Chirality in Biology
587
COOH
1,2
COOH
O
3
COOH
O
COOH
O
4
Fig. 7
Biosynthesis of jasmonic acid from
α
-linolenic acid via an
allene oxide. Enzymes are (1) lipoxygenase; (2) allene oxide synthase;
(3) allene oxide cyclase; (4) reduction and
β
-oxidation.
a profound action on the inactivation rate;
(
R
)-allenes are up to 200-fold more potent
than the (
S
)-allenes.
2.4
Amino Acids
Our biological world is often described
as homochiral – thus, the protein amino
acids belong
to
the
L
conFgurational
series.
If
only
weight
is
considered,
this is certainly true since the tonnage
of
L-amino
acids found in protein is
overwhelming; about 18% of the wet
weight
of
a
mammal
is
protein.
Of
the 20 ‘‘classical’’ amino acids involved
in ribosomal protein synthesis, glycine
is
achiral,
17
contain
a
single
chiral
center at the
α
-position, and threonine
and isoleucine contain a second chiral
center. The
necessary aminoacyl-tRNA
ligase enzymes are generally very speciFc
for L-amino acids during protein synthesis.
When examined
in vitro
, some reactivity
with
D-amino acids has been observed;
for example, the tyr-tRNA ligase from
Escherichia coli
can utilize D-tyrosine.
The so-called 21st ribosomally incor-
porated
amino
acid
is
selenocysteine,
HSe
−
CH
2
−
CH(NH
2
)
−
COOH, found in
bacteria, eukaryotes, Archae, and animals.
Its
formation
is
complex
involving
a
unique material, tRNA
Sec
.T
h
i
si
sl
i
g
-
ated with L-serine and the L-seryl-tRNA
Sec
is converted to selenocysteinyl-tRNA
Sec
with the aid of selenophosphate. Chi-
rality
at the
α
-position
of
L-serine
is
apparently lost in this pyridoxal phosphate-
dependent process, but it is usually as-
sumed that the selenocysteine in protein
has the L-conFguration. A 22nd ribosoma-
lly incorporated amino acid, pyrrolysine,
present in certain Archae and eubacteria, is
HOOC
−
CH(NH
2
)
−
(CH
2
)
4
−
NH
−
CO
−
R
(R
is
apparently
(4
R
,5
R
)-4-substituted-
pyrroline-5-carboxylate). The R group is
attached via the carboxyl group at the
ε
–NH
2
of lysine. The substituent at po-
sition 4 could be methyl, ammonium, or
hydroxyl. This amino acid is clearly of the
L-conFguration, derived from L-lysine. The
pyrroline ring structure may be related to
D-proline.
Some posttranslational racemization of
L
-t
o
D-amino acids is possible in pro-
teins. In long-lived proteins, such as lens
crystallins, tooth dentine and enamel, and
brain myelin, aspartate and serine racem-
ize at a rate of about 0.15% per annum
(for aspartate). This process can be used
to determine the age of fossil proteins and
of long-lived animals if care is taken to
