Post on 10-Apr-2018
8/8/2019 A.acid Metabolism
1/51
Amino Acid Metabolism
Hanley N. Abramson
Professor of Pharmaceutical Sciences
Wayne State University
December 2009
8/8/2019 A.acid Metabolism
2/51
2
Dynamics of Protein
And Amino Acid Metabolism
Dietary Proteins Digestion to Amino Acids
Transport in Blood to Cells
Protein Synthesis Functional Proteins
Protein Degradation In
Proteasomes Following
Tagging With Ubiquitin
Amino Acids
Metabolites
8/8/2019 A.acid Metabolism
3/51
3
Digestion of ProteinsStomach: Pepsinogen Pepsin (max. act. pH 2)
Small Intestine: Trypsinogen Trypsin
Trypsin cleaves:Chymotrypsinogen to chymotrypsin
Proelastase to elastase
Procarboxypeptidase to carboxypeptidase
Aminopeptidases (from intestinal epithelia)
Enteropeptidase
8/8/2019 A.acid Metabolism
4/51
4
Lumen Amino Acids Oligopeptides
Intestinal Absorption
Oligopeptides
Amino Acids
Peptidases
Blood
Transport
Protein
8/8/2019 A.acid Metabolism
5/51
5
Incorporation of NH4+
Into Organic
Compounds1) NH4
++ HCO3
-+ 2 ATP NH2CO2PO3
-2+ 2 ADP +
Carbamoyl Phosphate Pi+ 2 H+
2) NH4+
+
Carbamoyl
Phosphate
Synthase I
(CPS-I)
Glutamate
dehydrogenaseO
- O 2CCH 2CH 2CCO 2-
E-Ketoglutarate Glutamate
3+
-2 2 2 2
-
T A ycle
NADP+NADPH +H
+
mitochondria
8/8/2019 A.acid Metabolism
6/51
6
Incorporation of NH4+
Into Organic
Compounds (Cont.)
NH +
O
CCH
CH
CHCO
+ NH4+
+ 2 ATP
+
2 2 2 2
Glutamine
Glutamate GlutamineSynthase
Mg++
of glutamine donated to other compounds
in synthesis of purines, pyrimidines,
and other amino acids
)
8/8/2019 A.acid Metabolism
7/51
7
Biosynthesis of Amino Acids:
Transaminations
Amino Acid1 +E-Keto Acid2 Amino Acid2+E-Keto Acid1
+
-2 2 2 2
-
Glutamate
R- 2-+
O
-O 2 CCH 2 CH 2 CCO 2
-
E-Ketoglutarate
+
Pyridoxal phosphate (PLP)
Dependent Aminotransferase
8/8/2019 A.acid Metabolism
8/51
8
Transaminations: ole of PLP
2 P 3-2
3
2 2 2-
3+
2-
2 P 3-2
3
2
2 P 3-2
3
2 2 2-
2-
2 2
2 P 3-2
3
2
- 2 2 2 2-
+ +
++
- 2 2 2 2
-
2
Tautomerization
8/8/2019 A.acid Metabolism
9/51
9
Transaminations
Glutamate E-Ketoglutarate
+ +
Pyruvate Alanine
Glutamate E-Ketoglutarate
+ +Oxaloacetate Aspartate
Glutamate-Pyruvate
Aminotransferase
(Alanine Transferase ALT)
Glutamate-Oxaloacetate
Aminotransferase
(Aspartate Transferase AST)
Blood levels of these aminotransferases, also called transaminases,
are important indicators of liver disease
8/8/2019 A.acid Metabolism
10/51
10
Metabolic Classification of the
Amino Acids
Essential and Non-essential
Glucogenic andKetogenic
8/8/2019 A.acid Metabolism
11/51
11
Non-Essential Amino Acids in
Humans Not required in diet
Can be formed from E-keto acids by
transamination and subsequent reactions
Alanine
Asparagine
Aspartate Glutamate
Glutamine
Glycine
Proline
Serine Cysteine (from Met*)
Tyrosine (from Phe*)
*Essential amino acids
8/8/2019 A.acid Metabolism
12/51
12
Essential Amino Acids in Humans equired in diet
Humans incapable of forming requisite
carbon skeleton
Arginine*
Histidine*
Isoleucine Leucine
Valine
Lysine
Methionine
Threonine Phenylalanine
Tryptophan
*Essential in children, not in adults
8/8/2019 A.acid Metabolism
13/51
8/8/2019 A.acid Metabolism
14/51
14
Ketogenic Amino Acids Metabolized to acetyl CoA or acetoacetyl CoA
Animals cannot convert acetyl CoA oracetoacetyl CoA to pyruvate
Isoleucine
Leucine * Lysine *
Threonine
Tryptophan
Phenylalanine Tyrosine
*Leucine and lysine are only ketogenic
8/8/2019 A.acid Metabolism
15/51
15
Amino Acids Formed From E-
Ketoglutarate-
-
-
-
-
Transamination or
Glutamatedehydrogenase
E-Keto-
glutarate
Glutamate
Glutamine
Glutamine
synthase
! 2-
"
4 Steps
Proline
#
$
# 2 2 2 2 - rnithine
5 Steps
Arginine
Urea ycle
%
&
2
2N= - N 2 2 2 2 -
Guanidino group
8/8/2019 A.acid Metabolism
16/51
16
GABA Formation
H 3+
-O 2 CCH 2 CH 2 CHCO 2
-
H 3+
-O 2 CCH 2 CH 2 CH 2
Glutamate Gamma-aminobutyrate
(GABA)
GABA is an important inhibitory neurotransmitterin the brain
Drugs (e.g., benzodiazepines) that enhance the effects
of GABA are useful in treating epilepsy
Glutamatedecarboxylase
CO2
8/8/2019 A.acid Metabolism
17/51
17
Arginine Synthesis: The Urea Cycle
H 3+
-O2 CCH 2 CH 2 CHCO 2
-
HCOCH 3-O 2CCH 2CH 2CHCO 2
-
H 3+
H3 CH 2 CH 2 CH 2 CHCO 2-
+
H4
++
H
CO
3
-H
2CO
2PO
3
-2
Glutamate -Acetylglutamate
Ornithine
H 3+
H 2 CO H CH 2 CH 2 CH 2 CHCO 2-
-Acetylglutamate
synthase
CoASAc
4 Steps
CPS-I
Ornithine
Transcarbamoylase (OTC)
(mitochondria)
Citrulline
Ureido group
Carbamoyl
phosphate
Activates
8/8/2019 A.acid Metabolism
18/51
8/8/2019 A.acid Metabolism
19/51
19
Urea Formation
Occurs primarily in liver; excreted by kidney
Principal method for removing ammonia
Hyperammonemia:
Defects in urea cycle enzymes (CPS, OTC, etc.)
Severe neurological defects in neonates
Treatment:
Stop protein intake
Dialysis
Increase ammonia excretion: Na benzoate, Na phenylbutyrate, L-
arginine, L-citrulline
8/8/2019 A.acid Metabolism
20/51
20
Blood Urea Nitrogen Normal range: 7-18 mg./dL
Elevated in amino acid catabolism
Glutamate N-acetylglutamate
CPS-1 activation
Elevated in renal insufficiency Decreased in hepatic failure
8/8/2019 A.acid Metabolism
21/51
21
Synthesis of Nitric OxideNH3
+NH2
H2N=C-HN CH 2CH2CH2CHCO 2-
+
NH 3+
NH 2CONH CH 2CH 2CH 2CHCO 2- + NO
Nitric oxide synthase (NOS)
Arginine
Citrulline
8/8/2019 A.acid Metabolism
22/51
22
Nitric Oxide Cell messenger
Implicated in a wide range of physiological
and pathophysiological events:
Vasodilation: Activates guanylyl cyclase cGMP
Nitroglycerin Glycerin + NO
Sildenafil (Viagra): in vascular smooth muscle:
NO cGMP GMP
Phospho-
diesterase-5
Blocks
8/8/2019 A.acid Metabolism
23/51
23
Formation of Serine
D EE
F E
G
D H D
I
P G
F
F D
G
P
F E
G
D H D
I
P G
F D
G
P
F = D
Q R
S
+R
T R
U
V W V
S
X U
T
T V
U
X
Q R
S
+R
T R
U
V R
T V
U
X
T
GlucoseGlycolysis
hospho
glyceratehospho
hydroxypyruvate
hosphoserineSerine (Ser)
yruvate
Dehydrogenase
Y AD+ Y AD +
+
Glutamate
E-Ketoglutarate
Transaminase
Phosphatase
3 Steps
Inhibits
8/8/2019 A.acid Metabolism
24/51
24
Conversion of Serine to Glycine
N
N
N
NH2N
OH
CH2NHR
H
H
N
H
NCH2
N
H2C
Folate
Tetrahydrofolate
(FH4)
Dihydrofolate
reductase
N5, N10-Methylene FH4
NH 3+H
CH 2a H
CO 2-
C Serine
NH3+H
H
CO 2-
C
Glycine
Serine hydroxymethyl
transferase (PLP-dep.)
Key intermediate
in biosynthesis of
purines and
formation of
thymine Important in
biosynthesis of heme,porphyrins, and purines
8/8/2019 A.acid Metabolism
25/51
Sarcosine (N-Methylglycine)
25
H3N
O
O
H2N
H3
C
O
O
G lycine N -M ethyltransferase
Glycine Sarcosine
Sarcosine as a possible biomarker for prostate cancer:
See Nature Feb. 12, 2009
8/8/2019 A.acid Metabolism
26/51
26
Sulfur-Containing Amino Acids
b
+
C S C C C C -
c
+
S Cd
Cd
C Cd
-
e
+
Cf
C Cf
-
e
+
S Cg
Cg
C Cg
-h
+
S Ci
C Ci
-C
p
C Cq
Cq
-
Methionine
(Essential)
L- omocysteine
Methionine
Synthase
(Vit. B1 -dep.)+ F 4
+ 5-Methyl
F 4
r s
t
+s
Csu
v s
Cv
u
-
C Serine
Cystathionine
Cystathionine
F-synthase
(PLP-dep.)
Cystathionine
lyase
Cysteine
(Non-essential)
+
F-Hydroxy-butyrate
8/8/2019 A.acid Metabolism
27/51
27
HomocysteineHomocysteinuria
are; deficiency of cystathionineF-synthase
Dislocated optical lenses
Mental retardation Osteoporosis
Cardiovascular disease death
High blood levels of homocysteine associated with
cardiovascular disease May be related to dietary folate deficiency
Folate enhances conversion of
homocysteine to methionine
8/8/2019 A.acid Metabolism
28/51
28
Methionine Metabolism: Methyl
Donationw
w
w
w
x
x
y
x
y
-x 2
y
y
2 y
2- -y
2
w y
2
w y
3 + y
3
+3
+
3 2 2 2 -
w
w
w
w
x
x
y
x
y
-x 2
y
y
2 y
2- -y
2
w y
2
w y
3+
3
2
2
2- -
2
2
3
+
-Adenosyl methionine
synthase
ATP
-Adenosyl
Methionine
( AM)
-Adenosyl
homocysteine
Methyl-
transferases
DecarboxylatedAM
AM
Decarboxylase
2
Methionine
-
- 3+
8/8/2019 A.acid Metabolism
29/51
8/8/2019 A.acid Metabolism
30/51
30
Polyamines Spermidine and spermine found in virtually
all procaryotic and eucaryotic cells
Precise role undefined
Bind to nucleic acids
Inhibition of biosynthetic pathway:
E-Difluoromethyl-
ornithine (DFMO)
(Eflornithine) - inhibits ODC;
used to treat
Pneumocystis carinii infectons
8/8/2019 A.acid Metabolism
31/51
31
Creatine and CreatinineH 3
+H 2+
H2 =C-H CH 2CH 2CH 2CHC 2-
ArginineGlycine rnithine
Arginine-glycine
transamidinase
(Kidney)H 2
H2 =C-H CH 2C 2-
+
Guanidoacetate
HPO3
-2
CH 3
+H2 =C- CH 2C 2
-
Guanidoacetate
Methyltransferase
(Liver)
SAM + ATP
S-Adenosyl-
homocysteine
+ ADP
Phosphocreatine
H
CH 3
H
O
Creatinine
(Urine) on-enzymatic(Muscle)
H 2
CH 3
H2 =C- CH 2C 2-
+
Creatine kinase
(Muscle)
ATP
Creatine ADP+ Pi
8/8/2019 A.acid Metabolism
32/51
32
Creatine and Creatinine
Creatine:
Dietary supplement
Used to improve athletic performance
Creatinine:
Urinary excretion generally constant;
proportional to muscle mass
Creatinine Clearance Test:
Compares the level of creatinine in urine (24 hrs.)
with the creatinine level in the blood Used to assess kidney function
Important determinant in dosing of several drugs
in patients with impaired renal function
8/8/2019 A.acid Metabolism
33/51
33
Histidine Metabolism:
Histamine FormationH
H H
H
HH H H
Histidine Histamine
Histidine
decarboxylase
Histamine:
Synthesized in and released by mast cells
Mediator of allergic response: vasodilation, bronchoconstriction
(H1 receptors)
H1 blockers: Diphenhydramine (Benadryl)
Loratidine ( laritin)
Stimulates secretion of gastric acid (H
receptors)
H
blockers: imetidine (Tagamet); ranitidine (Zantac)
8/8/2019 A.acid Metabolism
34/51
34
Phenylalanine and Tyrosine
+
+
Phenylalanine
(Essential)
Tyrosine
( on essential)
Phenylalanine 4
Monooxygenase
(Phenylalanine
hydroxylase)
+
+
ADP + +
ADP+
Tetrahydrobiopterin
(B 4)
Dihydrobiopterin
8/8/2019 A.acid Metabolism
35/51
35
Normal Utilization of Phenylalanine
Phenylalanine
Protein (~25%)
Tyrosine (~75%)
8/8/2019 A.acid Metabolism
36/51
36
Phenylketonuria (PKU) Disease
Deficiency of Phe hydroxylase
Occurs in 1:20,000 live births in U.S.
Seizures, mental retardation, brain damage
Treatment: limit phenylalanine intake
Screening of all newborns mandated in all
states
CH 2CCO 2-
O
Phe
Tyr
Transamination
Phenylpyruvate
(urine)
8/8/2019 A.acid Metabolism
37/51
37
Sapropterin (Kuvan)
Synthetic tetrahydrobiopterin
Indicated for reduction of phenylalanine blood
levels in patients with BH4-responsive PKU
8/8/2019 A.acid Metabolism
38/51
38
Catecholamine Biosynthesis
C
C C
j
k
+
Cl
C Cl
m
n
+
C o C o o
C C o o
C C o C
Tyr hydroxylase
l
Tyrosine Dihydroxyphenylalanine
(D PA)
Dopamine
D PA
decarboxylase Cl
Dopamine
hydroxylase
orepinephrine
Catechol
Epinephrine
(Adrenaline)
SAM
S Adenosyl
homocysteine
Methyl
transferase
D PA, dopamine, norepinephrine,
and epinephrine are all neurotransmitters
8/8/2019 A.acid Metabolism
39/51
39
L-DOPA in ParkinsonismBlood Brain
Blood Brain Barrier
L-DOPA L-DOPA Dopamine
Dopamine
HO
HOCH 2-C-CO 2H
CH3
H
H 2Carbidopa
Blocks
Parkinsonism associated with
dopamine in brain through loss of
neurons in basal ganglia.
Carbidopa + L-DOPA
8/8/2019 A.acid Metabolism
40/51
40
Monoamine Oxidase (MAO)MAO
(in mitochondria)
OH H Norepi
OH CH3 Epi
H H Dopamine
CHCH 2NH '
HO
HO CHCHO
HO
HO
CHCO 2H
HO
HO
Urinary
metaboliteMAO inhibitors (e.g., tranylcypromine) are useful
in the treatment of depression
Brain levels of dopamine and norepi.; also
serotonin
Aldehyde
dehydrogenase
=OH Vanillylmandelic acid (VMA)
=H Homovanillic acid (HVA)
8/8/2019 A.acid Metabolism
41/51
41
TyramineOH
CH 2CH 2 H 2
Tyramine
OH
CH 2CHO
MAO
Tyramine found naturally in several types of cheese;also beer and red wine.
Tyramine intake can cause hypertensive crisis in
persons taking a MAO inhibitor ( norepi release)
( blood pressure)
8/8/2019 A.acid Metabolism
42/51
42
Catechol-O-Methyl Transferase
(COMT)
C C 2 R'
O
O
R
C C 2 R'
O
C 3 O
R
COMT
Inactive
metabolite
SAM S-Adenosyl-
homocysteine
COMT found in cytoplasm Terminates activity of catecholamines
Catecholamine excretion products result from
combined actions of MAO and COMT
Inhibitors of COMT (e.g., tolcapone) usefulin Parkinsons disease
Active
catecholamine
8/8/2019 A.acid Metabolism
43/51
43
Homogentisic Acid Formation
H
H
H
+
H
H
H
H
Transamination
Tyrosinep Hydroxyphenyl
pyruvate
Homogentisate
p Hydroxyphenyl
pyruvate
dioxygenase(ascorbate dep.)
H
H
Homogentisate
dioxygenase
leavage of
aromatic ring
Fumarate + acetoacetate
Deficient in
alkaptonuria
8/8/2019 A.acid Metabolism
44/51
44
Alkaptonuria
Deficiency of homogentisate dioxygenase
Urine turns dark on standing
Oxidation of homogentisic acid
Asymptomatic in childhood
Tendency toward arthritis in adulthood
8/8/2019 A.acid Metabolism
45/51
45
Melanin Formation
2 2-
3+
2 2-
3+
ighly colored
polymericintermediates
Melanin
(Black polymer)
Tyr hydroxylase
D PA
Dopaquinone
2 2-
3+
Tyrosine
Tyrosinase
Melanin formed in skin (melanocytes), eyes, and hair
In skin, protects against sunlight
Albinism: genetic deficiency of tyrosinase
2
8/8/2019 A.acid Metabolism
46/51
46
Tryptophan Metabolism: Serotonin
Formation
z
{
| { 2| { | } 2-
z { 3
+
z
{
| { 2 | { | } 2-
z { 3
{ }
+
z
{
| { 2 | { 2z { 2
{ }
Tryptophan
(Trp)
Indole ring
Trp
hydroxylase
2
5- ydroxy-
tryptophan
Decarboxylase
2 5- ydroxy-
tryptamine (5- T);
Serotonin
8/8/2019 A.acid Metabolism
47/51
47
Serotonin
Serotonin formed in: Brain (neurotransmitter; regulation of sleep, mood, appetite)
Platelets (platelet aggregation, vasoconstriction)
Smooth muscle (contraction)
Gastrointestinal tract (enterochromaffin cells - major storage site)
Drugs affecting serotonin actions used to treat: DepressionSerotonin-selective reuptake inhibitors (SS I)
Migraine
Schizophrenia
Obsessive-compulsive disorders
Chemotherapy-induced emesis
Some hallucinogens (e.g., LSD) act as serotonin agonists
8/8/2019 A.acid Metabolism
48/51
48
Food supplement promoted for serotonin effects
L-Tryptophan disaster (1989):
Eosinophilia-myalgia syndrome (EMS)
Severe muscle and joint pain
Weakness
Swelling of the arms and legs
Fever
Skin rash
Eosinophilia Many hundreds of cases; several deaths
Traced to impurities
L-Tryptophan
8/8/2019 A.acid Metabolism
49/51
49
Serotonin Metabolism: 5-HIAA
H
CH 2CH 2 H 2
HO
H
CH2CHO
HO
H
CH2CO 2H
HO
Serotonin
MAO
Dehydrogenase
5-Hydroxyindole acetic
acid (5-HIAA) (Urine)
Carcinoid tumors:
Malignant GI tumor type
Excretion of large amounts of 5-HIAA
8/8/2019 A.acid Metabolism
50/51
50
Serotonin Metabolism: Melatonin
NH
CH2CH2NHCOCH 3
H3 CO
NH
CH2CH2NH2
HO
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
8/8/2019 A.acid Metabolism
51/51
51
Tryptophan Metabolism:
Biosynthesis of Nicotinic Acid
N
~ ~
-
N
Tryptophan
N
~
Nicotinic acid (Niacin)
Several steps
Nicotinamide adenine
dinucleotide (NAD)