exogenous asparaginase and
  glutaminase had been tested as antitumor agents

84
Alanine-glucose cycleFigure 9-8

• Muscles generate over half of the total
  metabolism pool of amino acids. The ammonia
  produced in catabolism of amino acids in muscle
  is accepted by pyruvate to form alanine, which is
  released into the blood.
• Alanine appears to be the vehicle of ammonia for
  transport in the blood
• The liver takes up the alanine and converts it
  back into pyruvate by transamination
• The resulting pyruvate can be converted to
  glucose by the gluconeogenesis pathway and an
  amino group eventually appears as urea.
• Glucose formed in gluconeogenesis is released
  into the blood and taken up by muscles.
• Glycolysis of glucose produces pyruvate, which is
  then resynthesized alanine. This is called
  alanine-glucose cycle

85
Formation of Urea (Urea Cycle)

86
Urea Cycle

• The urea cycle takes place partly in the cytosol
  and partly in the mitochondria, and the
  individual reactions are as follows

87
Urea Cycle

• carbamyl phosphate synthetase 1 CPS1
• This liver mitochondrial enzyme converts the
  ammonia produced by glutamate dehydrogenase into
  carbamyl phosphate (carbamoyl phosphate) which
  is an unstable high energy compound. It is the
  mixed acid anhydride of carbamic acid and
  phosphoric acid, and requires two molecules of
  ATP to drive its synthesis.

88
Urea Cycle

CPS1 is an allosteric enzyme and is absolutely
dependent up on N-acetylglutamic acid for it
activity
89
Urea Cycle

• CPS1 deficiency results in hyperammonemia. The
  neonatal cases are usually lethal, but there is
  also a less severe, delayed-onset form.
  Ammonia-dependent CPS1 is present only in the
  liver mitochondrial matrix space. It should be
  distinguished from a second cytosolic
  glutamine-dependent carbamyl phosphate synthetase
  CPS2 which is found in all tissues and is
  involved in pyrimidine biosynthesis. Carbamyl
  phosphate synthesis is a major burden for liver
  mitochondria. This enzyme accounts for about 20
  of the total protein in the matrix space.
  Glutamate dehydrogenase is also present in very
  large amounts.

90
Urea Cycle

• The next reaction also takes place in the liver
  mitochondrial matrix space, where ornithine is
  converted into citrulline

ornithine transcarbamylase OTCase

91
Urea Cycle

• Citrulline is transported out of the mitochondria
  into cytosol by the mitochondrial inner membrane
  transport system. Once in the cytosol, citrulline
  condenses with aspartate and the reaction is
  driven by ATP. In this way aspartate contributes
  the second nitrogen atom to urea, the first
  having come from glutamate

92
Urea Cycle

• Production of arginino-succinate is an
  energetically expensive process, since the ATP is
  split to AMP and pyrophosphate. The pyrophosphate
  is then cleaved to inorganic phosphate using
  pyrophosphatase, so the overall reaction costs
  two equivalents of high energy phosphate per
  mole.

93
Urea Cycle

• Elimination of fumarate from
  arginino-succinate then yields arginine.
• arginino-succinate lyase

94
Urea Cycle

• Fumarate can be converted into oxaloacetate
  under catalysis of some enzymes as in the TCA
  cycle. Oxaloacetate can be converted to aspartate
  by transamination. The aspartate is then
  reutilized in the urea cycle

95
Urea Cycle

• Cleavage of arginine by arginase to produce
  urea regenerates ornithine, which is then
  available for another round of the cycle.

96
Urea Cycle

• Since humans can not metabolize urea, it is
  transported to the kidneys for excretion. Some
  urea that enters the intestinal tract is cleaved
  by bacteria urease, the resulting ammonia being
  absorbed and treated by the liver

97

• Note that of the two nitrogen atoms of
  urea, one comes from carbamoyl phosphate, being
  ultimately derived from ammonia. The other
  nitrogen is derived from the a-amino group of
  aspartate which in turn is obtained from
  transamination of oxaloacetate with glutamate.
  The formation of one molecule of urea requires
  the hydrolysis of four high-energy phosphate
  groups from 3 molecules of ATP
• The overall reaction is as follows
• 2NH3 CO2 3ATP 3H2O -gt H2N-CO-NH2 2ADP AMP
  4Pi

98
Urea Cycle (review)

• 1. Occurs in the liver mitochondria and cytosol

2. Starts with carbamoyl-PO4
3. Ends with arginine
4. Requires aspartate
5. Requires 3 ATPs to make one urea
99
Synthesis of Carbamoyl-PO4
NH4 HCO3- 2 ATP
2 ADP Pi
Carbamoyl phosphate Synthetase I
100
Citrulline
Aspartate
Carbamoyl-PO4
ATP
Urea Cycle
Ornithine
Argininosuccinate
Arginine
H2O
C
Urea
101
Reactions of Urea Cycle
Cytosol
Mitochondria
102
Cytosol

Fumarate
L-Arginine
L-Malate
Oxaloacetate
L-Aspartate
103
COO-
H3N-C-H
CH2
H2O
CH2

CH2
NH3


Urea
Ornithine
L-Arginine
Return to Mitochondria
104
Nitric Oxide

• Arginine also serves as a direct precursor of
  nitric oxide (NO). The free-radical gas NO is the
  potent muscle relaxant and short-lived signal
  molecule. Nitric oxide is formed by the catalysis
  of the cytosol enzyme nitric oxide synthase
  (NOS), which is a very complex enzyme with five
  cofactors NADPH, FAD, FMN, heme and
  tetrahydrobiopterin.
• The substrate in the reaction is arginine and
  products are citrulline and NO. Oxygen is
  required in the complex reaction. NO plays an
  important role in many physiologic and pathologic
  processes

105
Decarboxylation of Amino Acids

• Decarboxylation of amino acids forms amine.
  This reaction is catalyzed by decarboxylase,
  which contains pyridoxal phosphate as a cofactor.
  Amines always have potential physiological
  effects.

106
GABA

• gamma-Aminobutyric acid (GABA) is formed by
  pyridoxal phosphate-dependent enzyme, L-glutamate
  decarboxylase, which is principally present in
  brain tissue. GABA functions as inhibitory
  neurotransmitter. GABA, catalyzed by
  gamma-aminobutyrate transaminase, forms succinate
  and semialdehyde, which may be oxidized to form
  succinate and via TCA cycle to form CO2 and H2O

107
Histamine

• Decarboxylation of histidine forms histamine,
  a reaction catalyzed by histidine decarboxylase.
  Histamine has many physiological roles, including
  vasodilation and constriction of certain blood
  vessels. An overreaction of histamine can lead to
  bronchial asthma and other allergic reactions. In
  addition, histamine stimulates secretion of both
  pepsin and hydrochloric acid by the stomach, and
  is useful in the study of gastric activity

108
Serotonin

• 5-hydroxytryptamine (5-HT), also known as
  serotonin, results from hydroxylation of
  tryptophan by a tetrahydrobiopterin-dependent
  enzyme, hydroxylase and decarboxylation by a
  pyridoxal phosphate-containing decarboxylase.
  5-HT is a neurotransmitter in the brain and
  causes contraction of smooth muscle of arterioles
  and bronchioles.

109
polyaminesFigure 9-12

• Polyamines are important in cell
  proliferation and tissue growth. They are growth
  factors for cultured mammalian cells and
  bacteria. Since polyamines bear multiple positive
  charges that can interact with polyanions such as
  DNA and RNA, and thus can stimulate synthesis of
  nucleic acid and protein.