1 amino acid metabolism

Amino Acid Metabolism

2

Incorporation of NH4+ Into

Organic Compounds

1) NH4+

+ HCO3

- + 2 ATP NH2CO2PO3

-2 + 2 ADP +

Carbamoyl Phosphate Pi + 2 H+

2) NH4+

+

Carbamoyl

Phosphate

Synthase I

(CPS-I)

Glutamate

dehydrogenase O

-O2CCH2CH2CCO 2-

a-Ketoglutarate Glutamate

NADP +NADPH +

H+

NH3+

-O2CCH 2CH2CHCO 2-

TCA Cycle

3

Incorporation of NH4+ Into

Organic Compounds (Cont.)

NH3+

-O2CCH 2CH2CHCO 2- + NH4

+ + 2 ATP

NH3+O

H2NCCH2CH2CHCO 2-

Glutamine

Glutamate Glutamine

Synthase Mg++

N of glutamine donated to other compounds

in synthesis of purines, pyrimidines,

and other amino acids

3)

4

Biosynthesis of Amino Acids:

Transaminations

Amino Acid1 +a-Keto Acid2 Amino Acid2 +a-Keto Acid1

NH3+

-O2CCH 2CH2CHCO 2-

Glutamate

O

R-CCO 2-+

O-O2CCH2CH2CCO 2

-

a-Ketoglutarate

NH2

R-CHCO 2-

+

Pyridoxal phosphate (PLP)-

Dependent Aminotransferase

5

Transaminations: Role of PLP

N

C

CH2OPO3-2HO

H3C

N CHCH2CH2CO2-

N

HNH3+

CO2-

CHO

CH2OPO3-2HO

H3C

H

N

H

CH2

CH2OPO3-2HO

H3C

N CCH2CH2CO2-

H

CO2-

N

CH2NH2

CH2OPO3-2HO

H3C

O

H

H2O

-O2CCH 2CH 2CCO 2-

+ +

++

-O2CCH 2CH 2CHCO 2-

H2O

Tautomerization

6

Transaminations

Glutamate a-Ketoglutarate

+ +

Pyruvate Alanine

Glutamate a-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

7

Metabolic Classification of

the Amino Acids

• Essential and Non-essential

• Glucogenic and Ketogenic

8

Non-Essential Amino Acids

in Humans

• Not required in diet

• Can be formed from a-keto acids by

transamination and subsequent reactions

• Alanine

• Asparagine

• Aspartate

• Glutamate

• Glutamine

• Glycine

• Proline

• Serine

• Cysteine (from Met*)

• Tyrosine (from Phe*)

* Essential amino acids

9

Essential Amino Acids in

Humans

• Required 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

10

Glucogenic Amino Acids

• Metabolized to a-ketoglutarate,

pyruvate, oxaloacetate, fumarate, or

succinyl CoA

• Aspartate

• Asparagine

• Arginine

• Phenylalanine

• Tyrosine

• Isoleucine

• Methionine

• Valine

• Glutamine

• Glutamate

• Proline

• Histidine

• Alanine

• Serine

• Cysteine

• Glycine

• Threonine

• Tryptophan

11

Ketogenic Amino Acids

• Metabolized to acetyl CoA or

acetoacetate

• Isoleucine

• Leucine

• Threonine

• Tryptophan

• Lysine

• Phenylalanine

• Tyrosine

12

Amino Acids Formed From

a-Ketoglutarate O

-O2CCH2CH2CCO 2-

NH3+

-O2CCH 2CH2CHCO 2-

NH3+O

H2NCCH2CH2CHCO 2-

Transamination or

Glutamate

dehydrogenase

a-Keto-

glutarate

Glutamate

Glutamine

Glutamine

synthase

N

H H

CO2-

+

4 Steps

Proline

NH3+

+H3NCH2CH2CH2CHCO 2

- Ornithine

5 Steps

Arginine

Urea Cycle

NH3+NH2

H2N=C-HNCH2CH2CH2CHCO 2-

+

Guanidino group

13

GABA Formation

NH3+

-O2CCH 2CH2CHCO 2-

NH3+

-O2CCH 2CH2CH2

Glutamate Gamma-aminobutyrate

(GABA)

GABA is an important inhibitory neurotransmitter

in the brain

Drugs (e.g., benzodiazepines) that enhance the effects

of GABA are useful in treating epilepsy

Glutamate

decarboxylase

CO2

14

Arginine Synthesis: The Urea Cycle

NH3+

-O2CCH2CH2CHCO 2-

NHCOCH3

-O2CCH2CH2CHCO 2-

NH3+

H3NCH2CH2CH2CHCO 2-

+

NH4+

+ HCO3- NH2CO2PO3

-2

Glutamate N-Acetylglutamate

Ornithine

NH3+

NH2CONHCH2CH2CH2CHCO 2-

N-Acetylglutamate

synthase

CoASAc

4 Steps

CPS-I

Ornithine

Transcarbamoylase (OTC)

(mitochondria)

Citrulline Ureido group

Carbamoyl

phosphate

Activates

15

The Urea Cycle (Contd.)

NH3+

NH-CHCH2CO2-

CO2-

+H2N=C-HNCH2CH2CH2CHCO 2

-

NH3+

NH2CONHCH2CH2CH2CHCO 2-

Citrulline

Arginosuccinate

Arginosuccinate

synthase

NH3+NH2

H2N=C-HNCH2CH2CH2CHCO 2-

+

Arginine

NH3+

H3NCH2CH2CH2CHCO 2-

+

Ornithine

CO2-

-O2C

H

H

Fumarate

TCA Cycle

Arginase

H2NCONH2

Urea

Argino-

succinase

Ornithine

Transcarbamoylase

(mitochondria)

NH3-CHCH2CO2-

CO2-

+ Asp

16

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

17

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

18

Synthesis of Nitric Oxide

NH3+NH2

H2N=C-HNCH2CH2CH2CHCO 2-

+

NH3+

NH2CONHCH2CH2CH2CHCO 2- + NO

Nitric oxide synthase (NOS)

Arginine

Citrulline

19

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

20

Review Questions

21

Which one of the following does

not belong on this list?

A. Carbamoyl phosphate

B. Urea

C. Glutamate

D. Glutamine

Explain your answer:

22

A glucose metabolite formed in the

transamination of alanine

___________

Derivatives of this vitamin are

involved in transamination

reactions ______________

An amino acid, not found in

peptides, that is formed in the

urea cycle ______________

The amino acid that is the

immediate precursor of urea and

NO ____________

23

Decarboxylation of ___________

affords ___________, an important

inhibitor of neurotransmission in the

CNS

Explain the importance of the Blood

Urea Nitrogen (BUN) test:

__________________________________

__________________________________

24

Categorize the following amino acids

as glucogenic (G) or ketogenic (K);

essential (E) or non-essential (NE):

Leucine: G K E NE

Phenylalanine: G K E NE

Glutamate: G K E NE

25

Formation of Serine

OHH

CH2OPO3-2

C

CO 2-

CH2OPO3-2

CO 2-

C=O

NH3+H

CH2OPO3-2

C

CO 2-

NH3+H

CH2OH

CO 2-

C

Glucose Glycolysis

3-Phospho-

glycerate 3-Phospho-

hydroxypyruvate

3-Phosphoserine Serine (Ser)

Pyruvate

Dehydrogenase

NAD+ NADH +

H+

Glutamate

a-Ketoglutarate

Transaminase

Phosphatase

3 Steps

Inhibits

26

Conversion of Serine to Glycine

N

N

N

NH2N

OH

CH2NHR

H

H

N

H

N CH2

NH2C

Folate

Tetrahydrofolate

(FH4)

Dihydrofolate

reductase

N5, N10-Methylene FH4

NH3+H

CH2OH

CO 2-

C Serine

NH3+H

H

CO 2-

CGlycine

Serine hydroxymethyl

transferase (PLP-dep.)

Key intermediate

in biosynthesis of

purines and

formation of

thymine Important in

biosynthesis of heme,

porphyrins, and purines

27

Sulfur-Containing Amino Acids

NH3+

CH3SCH2CH2CHCO 2-

NH3+

HSCH2CH2CHCO 2-

NH3+

CH2CHCO2-

NH3+

SCH2CH2CHCO 2-NH3

+

HSCH2CHCO 2-

OH

CH3CHCH 2CO2-

Methionine

(Essential)

L-Homocysteine

Methionine

Synthase

(Vit. B12-dep.) + FH4

+ 5-Methyl

FH4

NH3+H

CH2OH

CO 2-

C Serine

Cystathionine

Cystathionine

b-synthase

(PLP-dep.)

Cystathionine

lyase

Cysteine

(Non-essential)

+

b-Hydroxy-

butyrate

28

Homocysteine

Homocysteinuria

• Rare; deficiency of cystathionine b-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

29

Methionine Metabolism:

Methyl Donation N

N N

N

O

OHOH

-O2CCHCH2CH2-S-H2C

NH2

NH3+ CH3

+

NH3+

CH3SCH2CH2CHCO 2-

N

N N

N

O

OHOH

-O2CCHCH2CH2-S-H2C

NH2

NH3+

N

N N

N

O

OHOH

H3NCH2CH2CH2-S-H2C

NH2

CH3

+

S-Adenosyl methionine

synthase

ATP

S-Adenosyl

Methionine

(SAM)

S-Adenosyl

homocysteine

Methyl-

transferases

Decarboxylated

SAM

SAM

Decarboxylase

CO2

Methionine

R-H

R-CH3 +

30

Polyamine Biosynthesis

NH3+

H3NCH2CH2CH2CHCO 2-

+

H3N

H

H

H

NH

N NH3

++++

H3NNH3

++

Ornithine

(from urea cycle)

Putrescine

CO2

Ornithine

decarboxylase

(ODC)

(PLP-dep.)

Decarboxylated

SAM

Spermidine

synthase

5’-Methylthio-

adenosine

H3NNH

NH3

H

+++

Spermidine

Spermine

Decarboxylated

SAM

Spermine

synthase

5’-Methylthio-

adenosine

31

Polyamines

• Spermidine and spermine found in virtually

all procaryotic and eucaryotic cells

• Precise role undefined

• Bind to nucleic acids

• Inhibition of biosynthetic pathway:

H2NNH2

CO2H

CHF2

a-Difluoromethyl-

ornithine (DFMO)

(Eflornithine) - inhibits ODC;

used to treat

Pneumocystis carinii infectons

32

Creatine and Creatinine

NH3+NH2

+H2N=C-HNCH2CH2CH2CHCO 2

-

Arginine Glycine Ornithine

Arginine-glycine

transamidinase

(Kidney) NH2

H2N=C-HNCH2CO 2-

+

Guanidoacetate

NHPO3-2

CH3

+H2N=C-NCH2CO 2

-

Guanidoacetate

Methyltransferase

(Liver)

SAM + ATP

S-Adenosyl-

homocysteine

+ ADP

Phosphocreatine

N

NH

CH3

HN

O

Creatinine

(Urine) Non-enzymatic

(Muscle)

NH2

CH3

H2N=C-NCH2CO 2-

+

Creatine kinase

(Muscle)

ATP

Creatine ADP

+ Pi

33

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

34

Histidine Metabolism:

Histamine Formation

N

NH

CH2CHCO2-

NH3

+

N

NH

CH2CH2NH2

Histidine Histamine

Histidine

decarboxylase

CO2

Histamine:

• Synthesized in and released by mast cells

• Mediator of allergic response: vasodilation, bronchoconstriction

(H1 receptors)

• H1 blockers: Diphenhydramine (Benadryl)

Loratidine (Claritin)

• Stimulates secretion of gastric acid (H2 receptors)

• H2 blockers: Cimetidine (Tagamet); ranitidine (Zantac)

35

Review Questions

36

This amino acid, formed from serine using

a folate-derived coenzyme, is a key

building block in the biosynthesis of

heme and purines ________________

37

High blood levels of this amino acid, which is

not found in peptides, are associated with

increased risk of cardiovascular disease

________________

Explain why a person found to have high

levels of this amino acid might benefit from

the use of folic acid __________________

38

N

N N

N

O

OHOH

-O2CCHCH2CH2-S-H2C

NH2

NH3+ CH3

+

What is the name of the above compound?

Explain the importance of the above

compound in the cell:

39

Name:

An arginine metabolite, formed mainly in

muscle and excreted in the urine, that is

sometimes used as a dietary supplement

to improve athletic performance ___________

An amino acid, which upon decarboxylation,

produces a compound some of whose biological

effects are blocked by cimetidine (Tagamet)

_________________

40

Phenylalanine and Tyrosine

CH2CHCO2-

NH3+

CH2CHCO2-

NH3+

HO

HN

N

NH

NH

H2N

O

H

H

CHCHCH3

HO OH

HN

N

NH

NH2N

O

CHCHCH3

HO OH

Phenylalanine

(Essential)

Tyrosine

(Non-essential)

Phenylalanine-4-

Monooxygenase

(Phenylalanine

hydroxylase)

O2

H2O

+

+

NADPH + H+

NADP+

Tetrahydrobiopterin

Dihydrobiopterin

41

Phenylketonuria (PKU) Disease

• Deficiency of Phe hydroxylase

• Occurs in 1:16,000 live births in U.S.

• Seizures, mental retardation, brain

damage

• Treatment: limit phenylalanine intake

• Screening of all newborns mandated

in all states

CH2CCO2-

O

Phe

Tyr

Transamination

Phenylpyruvate

(urine)

42

Catecholamine Biosynthesis

CH2CHCO2-

NH3+

HO

CH2CHCO2-

NH3+

HO

HO

CH2CH2NH2

HO

HO

CHCH2NH2

HO

HO

OH

CHCH2NHCH3

HO

HO

OH

Tyr hydroxylase

O2

Tyrosine Dihydroxyphenylalanine

(DOPA)

Dopamine

DOPA

decarboxylase CO2

Dopamine

hydroxylase

Norepinephrine

Catechol

Epinephrine

(Adrenaline)

SAM

S-Adenosyl-

homocysteine

Methyl

transferase

DOPA, dopamine, norepinephrine,

and epinephrine are all neurotransmitters

43

L-DOPA in Parkinsonism Blood Brain

Blood Brain Barrier

L-DOPA L-DOPA Dopamine

Dopamine

HO

HOCH2-C-CO2H

CH3

NHNH2Carbidopa

Blocks

Parkinsonism associated with

dopamine in brain through loss of

neurons in basal ganglia.

Carbidopa + L-DOPA

44

Monoamine Oxidase (MAO)

MAO

(in mitochondria)

R R’

OH H Norepi

OH CH3 Epi

H H Dopamine

CHCH2NHR'

HO

HO

R

CHCHO

HO

HO

R

CHCO2H

HO

HO

RUrinary

metabolite MAO inhibitors (e.g., tranylcypromine) are useful

in the treatment of depression

Brain levels of dopamine and norepi.; also

serotonin

Aldehyde

dehydrogenase

R=OH Vanillylmandelic acid (VMA)

R=H Homovanillic acid (HVA)

45

Tyramine OH

CH2CH2NH2

Tyramine

OH

CH2CHO

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)

46

Catechol-O-Methyl

Transferase (COMT)

CHCH2NHR'

HO

HO

R

CHCH2NHR'

HO

CH3O

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) useful

in Parkinson’s disease

Active

catecholamine

47

Homogentisic Acid Formation

CH2CHCO2-

NH3+

HO

OH

OH

CH2CO2-

Transamination

Tyrosine p-Hydroxyphenyl-

pyruvate

Homogentisate

p-Hydroxyphenyl-

pyruvate

dioxygenase

(ascorbate-dep.)

O2

CO2

CH2CCO2-

O

HO

Homogentisate

dioxygenase

O2

Cleavage of

aromatic ring

Fumarate + acetoacetate

Deficient in

alkaptonuria

48

Alkaptonuria

• Deficiency of homogentisate dioxygenase

• Urine turns dark on standing

• Oxidation of homogentisic acid

• Asymptomatic in childhood

• Tendency toward arthritis in adulthood

49

Melanin Formation

CH2CHCO2-

NH3

O

O

+

CH2CHCO2-

NH3+

HO

HO

Highly colored

polymeric

intermediates

Melanin

(Black polymer)

Tyr hydroxylase

DOPA

Dopaquinone

CH2CHCO2-

NH3+

HO

Tyrosine

Tyrosinase

Melanin formed in skin (melanocytes), eyes, and hair

In skin, protects against sunlight

Albinism: genetic deficiency of tyrosinase

O2

50

Tryptophan Metabolism:

Serotonin Formation

NH

CH2CHCO2-

NH3

+

NH

CH2CHCO2-

NH3

HO

+

NH

CH2CH2NH2

HO

Tryptophan

(Trp)

Indole ring

Trp

hydroxylase

O2

5-Hydroxy-

tryptophan

Decarboxylase

CO2 5-Hydroxy-

tryptamine (5-HT);

Serotonin

51

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:

• Depression

•Serotonin-selective reuptake inhibitors (SSRI)

• Migraine

• Schizophrenia

• Obsessive-compulsive disorders

• Chemotherapy-induced emesis

• Some hallucinogens (e.g., LSD) act as serotonin agonists

52

• 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

53

Serotonin Metabolism: 5-HIAA

NH

CH2CH2NH2

HO

NH

CH2CHO

HO

NH

CH2CO2H

HO

Serotonin

MAO

Dehydrogenase

5-Hydroxyindole acetic

acid (5-HIAA) (Urine)

Carcinoid tumors:

• Malignant GI tumor type

• Excretion of large amounts of 5-HIAA

54

Serotonin Metabolism:

Melatonin

NH

CH2CH2NHCOCH3

H3CO

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

55

Tryptophan Metabolism:

Biosynthesis of Nicotinic Acid

NH

CH2CHCO2-

NH3

+

Tryptophan

N

CO2H

Nicotinic acid (Niacin)

Several steps

Nicotinamide adenine

dinucleotide (NAD)

56

Review Questions

57

This neurotransmitter is formed by oxidation of

tryptophan, followed by decarboxylation ____________

This natural catecholamine is used as a drug to

treat Parkinson’s disease ____________________.

This drug’s effectiveness can be enhanced by

using ________________,

a potent decarboxylase inhibitor.

This compound is found in high levels in the blood

of patients with PKU disease ___________

58

CHCH2NHR'

HO

HO

R

CHCH2NHR'

HO

CH3O

R

CH2CHCO2-

NH3+

HO

HO CH2CHCO2-

NH3

O

O

+

Name the enzyme that catalyzes each of the

following reactions and explain the importance

of each reaction