Page 71 - Color Atlas of Biochemistry
P. 71
62 Biomolecules
Non-proteinogenic amino acids The ε-amino group of lysine residues is sub-
ject to a particularly large number of modifi-
In addition to the 20 proteinogenic amino cations. Its acetylation (or deacetylation) is an
acids(seep. 60),there arealso manymore important mechanism for controlling genetic
compounds of the same type in nature. These activity (see p. 244). Many coenzymes and
arise during metabolic reactions (A)or as a cofactors are covalently linked to lysine resi-
result of enzymatic modifications of amino dues. These include biotin (see p.108), lipoic
acid residues in peptides or proteins (B). The acid (see p.106), and pyridoxal phosphate
“biogenic amines” (C) are synthesized from α- (see p.108), as well as retinal (see p. 358).
amino acids by decarboxylation. Covalent modification with ubiquitin marks
proteins for breakdown (see p.176). In colla-
gen, lysine and proline residues are modified
A. Rare amino acids
by hydroxylation to prepare for the formation
Only a few important representatives of the of stable fibrils (see p. 70). Cysteine residues
non-proteinogenic amino acids are men- form disulfide bonds with one another (see
tioned here. The basic amino acid ornithine p. 72). Cysteine prenylation serves to anchor
is an analogue of lysine with a shortened side proteins in membranes (see p. 214). Covalent
chain. Transfer of a carbamoyl residue to or- bonding of a cysteine residue with heme oc-
nithine yields citrulline.Both of these amino curs in cytochrome c.Flavins are sometimes
acids are intermediates in the urea cycle (see covalently bound to cysteine or histidine res-
p.182). Dopa (an acronym of 3,4-dihydroxy- idues of enzymes. Among the modifications of
phenylalanine) is synthesized by hydroxyla- tyrosine residues, conversion into iodinated
tion of tyrosine. It is an intermediate in the thyroxine (see p. 374) is particularly interest-
biosynthesis of catecholamines (see p. 352) ing.
and of melanin. It is in clinical use in the
treatment of Parkinson’s disease. Selenocys-
teine, a cysteine analogue, occurs as a compo- C. Biogenic amines
nent of a few proteins—e. g., in the enzyme Several amino acids are broken down by de-
glutathioneperoxidase (seep. 284). carboxylation. This reaction gives rise to what
are known as biogenic amines, which have
various functions. Some of them are compo-
B. Post-translational protein modification
nents of biomolecules,suchas ethanolamine
Subsequent alteration of amino acid residues in phospholipids (see p. 50). Cysteamine and
in finished peptides and proteins is referred b-alanine are components of coenzyme A (see
to as post-translational modification. These re- p.12) and of pantetheine (see pp.108, 168).
actions usually only involve polar amino acid Other amines function as signaling substan-
residues, and they serve various purposes. ces. An important neurotransmitter derived
The free α-amino group at the N-terminus from glutamate is γ-aminobutyrate (GABA,
is blocked in manyproteinsbyan acetyl res- see p. 356). The transmitter dopamine is also
idue or a longer acyl residue (acylation). N- a precursor for the catecholamines epineph-
terminal glutamate can cyclize into a pyroglu- rine and norepinephrine (see p. 352). The bio-
tamate residue, while the C-terminal carbox- genic amine serotonin, a substance that has
ylategroup can bepresent in an amidated many effects, is synthesized from tryptophan
form (see TSH, p. 380). The side chains of ser- via the intermediate 5-hydroxytryptophan.
ine and asparagine residues are often linked Monamines are inactivated into aldehydes
to oligosaccharides (glycosylation, see p. 230). by amine oxidase (monoamine oxidase,
Phosphorylation of proteins mainly affects “MAO”) with deamination and simultaneous
serine and tyrosine residues. These reactions oxidation. MAO inhibitors therefore play an
have mainly regulatory functions (see p.114). important role in pharmacological interven-
Aspartate and histidine residues of enzymes tions in neurotransmitter metabolism.
are sometimes phosphorylated, too. A special
modification of glutamate residues, J-carbox-
ylation, is found in coagulation factors. It is
essential for blood coagulation (see p. 290).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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