Page 361 - Color Atlas of Biochemistry
P. 361
352 Tissues and organs
Neurotransmitters This provides better protection against break-
down by peptidases.
Neurotransmitters in the strict sense are sub- Endorphins, dynorphins, and enkephalins
stances that are produced by neurons, stored are a particularly interesting group of neuro-
in the synapses, and released into the synap- peptides. They act as “endogenous opiates” by
tic cleft in response to a stimulus. At the post- producing analgetic, sedative, and euphoriant
synaptic membrane, they bind to special re- effects in extreme situations. Drugs such as
ceptors and affect their activity. morphine and heroin activate the receptors
for these peptides (see p. 354).
Purine derivatives with neurotransmitter
A. Important neurotransmitters
function are all derived from adenine-con-
Neurotransmitters can be classified into sev- taining nucleotides or nucleosides. ATP is re-
eral groups according to their chemical struc- leased along with acetylcholine and other
ture. The table lists the most important rep- transmitters, and among other functions it
resentatives of this family, which has more regulates the emission of transmitters from
than 100 members. its synapse of origin. The stimulatory effect
Acetylcholine, the acetic acid ester of the of caffeine is mainly based on the fact that it
cationic alcohol choline (see p. 50) acts at binds to adenosine receptors.
neuromuscular junctions, where it triggers
muscle contraction (see p. 334), and in certain
parts of the brain and in the autonomous B. Biosynthesis of catecholamines
nervous system. The catecholamines are biogenic amines that
Several proteinogenic amino acids (see have a catechol group. Their biosynthesis in
p. 60) have neurotransmitter effects. A partic- the adrenal cortex and CNS starts from tyro-
ularly important one is glutamate, which acts sine.
as a stimulatory transmitter in the CNS. More [1] Hydroxylation of the aromatic ring ini-
than half of the synapses in the brain are tially produces dopa (3,4-dihydroxyphenyl-
glutaminergic. The metabolism of glutamate alanine). This reaction uses the unusual
and that of the amine GABA synthesized from coenzyme tetrahydrobiopterin (THB). Dopa
it (see below) are discussed in more detail on (cf. p. 6) is also used in the treatment of Par-
p. 356. Glycine is an inhibitory neurotransmit- kinson’s disease.
ter with effects in the spinal cord and in parts [2] Decarboxylation of dopa yields dopa-
of the brain. mine, an important transmitter in the CNS. In
Biogenic amines arise from amino acids by dopaminergic neurons, catecholamine syn-
decarboxylation (see p. 62). This group in- thesis stopsatthispoint.
cludes 4-aminobutyrate (γ-aminobutyric [3] The adrenal gland and adrenergic neu-
acid, GABA), which is formed from glutamate rons continue the synthesis by hydroxylating
and is the most important inhibitory trans- dopamine into norepinephrine (noradrena-
mitter in the CNS. The catecholamines norepi- line). Ascorbic acid (vitamin C; see p. 368)
nephrine and epinephrine (see B), serotonin, acts as a hydrogen-transferring coenzyme
which is derived from tryptophan, and hista- here.
mine also belong to the biogenic amine group. [4] Finally, N-methylation of norepineph-
All of them additionally act as hormones or rine yields epinephrine (adrenaline). The
mediators (see p. 380). coenzyme for this reaction is S-adenosylme-
Peptides make up the largest group among thionine (SAM; see p.110).
the neurosecretions. Many peptide hormo-
nes—e. g., thyroliberin (TRH) and angiotensin The physiological effects of the catechol-
II—simultaneously act as transmitters. Most amines are mediated by a large number of
neuropeptides are small (3–15 AA). At their different receptors that are of particular inter-
N-terminus, many of them have a glutamate est in pharmacology. Norepinephrine acts in
residue that has been cyclized to form the autonomic nervous system and certain
pyroglutamate (5-oxoproline, <G), while the areas of the brain. Epinephrine is also used
C-terminus is often an acid amide (–NH 2 ). as a transmitter by some neurons.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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