Chemistry Reference
In-Depth Information
seryl residue of Ser-tRNA
Sec
is then converted to Sec-tRNA
Sec
. From sequencing of selenoproteins and cloned
DNAs, it was clear that Sec in selenoproteins was coded by the codon TGA (UGA in the corresponding
mRNA). How Sec is incorporated in response to an in-frame UGA codon, which should normally signal
termination of translation, informing the ribosomes not to stop at this position on the mRNA has been
progressively deciphered, first in bacteria and then in eukaryotes. This involves a complex recoding machinery,
involving a stem
loop in the 3
0
-untranslated region of all selenoprotein mRNAs, the selenocysteine insertion
sequence, or SECIS, SECIS-binding proteins (SBPs), and a number of other factors.
Figure 18.9
presents
e
FIGURE 18.9
A model for nucleocytoplasmic trafficking of SBP2. Shuttling of the SPS1/SECp43/EFSec/Sec-tRNA
Sec
complex into the
nucleus and association with SBP2 and the SECIS element are depicted. Cytoplasmic export of the SECIS-bound complex is shown on the left.
(From
Allmang et al., 2009
.
Copyright 2009 with permission from Elsevier.)
a model of how the SPS1/SECp43/EFSec/Sec-tRNA
Sec
complex depicted in
Figure 18.8
is shuttled into the
nucleus where it associates with SBP2 and the SECIS element on a selenoprotein mRNA before being
re-exported to the cytosol.
In summary, the codon TGA, which normally codes for termination of protein synthesis is programmed
from a distance to encode the “21st amino acid” selenocysteine: a special tRNA is loaded in a unique and
unorthodox way, incorporating a selenocysteine residue which is synthesised de novo at the tRNA level by
special enzymes; highly specialised proteins are required to recognise and bind the secondary mRNA struc-
tures and the tRNA
sec
; specialised elongation factors have to compete with canonical ones and with release
factors. No wonder that chemists and biochemists ask what are the unique properties of selenocysteine
compared to cysteine (
Arn´r, 2010
)
to justify the involvement of so many molecular partners (
Allmang, Wurth,
& Krol, 2009
).
Chlorine and Iodine
The halogens as a group in the Periodic Table are characterised by their presence in the biological system as the
halide anions F
,Cl
,Br
, and I
. We have already briefly discussed F
and Br
in Chapter 1 and we discuss Cl
and I
, here. The concentration of Cl
in biological systems is, in general, quite high. It is the principal ionic
component of seawater (Cl
55% compared with Na
þ
30%, SO
2
7.7%, Mg
2
þ
3.7%, Ca
2
þ
1.2%, K
þ
1.1%)
this
e
