PHAR2811Lecture

Amino acids as drug targets

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Amino Acid derivatives

• Adrenalin/epinephrine

• Serotonin

• GABA

• Histamine

• Dopa & dopamine

• thyroxin

The Catecholamine Family

• L-dopa• Dopamine• Noradrenalin or

Norepinephrine• Adrenalin or

epinephrine

OH

OH

Catechol

The Catecholamine Family

H3N+ CH C

CH2

O-

O

OH

Tyrosine

H3N+ CH C

CH2

O-

O

OH

HO

L-Dopa

Tyrosinehydroxylase

The Catecholamine Family

+NH3

CH2

CH2

OH

HO

L-Dopamine

+NH3

CH2

CH

OH

HO

Noradrenalin

HO

+NH2

CH2

CH

OH

HO

Adrenalin

HO

CH3

decarboxylation hydroxylation methylation

The Catecholamine Family

• Family members act as neurotransmitters in the brain and hormones in the circulatory system

• Produced in adrenal medulla and sympathetic neurons

• The pools are kept separate by the blood brain barrier

Acting as hormones

• The flight or fight response

• Adrenalin and noradrenalin are produced in the adrenal medulla and stored in granules

• Released into the circulation by stimuli from the sympathetic nervous system

• They bind to specific receptors

Adrenergic receptors

• Glycoproteins which span the membrane

• Known as G-coupled protein receptors

• 4 classes of adrenergic receptors: 1, 2, 1, 2

• Some are stimulatory (β) some inhibitory (α)

• Activate or inhibit adenylyl cyclase cAMP

Adrenergic receptors

• Have short term and long term effects

• The alpha receptors stimulate smooth muscle contraction in peripheral organs

• the beta receptors mobilise fuels, relax smooth muscles of the bronchi and blood vessels supplying skeletal muscles and increase heart rate.

Adrenergic receptors

• The end result of these actions is to mobilise and shunt energy reserves to where they are most needed

• prepare for action!

Used as a drug: Adrenalin

• Treatment for cardiac arrest and anaphylactic reactions

• Bronchodilator properties used in asthma

• Agonists and antagonists to adrenergic receptors are also used as drugs

Acting as neurotransmitters

• Noradrenalin is uniquely found between the junctions of sympathetic neurons and smooth muscle cells

• Decreased levels of noradrenalin in the brain are associated with some forms of clinical depression

Noradrenalin and depression

Strategies to increase noradrenalin levels in the brain:

• Inhibit inactivation (monoamine oxidase inhibitors or MAO inhibitors)

OR

• Inhibit reuptake (tricyclics)

Other Catecholamines

• L-dopa and dopamine

• Dopamine is also a neurotransmitter in synapses in localised areas of the brain stem

• Parkinson’s disease is caused from the degeneration of these dopaminergic neurons.

• The psychotic symptoms of Schizophrenia are associated with elevated dopamine

Parkinson’s disease

• Parkinson patients are treated with L-dopa or levadopa

• Although it is dopamine that is deficient it cannot cross the blood brain barrier

• L-dopa crosses the barrier (on amino acid transporters), where it is decarboxylated to produce dopamine

• It is usually administered with a peripheral decarboxylase inhibitor; carbidopa (to prevent the L-

Dopa going to dopamine before it gets to the brain).

L-Dopa and dopamine+NH3

CH2

CH2

OH

HO

L-Dopamine

H3N+ CH C

CH2

O-

O

OH

HO

L-Dopa

decarboxylationCrosses the blood brain barrier

This reaction must be inhibited before it gets to the blood brain barrier

carbidopa

H3N+ CH C

CH2

O-

O

OH

HO

L-Dopa

HNH2N C C

CH2

OH

O

OH

CH3

HO

Carbidopa, an inhibitor of levadopa decarboxylation

Tyrosine Hydroxylase (TH)

• The rate limiting step in the pathway

• Adding another –OH to the aromatic ring

• It requires O2 and biopterin (this moiety also

makes up folates – we obtain it from our diet or microorganisms in the gut)

• This is a tricky reaction

• There are only a few examples of this in life!

Tyrosine Hydroxylase

H3N+ CH C

CH2

O-

O

OH

Tyrosine

H3N+ CH C

CH2

O-

O

OH

HO

L-Dopa

Tyrosinehydroxylase

O2

Biopterin

NH

HNN

N

H2N

O

CH3

OH

OH

OH

Tyrosine hydroxylase

• Although it is the rate limiting step in synthesis it is not a great site for drug action.

• There are drugs that inhibit the enzyme but these are rarely used

• Regulation of adrenalin is done at the release phase

• Most drugs work on the receptor

Tyrosine hydroxylase

• There are 3 isoforms produced by alternative splicing although the biological significance of these is not entirely clear.

• Another example of a similar reaction is the conversion of phenylalanine to tyrosine

• The enzyme here is phenylalanine hydroxylase

• This reaction also requires biopterin

Phenylketonuria (PKU)

• The conversion of phenylalanine to tyrosine is a similar hydroxylation

• Catalysed by phenylalanine hydroxylase

• If this enzyme is defective the phenylalanine has to go elsewhere

• Accumulates as phenylpyruvate

Phenylalanine hydroxylase

H3N+ CH C

CH2

O-

O

H3N+ CH C

CH2

O-

O

OH

O2

Phenylalanine

Tyrosine

Phenylalanine hydroxylase

PKU: diversion

H3N+ CH C

CH2

O-

O

Phenylalanine

C C

CH2

O-

O

Phenylpyruvate

O

Oxidative deamination

Build up causes severe mental retardation in developing brain

PKU diet

Thyroxin

+NH3

CH

C

H2C

O-

O

OHO

I I

II

Synthesised from 2 tyrosine residues

Thyroxin

CH2

OH

H2CHO

N

C

Protein, thyroglobulin

Thyroxin

CH2

OH

H2CHO

I I

I

I

Iodination

Oxidative coupling

Thyroxin

+NH3

CH

C

H2C

O-

O

OHO

I I

II

CH2OHO

I I

II

+NH3

CH

C

H2C

O-

O

OHO

I I

II

proteolysis

+NH3

CH

C

H2C

O-

O

OHO

I I

II

+NH3

CH

C

H2C

O-

O

OHO

I I

II

+NH3

CH

C

H2C

O-

O

OHO

I I

II

5 – 6 thyroxins released per thyroglobulin

Hypothyroidism:

• thyroxin levels too low; individuals present with lethargy, cold skin and often overweight.

• low iodine in the diet often the result of low iodine in the soil….leads to a goiter

Cretinism

• Thyroxin is essential for normal growth and development

• A deficiency will result in cretinism, a condition defined by mental retardation, stunted growth

Serotonin• A derivative synthesised

by the decarboxylation of tryptophan.

• A neurotransmitter• low levels of serotonin

have also been linked to depression.

• Prozac acts to inhibit the degradation of serotonin.

Synthesis of Serotonin

H3N+ CH C

CH2

O-

O

HN

Tryptophan

H3N+ CH2

CH2

HN

OH

Serotonin

Histamine

• Produced by the decarboxylation of histidine

• A common product of allergic reactions, produced by mast cells

• Neurotransmitter• Involved in sleep regulation• Regulates acid secretions in the stomach• Antihistamines are used to relieve the

symptoms of an allergic reaction

Histamine synthesis

H3N+ CH C

CH2

O-

O

N

NH

Histidine

H3N+ CH2

CH2

N

NH

Histamine

decarboxylation

GABA

• Gamma amino butyric acid

• Produced by the decarboxylation of glutamate

• Major inhibitory neurotransmitter

• Released by 30% of synapses

GABA synthesis

H3N+ CH C

CH2

O-

O

CH2

C

OH

O

CH2

CH2

CH2

C

OH

O

+NH3

Glutamate GABA

Decarboxylation

Amino Acid derivatives

• Adrenalin/epinephrine

• Serotonin

• GABA

• Histamine

• Dopa & dopamine

• thyroxin