A.acid Metabolism

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Amino Acid Metabolism

Hanley N. Abramson

Professor of Pharmaceutical Sciences

Wayne State University

December 2009


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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


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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


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Lumen Amino Acids Oligopeptides

Intestinal Absorption

Oligopeptides

Amino Acids

Peptidases

Blood

Transport

Protein


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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


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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

)


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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


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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


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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


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Metabolic Classification of the

Amino Acids

Essential and Non-essential

Glucogenic andKetogenic


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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+


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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


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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


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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


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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)


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Phenylalanine and Tyrosine

+

+

Phenylalanine

(Essential)

Tyrosine

( on essential)

Phenylalanine 4

Monooxygenase

(Phenylalanine

hydroxylase)

+

+

ADP + +

ADP+

Tetrahydrobiopterin

(B 4)

Dihydrobiopterin


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Normal Utilization of Phenylalanine

Phenylalanine

Protein (~25%)

Tyrosine (~75%)


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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)


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Sapropterin (Kuvan)

Synthetic tetrahydrobiopterin

Indicated for reduction of phenylalanine blood

levels in patients with BH4-responsive PKU


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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


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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


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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)


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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)


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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


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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


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Alkaptonuria

Deficiency of homogentisate dioxygenase

Urine turns dark on standing

Oxidation of homogentisic acid

Asymptomatic in childhood

Tendency toward arthritis in adulthood


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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


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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


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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


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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


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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


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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


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Tryptophan Metabolism:

Biosynthesis of Nicotinic Acid

N

~ ~

-

N

Tryptophan

N

~

Nicotinic acid (Niacin)

Several steps

Nicotinamide adenine

dinucleotide (NAD)

A.acid Metabolism

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