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Protein metabolism
BY
Dr. NAGLAA IBRAHIM AZAB
Lecturer Of Medical Biochemistry
&Molecular Biology
BENHA FACULTY OF MEDICINE
Protein structure
α- a.a ---- α- a.a ---- α- a.a ---- α- a.a ---------- α- a.a
1
2
3
4
n
Peptide linkages
α- amino acid
R—CH—COOH
NH2
R1
R2
Rn
NH2 -- CH—CO---- NH-- CH—CO----------CO---- NH- CH—COOH
Amino terminus
peptide linkages
carboxyl terminus
Classification of amino acids
• Structural classification : according to
the chemical structure of the side chain
(R)
• Nutritional classification
•
•
(essensial & non essential a.as)
• Metabolic classification : according to
the fate of amino acids inside the body
(glucogenic,ketogenic and mixed a.as)
•
Classification
According to
R
Aliphatic a.as
No ring structure
Aromatic a.as
contain Phenyl ring
Heterocyclic a.as
Contain heterocyclic
ring
Aliphatic a.as
Neutral a.as
Amidic a.as
Contain
Contain
One NH2
one COOH
One NH2
one COOH
One NH2CO(amidic group)
Acidic a.as
Basic a.as
Contain
Contain
One NH2
two COOH
>One NH2
one COOH
Aliphatic neutral a.as
WITH HYDROCARBON SIDE CHAIN
-------------------------------------------------1-Glycine(2C)
CH2–
COOH
2-Alanine(3C)
3-Valine(5C)
4-Leucine(6C)
5-Isoleucine(6C)
I
NH2
CH3–CH–COOH
I
NH2
CH3–CH–CH–COOH
I
I
CH3NH2
CH3–CH–CH2–CH–COOH
I
I
CH3
NH2
CH3–CH2–CH–CH–COOH
I
I
CH3 NH2
WITH HYDROXYL (OH ) CONTAINING SIDE CHAIN
-----------------------------------------------------------------------
1-Serine(3C)
CH2–CH–COOH
I
I
OH
2-Homoserine(4C)
NH2
CH2–CH2–CH–COOH
I
I
OH
3-Threonine(4C)
NH2
CH3–CH–CH–COOH
I
I
OH NH2
WITH SULFUR (S) CONTAINING SIDE CHAIN
--------------------------------------------------------------1-Cysteine(3C)
2-Homocysteine(4C)
3-Methionine(4C)
4-Cystine
CH2–CH–COOH
I
I
SH NH2
CH2–CH2–CH–COOH
I
SH
CH2–CH2–CH–COOH
I
I
NH2
S-CH3
CH2–CH–COOH
I
S
NH2
S
CH2–CH–COOH
I
NH2
Aliphatic
acidic a.as
Aspartic acid (4C)
HOOC–CH2–CH–COOH
I
NH2
Aliphatic
Amidic a.as
Aspargine (4C)
NH2–OC–CH2–CH–
I
COOH
NH2
Glutamic acid (5C)
Glutamine (5C)
HOOC–CH2–CH2–CH–COOH
I
NH2
NH2–OC–CH2–CH2–CH–COOH
I
NH2
Aliphatic Basic a.as
1-Lysine (6C)
CH2–CH2–CH2–CH2–CH–COOH
I
I
NH2
NH2
2-Arginine (5C+1)
Guanido group
CH2–CH2–CH2–CH–COOH
I
NH
NH2
I
NH =C─ NH2
3-Ornithine (5C)
4-Citrulline (5C+1)
Ureido group
CH2–CH2–CH2–CH–COOH
I
I
NH2
NH2
CH2–CH2–CH2–CH–COOH
I
NH
NH2
I
O= C─ NH2
Ornithine and citrulline are not found in proteins but are formed in urea cycle
Aromatic a.as
Phenylalanine
CH2–CH–COOH
I
NH2
Tyrosine = Parahydroxy phenylalanine
CH2–CH–COOH
I
NH2
OH
Heterocyclic a.as
Tryptophan
CH2–CH–COOH
I
NH2
N
H
Histidine
N
NH
CH2–CH–COOH
I
NH2
Proline (imino acid)
N
H
COOH
.
Proteins
Parietal cells
Gastric HCL
1st
Chief cells
HOW?
Pepsinogen
Denatured proteins
Pepsin
Inactive zymogen
Active enzyme
then
Endopeptidase with broad
Specificity (cleaves the
internal peptide bonds in
which the carboxylic group
is of aromatic a.as or leucine)
Rennin in infants
Milk
caseinogen
Soluble casein
Denaturation
Further digested by pepsin
and other proteases
Ca
Ca paracaseinate
(milk clot)
Importance?
Large peptides + some free a.as
Large peptides + some free a.as
Pancreatic endopeptidases
including
Intestinal cells
*Trypsinogen
(inactive)
enteropeptidase
*Chymotrypsinogen
(inactive)
*Proelastase
(inactive)
*Collagenase
Trypsin
(active)
Cleaves peptide bonds in which the COOH
is of Basic a.as (arginine and lysine)
Chymotrypsin
(active)
Cleaves peptide bonds in which the COOH
is of Aromatic a.as or leucine.
Elastase
(active)
Cleaves peptide bonds in which the COOH
is of small non polar a.as (a.as with small
uncharged R as glycine, alanine & serine)
Catalyzes the hydrolysis of collagen
Smaller peptides + free a.as
Exopeptidases
Smaller peptides + free a.as
Pancreatic carboxypeptidases
*Procarboxypeptidase A
(inactive)
including
Carboxy peptidase A
(active)
Cleaves the aromatic a.as from the
C- terminal end of the peptide
Carboxy peptidase B
(active)
Cleaves the basic a.as from the
C- terminal end of the peptide
Trypsin
*Procarboxypeptidase B
(inactive)
Intestinal proteases
*Aminopeptidases
Cleaves one a.a from the
N- terminal end of the peptide
*Dipeptidases and tripeptidases
Cleaves a.as from the dipeptides
or tripeptides
a.as
In infants, Ig A in the clostrum of milk is
absorbed without digestion by
pinocytosis
giving immunity to the babies
Intestinal cell
Intestinal lumen
IgA
IgA
[gA
[gA
Blood
[gA
IgA
IgA
Aa.s resulting from protein digestion
are absorbed from the small intestine by:
■ passive transport mechanism (For D-aas).
■ Active transport mechanism (For L-aas and
dipeptides):
► Carrier protein transport system
( sodium – amino acid carrier system ).
► Glutathione transport system
(γ-glutamyl cycle)
Carrier protein transport system
( sodium – amino acid carrier system )
Intestinal lumen
K+
Na+
ADP+Pi
Cell membrane
Na+
a.a.
a.a.
Na/K ATPase
Carrier
ATP
Intestinal cell
K+
Na+
a.a.
Cytoplasm
Portal blood
a.a.
*This system transport the a.a. against its conc.
gradiant using energy derived from Na/K+ pump.
*Here a.as are absorbed by specific carrier protein
in the cell membrane of the small intestinal
cells.This carrier protein has one site for the a.a.
and another site for the Na+. It transports them
from the intestinal lumen across the cell
membrane to the cytoplasm. Then the a.a.
passes to the blood down its conc. Gradient,
while the Na+ is pumped out from the cell to the
intestinal lumen by Na/K+ pump utilizing ATP as a
source of energy derived from Na/K+ pump.
Glutathione transport system
(γ-glutamyl cycle)
R
NH2—CH—COOHa.a.
γ-glutamyl transpeptidase(transferase) (GGT)
Cell membrane
γ-glutamyl- cysteinylglycine
CO-NH-CH-COOH
CH2
R
CH2
cysteinylglycine
CH-NH2
ADP+Pi
glutathione
synthetase
ATP
dipeptidase
COOH
γ-glutamyl a.a.
glycine
γ-glutamyl- cysteine
ADP+Pi
γ-Glutamyl
cyclotransferase
γ-Glutamylcysteine
synthetase
cysteine
a.a.
5-oxoproline
ATP
Glutamic acid
Oxoprolinase
H2C
CH2
H2O
O=C
ADP+Pi
ATP
CH-COOH
N
I
H
►This transport system is for:
the transport of a.a.s from the extracellular space to the
cytoplasm in the intestine,kidney, brain & liver(bile ductule cells)
So it is not important only for the uptake of a.as from the
intestinal lumen (a.a. absorption) .
► 3 ATP molecules are utilized for the transfer of one
a.a.
►Clinical notes:
■The blood conc. of GGT enzyme is increased in
cholestasis & chronic alcholism ( so used as a liver
function test).
■Oxoprolinuria: Inherited deficiency of glutathione
synthetase enzyme , leading to increase levels of 5
oxoproline in blood &urine
acidosis &
neurological damage
Quiz
The basis of allergic reactions to
food
Fate of absorbed a.as
Enter in the formation of a.a. pool
► Definition: It include the free a.as distributed throughout the body
- The a.a. pool contains 100 gm a.as.50% of these a.as are in the form of
glutamate & glutamine (Why?)
- In contrast to the amount of protein in the body (about 12 Kg in 70 Kg man),
the a.a. pool is small (only 100 gm)
►Sources & fate of the
Diatery proteins
a.a. pool:
Non essential a.a.s
synthesized in the body
Tissue proteins
a.a.s
Amino acid pool
Anabolism
Catabolism(Deamination)
Synthesis of
Fate of deamination products
Proteins
Other nitrogenous compounds
►Tissue proteins
►Plasma proteins
►Enzymes
►Some hormones
►Milk
►Aminosugars
►Nitrogenous bases
of phospholipids
►Purines & pyrimidines
►Neurotransmitters
►Niacin
►Creatine
►Heme
α- keto acid
Ammonia
Krebs
cycle
glucose
Ketone bodies CO2+H2O
+ENERGY
Non essential
a.a.synthesis
Synthetic
pathway
Urea
Catabolic
pathway
glutamine
Excreted
in urine
Catabolism of the a.a.s occurs by
Deamination (removal of the amino group)
either by
General methods of deamination:
►Oxidative deamination
►Transamination
►Transdeamination
Or
Specific methods of deamination:
For certain a.a.s
Oxidative deamination
► Definition:
It is the
oxidation
(removal of hydrogen)
and deamination (removal of the amino group
which is liberated as free ammonia)
giving
α- ketoacid and ammonia (reversible reactions)
R— CH — COOH
l
NH2
► Site: In most tissues , mostly in the liver
and kidney
► Enzymes involved:
1- L-glutamate dehydrogenase enzyme:
Present in the cytosol & mitochondria of most tissues
NAD(P)
NAD(P)H+H
H2O
NH3
HOOC–CH2–CH2–CH–COOH ◄
►
l
NH2
L-GLUTAMATE
DEHYDROGENASE ENZYME
HOOC–CH2–CH2–C –COOH
ll
O
α- ketoglutaric acid
L-glutamic acid
Coenzyme is NAD or NADP
Regulation: The direction of the reaction depends on:
1- Availability of the substrates:
--Relative conc. Of (α-ketoglutarate &NH3) and (glutamate).
--Ratio of NADP : NADPH+H
2- Allosteric regulation:
--Activators : ADP or GDP.
-- Inhibitors : ATP ,GTP & NADH
QUIZ
---After a protein meal , in which direction
the reaction proceeds? Why?
---After a CHO meal ,
in which direction the
reaction proceeds?
Why?
2- D- & L- a.a. oxidases :
Present only in the liver and kidney in minimal amounts
They are of low activity in the mammalian tissue
N.B. a.a.s
L-a.as : mammalian proteins are formed of only L-a.as
D-a.as: are found in plants and the cell wall of
microorganisms but not used in the synthesis of mammalian proteins
L-a.a. oxidase : It deaminates most of the naturally occuring a.a.s.
H2O
R— CH — COOH
L-a.a. oxidase
l
NH3
R— C — COOH
ll
NH2
O
FMN
L- amino acid
FMNH2
H2O2
H2O2
Catalase
2 H2O + O2
O2
α- keto acid
D-a.a. oxidase :
It deaminates D-a.as present in diet
giving α- keto acids
that either transaminated to the coressponding L-a.as
or converted to glucose or F.as
or catabolized to CO2 + H2O + energy
H2O
R— CH — COOH
NH3
D-a.a. oxidase
R— C — COOH
l
ll
NH2
O
D- amino acid
H2O2
FAD
FADH2
H2O2
O2
Catalase
2 H2O + O2
α- keto acid
L- amino acid
α- keto acid
R— CH — COOH
l
NH2
L- amino acid
Transaminase
■Glucose
■f.as
■CO2+H2O
►Importance of oxidative deamination
L-glutamate dehydrogenase enzyme is the only a.a.
that undergoes oxidative deamination in the
mammalian tissue.
Oxidative deamination by L- glutamate dehydrogenase
is an essential component of transdeamination.
So it is important in deamination of most a.as.
Transamination
►Definition: It is the transfer of amino group
from one α- a.a. to α- keto acid
to form a new α- a.a & a new α- keto acid
(reversible reaction)
► Enzymes involved:
Transaminases or aminotransferases
Coenzyme: PLP (Pyridoxal phosphate)
R— C H — COOH
l
NH2
◄
►
R— C — COOH
ll
O
Transaminase
PLP
R— C — COOH
ll
O
◄
►R— C H
l
NH2
— COOH
►Site: In the cytosol or both the cytosol & the
mitochondria of most cells especially the
liver
► All a.as except
threonine,lysine,proline&hydroxy proline
may undergo transamination
► α-ketoglutarate & glutamate are often involved in
transamination reactions
α- a.a.
◄
►
α- keto glutarate
Glutamate transaminase
α- keto acid
◄
►
Glutamic acid
► Transaminases of clinical importance are:
■ Alanine transaminase(ALT) =
{ Glutamate pyruvate transaminase(GPT)}:
Glutamic acid + Pyruvic acid
ALT
PLP
CH3— C — COOH
ll
O
Alanine + α- keto glutarate
CH3— C H — COOH
l
NH2
■ Aspartate transaminase(AST)=
{ Glutamate oxaloacetate transaminase(GOT)}:
ALT
Glutamic acid + Oxaloacetic acid
HOOC–CH2–C–COOH
ll
O
PLP
Aspartic acid + α- keto glutarate
HOOC–CH2–CH–COOH
l
NH2
►Value of transamination:
■Function:
1- Degradation of a.as to form α- keto acids.
2- Synthesis of non essential a.as from
CHO.
■Diagnostic value:
Transaminases are normally intracellular enzymes.
They are elevated in the blood when damage to the
cells producing these enzymes occurs.
** Increase level of both ALT & AST
indicates
possible damage to the liver cells.
** Increase level of AST alone
suggest damage to
heart muscle ,skeletal muscle or kidney.
Transdeamination
►Definition:
It is the combination of transamination & oxidative deamination.
It includes the transamination of most a.as with α– keto
glutarate to form glutamate then the glutamate is oxidatively
deaminated reforming α– keto glutarate and giving ammonia.
This provides a pathway by which the amino group of most
a.as is released in the form of ammonia.
α- a.a. ◄
► α- keto glutarate ◄
L-GLUTAMATE
DEHYDROGENASE ENZYME
Glutamate transaminase
α- keto acid ◄
►
Glutamic acid
◄
► NH3
►NAD(P)H+H
NAD(P)
► H2O
On biochemical basis explain
■Removal of ammonia from a.as can not be explained alone
by transamination nor by oxidative deamination alone
It can not be explained by transamination alone as no free
ammonia is liberated nor by oxidative deamination alone
as oxid. Deamination works efficiently only on glutamic
acid as L- glutamate dehydrogenase is of high activity in
the mammalian tissue,while the L- amino acid oxidase
which works on most a.as is of low activity.
■The formation of NH3 from α– a.as occurs mainly via the α–
amino nitrogen of glutamate
As this occurs by transdeamination, and L- glutamate is the
only a.a that undergoes oxidative deamination at an
appreciable rate in the mammalian tissue.
Another importance of transdeamination is we
can form α– a.a. from ammonia
α- a.a. ◄
► α- keto glutarate ◄
L-GLUTAMATE
DEHYDROGENASE ENZYME
Glutamate transaminase
α- keto acid ◄
►
Glutamic acid
◄
► NH3
►NAD(P)H+H
NAD(P)
► H2O
Specific methods of deamination
1- L- glutamate dehydrogenase:
Said before
2- Glycine oxidase:
as the mechanism of action of D- amino acid oxidase
H2O
Glycine oxidase
CH2 — COOH
NH3
CH — COOH
l
ll
NH2
O
FAD
FADH2
Glycine
Glyoxylic acid
H2O2
H2O2
Catalase
2 H2O + O2
O2
3- Glycine cleavage system:
NAD
CH2 – COOH
|
NH2
Glycine
+ FH4
Tetrahydrofolic
acid
NADH+H
CH2─FH4 + NH3 + CO2
Methylene
tetrahydrofolic
acid
4- Histidase: ( Non oxidative deamination)
N
– CH2 – CH – COOH
|
NH
NH2
Histidine
Histidase
– CH = CH – COOH
NH3 N
NH
Urocanic acid
5-Dehydratases: (Non oxidative deamination)
For hydroxy containing a.as (serine& threonine)
CH2–CH–COOH
l
l
OH NH2
Serine
Serine
dehydratase
PLP
H2O
CH2=C–COOH
l
NH2
α–aminoacrylic acid
CH3–C–COOH
ll
NH
α–iminopropionic acid
H2O
Serine
dehydratase
NH3
CH3–C–COOH
ll
O
Pyruvic acid
CH3–CH–CH–COOH
l
l
OH NH2
Threonine
Threonine
dehydratase
PLP
NH3
CH3–CH2–C–COOH
ll
O
α–ketobutyric acid
6-Desulfhydrases:
The a.a. cysteine is deaminated by cysteine desulfhydrase
CH2–CH–COOH
l
l
SH NH2
Cysteine
Cysteine
desulfhydrase
PLP
H2S
CH2=C–COOH
l
NH2
α–aminoacrylic acid
CH3–C–COOH
ll
NH
α–iminopropionic acid\
H2O
NH3
Cysteine
desulfhydrase
CH3–C–COOH
ll
O
Pyruvic acid
7-Hydrolytic deaminases: Deamination by H2O
Glutaminase & asparginase which catalyze the
hydrolytic deamination of glutamine & aspargine
respectively.
H2O
OC–CH2–CH2–CH–COOH
l
l
NH2
NH2
Glutamine
NH3
Glutaminase
(in kidney, intestine)
HOOC–CH2–CH2–CH–COOH
l
NH2
Glutamic acid
8-Reductive deaminases:
By the action of intestinal bacteria on the a.as
(putrefaction) with the production of the
corresponding organic acids.
R— C H — COOH
l
NH2
α- a.a.
2H
NH3
R— C H2 — COOH
Corresponding organic acid
Deamination products
NH3
α- keto acid
Blood level:
Normally < 0.1 mg / dl
Urine level: 0.7 gm / day
Sources & fate:
Various nitrogenous compounds
as
Amino acids
Purines& pyrimidines
Deamination:
--Oxidative deamination
--Transdeamination
--Specific deamination
Methods
(amino groups attached
to the rings)
Catabolism
( MAIN SOURCE)
Some
neurotransmitters
as
Urea secreted into
the intestine
Monoamines:
--serotonin
Histamine
–epinephrine,norepinephrine,
dopamine & their metabolites
metanephrine, normetanephrine Histaminase
&3 methoxy tyramine
Intestinal bacterial
urease
Monoamine oxidase
(MAO)
NH3
Synthetic
pathway
Catabolic
pathway
Liver
Transdeamination
Non essential
a.a.synthesis
(MAIN FATE)
Urea
Urine
glutamine
Excreted
in urine
►Urea formation: is the main pathway by which the body gets rid of
NH3
►Glutamine formation:
--By the glutamine synthetase enzyme which is a mitochondrial enzyme
Glutamic acid + ammonia
glutamine synthetase
glutamine
-- Glutamine is produced in many extra renal tissues esp. important:
•In the muscle
•In the liver:The formation of glutamine can be considered as a
mechanism for scavenging NH3 that has not been incorporated into
urea.
•In the brain: It removes the toxic effect of NH3 in the brain .Then the
glutamine goes via the blood to the kidneys where it become
hydrolyzed by glutaminase into glutamic acid and NH3 which is
excreted in urine,(This accounts for 60% of the NH3 excreted in urine)
Glutamine
glutaminase
H2O
NH3
Glutamic acid
►NH3 produced from a.a. deamination in the kidney is directly
excreted in urine ( This acconts for 40%of NH3 excreted in urine)
■N.B. NH3 produced from a.a deamination in the kidney esp.
glutamine regulates acid base balance &preserve cations
Steps
CO2
NH3
Cytoplasm
Mitochondria
CO2
Carbamoyl-phosphate synthetase 1
+
NH3
2ATP
2 ADP+1 Pi
O
O
ll
ll
H2N – C – O – P – OH Carbamoyl-phosphate
l
Pi
OH
CH2–CH2–CH2–CH–COOH
l
l
Ornithine
NH2
NH2
O
ll
H2N–C–NH2
Ornithine
Urea
Arginase
CH2–CH2–CH2–CH–COOH
H2O
l
l
NH
NH2
Arginine
l
NH2–C=NH
HOOC–C–H
Fumarate
ll
H–C–COOH
Ornithine
transcarbamoylase
Citrulline
Citrulline
Arginino-succinate
synthetase
CH2–CH2–CH2–CH–COOH
l
l
NH
NH2
l
O=C–NH2
HOOC–CH2–CH–COOH
l
NH2
Aspartic acid
ATP
AMP+PPi
H2O
CH2–CH2–CH2–CH–COOH
l
l
Arginino succinate
NH
NH2
l
NH–C=NH
Arginino succinase
l
HOOC–CH2–CH–COOH
Carbamoyl- phosphate synthetase I is different from
carbamoyl phosphate synthetase II
CPS I
CPS II
Mitochondria
Cytoplasm
Pathway
Urea synthesis
Pyrimidine synthesis
+ve
effector
Inhibitor
N-acetyl
glutamate
Nil
Nil
CTP
Source
of N
Ammonia
Glutamine
Site
Sources of the atoms of urea:
O
ll
H2N–C–NH2
1- C & O: from CO2
2- 1st N atom: from NH3
3- 2nd N atom: from aspartate
Glutamate is usually the immediate precursor
of both NH3 &aspartate
H2O
Glutamate
Oxaloacetate
NAD
NADH+H
α- ketoglutarate
NH3
Oxidative
deamination
α- ketoglutarate
Transamination
Aspartate
Overall reaction:
NH3 + CO2 + Aspartate
Urea + fumarate
There in no net gain or loss of ornithine, citrulline ,
argininosuccinate or arginine.
Ornithine is regenerated with each turn of the urea cycle.
The release of the high energy phosphate of carbamoyl
phosphate as inorganic phosphate drives the reaction
in the forward direction.
Fate of fumarate & its link to TCA cycle
CO2
NH3
Oxaloacetate
Carbamoyl-phosphate
Malate
NADH+H
NAD
Fumarate
Ornithine
Citrulline
Ornithine
3ATP
TCA CYCLE
Citrulline
Aspartate
Urea
Arginine
Arginino succinate
Transamination
Fumarate
Cytoplasmic
fumarase
Malate
shuttle
NADP
NADPH+H
H2O
Malate
Fed state
Malic
enzyme
Pyruvic acid
Malate
dehydrogenase
NAD
or
Oxaloacetate
Fasting
state
PEP
Glucose
NADH+H
3ATP
Pyruvic acid
Oxaloacetate
Citrate
ATP
citrate
lyase
Citrate
Acetyl COA
f.a.
synthesis
Bioenergetics:
Urea cycle consumes four "high-energy" phosphate bonds (3 ATP hydrolyzed to 2
ADP and one AMP).
1 ATP
ADP + Pi
~P
1 ATP
ADP + Pi
Adenosine ~ P
1 ATP
AMP + Pi + Pi
~P
►However. One NADH+H molecule is produced by oxidative deamination of
glutamate to NH3 and α-ketoglutarate. Glutamate provides the NH3 used in the
initial synthesis of carbamoyl phosphate.
► Also fumarate in the cycle may be converted to malate in the cytosol . Malate then
oxidized to oxaloacetate gives 1 NADH+H equivalent to 3 ATP obtained from
3ADP,
So the net energy expenditure is only one high energy phosphate .
The two NADH+H produced can provide energy for the formation of 5 ATP, a net
production of one high energy phosphate bond for the urea cycle. However, if
gluconeogenesis is underway in the cytosol, the latter reducing equivalent is used
to drive the reversal of the glyceraldehyde 3-p dehydrogenase step instead of
generating ATP. So the net energy expenditure is only one high energy phosphate
.
Regulation:
Carbamoyl phosphate synthetase 1 is the key & it has an absolute requirement
for
N-Acetylglutamic acid which act as an allosteric activa tor.
The synthesis of N- Acetylglutamate from glutamic acid & acetyl CO A is ++ by
high protein diet & a.as especially arginine → ↑rate of urea cycle
.
Acetate
GLUTAMATE
Acetyl CO A
a.as esp arginine
Synthetase ++
N- ACETYL GLUTAMATE
ALLOSTERICALLY ++
carbamoyl phosphate synthetase 1
CO A
•
Structure:
Creatine (methyl guanido acetic acid)
CH2 – COOH
l
CH3 – N
l
HN = C – NH2
Phosphocreatine (Creatine phosphate)
CH2 – COOH
l
CH3 – N
O
l
ll
HN = C – NH ~ P– OH
l
OH
Creatinine
CH2 – CO
l
CH3 – N
l
HN = C ––– NH
Synthesis of creatine& creatine phosphate
& their degradation into creatinine
Creatine is formed from 3 a.as ( glycine, arginine & methionine )
Arginine
Glycine
+
CH2–CH2–CH2–CH–COOH
l
l
NH
NH2
l
NH = C–NH2
CH2 – COOH
|
NH2
Glycine
transamidinase
(kidney, pancreas)
Guanidoacetic
acid
CH2 – COOH
l
NH
l
HN = C – NH2
+
Ornithine
CH2–CH2–CH2–CH–COOH
l
l
NH2
NH2
S- adenosyl methionine
Methyl transferase
(liver)
S- adenosyl homocysteine
Creatine phosphate
Creatine phosphokinase
Creatine
(liver, brain, muscle)
CH2 – COOH
l
CH3 – N
O
l
ll
HN = C – NH ~ P– OH
l
Rapid
CH2 – COOH
l
CH3 – N
l
HN = C – NH2
(methyl guanido acetic acid)
Non enzymatic (spontaneous)
Slow
H2O
Pi
Creatinine
OH
Excreted in urine
CH2 – C O
l
CH3 – N
l
HN = C ––– N H
Importance of creatine:
►Creatine is converted to creatine phosphate using ATP as a phosphate donor. Creatine
phosphate acts as a store of high energy phosphate ,so it is used as a source of energy
in time of need (as it can give the phosphate group to ADP to form ATP).
►Creatine is present mainly in the muscle (98%) , but also in the brain & blood . Both the
muscle & the brain contain large amounts of creatine phosphate.
►During muscular relaxation,creatine becomes converted to creatine phosphate to be used
as a source of energy during muscular contraction.
(During ms contraction, ATP is mostly derived from glycolysis for which ms glycogen acts
as a substrate,However before ATP from glycolysis become available, phosphocreatine
acts as a temporary source of ATP.So it is especially important during the early stages
of ms exercise(1st few minutes),where the largest quantities of creatine –P are found ).
Level of creatine in the blood& urine:
►Plasma creatine level = 0.4 mg %
Whole blood creatine level = 2 - 4 mg %
► Normally little creatine is excreted in urine ( normally < 100 mg /day ) .This is more in
females & children due to low androgens which increase the uptake of creatine by ms.
Creatinuria
►Def.: It is the presence of large amounts of creatine in urine
(Normal level of creatine excreted in urine is < 100 mg / day.)
►Causes:
1- Physiological creatinuria :
■ in children: due to - ↓androgens → ↓ uptake of creatine by the muscle
- small mass of the ms
■ in female: due to - ↓ free androgen especially during pregnancy.
- after labour due to involution of the uterus.
2- Pathological creatinuria :
■ in males with ↓ androgens( hypogonadism )
■ in muscle atrophy as in myopathies.
■ ↑ protein catabolism in --hormonal disturbances as D.M. ,
hyperthyrodism and cushing syndrome,--also in fever and
starvation
Creatinine levels in serum& urine:
►Normal serum creatinine level = 0.5 - 1.5 mg%.
↑ serum creatinine level
Causes: 1- Impairment of kidney function : about 50% of kidney function must be
lost before serum creatinine is ↑.
2- ↑ ms mass : as acromegally & gigantism.
↓ serum creatinine level
Causes: ↓ ms mass as in - myasthenia gravis
- muscular dystrophy
- debilitation
► Normal urine creatinine level = 1 - 1.5 mg %.
-
It is more in male due to greater ms mass ---- male = 1.5 g / day.
---- female = 1 gm / day.
There is a steady production of constant amount of creatinine that is
proportional to the total amount of phosphocreatine & creatine in the
body , which is in turn proportional to the ms mass of the individual .
So, the serum & urine creatinine is constant for an individual &
approximately proportional to the ms mass.
Creatinine excretion rate (creatinine coefficient) :
•It is the amount of creatinine measured in mg / kg body weight / day.
•It is said to be remarkably constant (21 in male & 16 in female).(Why?)
So, creatinine excretion rate (creatinine coeffecient) can be used to
check the
accuracy of 24 hr collection of urine. If found lower than expected, it
indicates that part of the urine was discarded .
Creatinine clearance
-
-
-
-
-
The volume of serum or plasma that would be cleared of creatinine by one minute's excretion of
urine. value that reflects the body's ability to excrete creatinine; it is used to diagnose and
monitor renal function.
Urinary creatinine is 100% endogenous in origin i.e. not affected by diet being synthesized in
the body & no diurnal variation .So it can be used for calculation of urinary output of substance
amount of creatinine in urine.
Also creatinine is chiefly filtered by the kidney, though a small amount is actively secreted.
There is little-to-no tubular reabsorption of creatinine. If the filtering of the kidney is deficient,
blood levels rise. As a result, creatinine levels in blood and urine may be used to calculate the
creatinine clearance (CrCl), which reflects the glomerular filtration rate (GFR). The GFR is
clinically important because it is a measurement of renal function. However, in cases of severe
renal dysfunction, the creatinine clearance rate will be "overestimated" because the active
secretion of creatinine will account for a larger fraction of the total creatinine cleared. Ketoacids,
cimetidine and trimethoprim reduce creatinine tubular secretion and therefore increase the
accuracy of the GFR estimate, particularly in severe renal dysfunction. (In the absence of
secretion, creatinine behaves like inulin.)
Creatinine clearance (CCr) can be calculated if values for creatinine's urine concentration
(UCr), urine flow rate (V), and creatinine's plasma concentration (PCr) are known. Since the
product of urine concentration and urine flow rate yields creatine's excretion rate, creatinine
clearance is also said to be its excretion rate (UCr×V) divided by its plasma concentration. This
is commonly represented mathematically as :
Creatinine clearance(ml/min)=
Urinary creatinine conc.X urine volume in ml
Plasma creatinine conc X time of collection/min
NPN Compounds
Def: Are nitrogenous compounds not
precipitated by protein ppting
reagents.
Include:
a.as
Urea
Creatine
NH3
Uric acid
Creatinine
a.as
R-CH – CooH
Structure
‫׀‬
Ammonia
NH3
O
‫׀׀‬
H2N – C- NH2
NH2
Source



Proteins in diet.
Tissue protein catabolism.
Synthesis of non essential a.as.
(sources of a.a.pool)
Urea

a.a. deamination (main
source)

other nitrogonous
compounds as:
- purines & pyrimidmes.
 Co2.
 NH3.
 Aspartic acid
Through the
Urea cycle in liver.
- amines.
-urea splitted in the intestine.
■Synthesis of :

Proteins as:
- tissue Pr.
- Plasma Pr.
- Enzymes.
- Some hrs.
- Milk.

Other nitrogenous comp as creatine,heme,
neurotransmitters, purines & pyrimidinet
■catabolism (deamination) giving:
- NH3  Fate said with NH3.
- α -Keto-acids:→ glucose.
→ ketone bodies.
→ Co2 + H2o energy.




Blood level
4-6 mg/dl
< 0.1 mg/dl.
20-40 mg/dl
Urine level
0.7 g/day
0.7 gm/day
20-40 gm/day.
Clinical
implication
Amino acid uria
Ammonia intoxication
Bl. Urea is a krdnay
function test  in
renal failure.
Fate
Synthesis of a.as.
Urea formation.
Glutamine formation.
Excretion in urine.
Excretion in urine &
part
Pass
to
intestine.
Creatine
Structure
Source
Fate
Bl. Level
Urine level
Clinical
implications
CH2 – COOH
l
CH3 – N
l
HN = C – NH2
Creatinine
Uric acid
CH2 – C O
l
CH3 – N
l
HN = C ––– N H
- Arginine + glycine +
methionine.
( in kidneys and liver.)
-Creatine & creatine phosphate.
( in ms, brain and other tissues.)
Catabolism
(liver).
of
Uptake by ms and other
tissues.
Excretion in urine.
0.4 mg % plasma
2-4mg% blood
0.5-1.5 mg%
2-7 mg/dl
< 100 mg/day
1-1.5 gm/day
500 mg/day
Creatinuria
 serum creatinine:
- Renal failure.
- ↑ms mass as acromegally &
gigantism.
 serum creatinine in  ms mass as
ms dystrophy
deblitation
Creatinine coefficient for accuracy
of collecting 24 hrs volume of urine.
Creatinine clearance as measure of
GFR.
purines
Excretion in urine.
Hyperuricemia
(gout)
Hypouricemia.
Heterocyclic a.as
1- Tryptophan
Synthesis: essential a.a.
Catabolism: Both glucogenic and ketogenic.
Tryptophan
Tryptophan pyrrolase (oxygenase)
N- formyl kynurenine
Acetoacetic acid
-keto adipic acidα
α-aminomuconate
α-aminomuconate semialdehyde
Quinolinic acid
NAD&NADP
Nicotinic acid
2-Acrolyl 3- amino fumarate
3-Hydroxyanthranilic acid
Hydroxykynurenine
Kynurenine
Importance:
Synthesis of
1-nicotinic acid(niacin)→ NAD&NADP.
2-Serotonin
3-Melatonin
4-Indole &skatole
1-Nicotinic acid:
• It is produced from tryptophan in the liver since the enzyme picolinate carboxylase has low activity in the liver so some 2-acrolyl 3aminofumarate is cyclized in non enzymatic reaction to niacin.
•
Every 60 mg tryptophan gives 1 mg niacin. 50%of niacin is formed from tryptophan while the rest are obtained from diet.
Pellagra:
•Causes : 1- ↓tryptophan due to deficiency in diet as in maize eating areas or↓
absorption in hartnup disease
2-niacin deficiencyin diet or due to decrease its synthesis from tryptophan
in case of carcinoid syndrome .
3- Isoniazid drug administration in T.B.treatment leading to vitamin B6
deficiency which is important in formation
of niacin from tryptophan
• pellagra is characterized by 3Ds diarrhea,dermatitis and dementia