الفهرس 
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Abstract
I’
Selenocysteine (Sec) is recognized as the 21 st naturally occurring amino acid in
protein. The expansion of the genetic code to include Sec (Back et al., 1991a) was the first
addition to the code since the time it was deciphered and shown to be universal in the mid
1960s (Khorana et al, 1966; Nirenberg et al., 1966; Marshall et al., 1967). Sec occurs at
the active site in all Sec-containing proteins and this unique amino acid plays an important
role in catalyzing oxidation-reduction reactions (Lee et al., 1996; Low and Berry, 1996;
Stadtman 1996).
The structure of Sec is very similar to the amino acids cysteine and serine (Figure
i
I). The only difference is that Sec contains selenium in place of sul fur i~ cysteine and
. selenium in place of oxygen in serine. Another amino acid that is very similar in structure
to Sec is phosphoserine which has a phosphate group in place of selenium.
CH2SeH ClilSH CH20H CH2OP03H2
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HCNH2 HCNH2 HCNH2 HCN~
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c=o c=o c=o c=o
I I I I
0 0 0 0
I I I I
H H H H
Selenocystelne Cysteine Serine Phosphoserine
Figure I: Structures of Sec and its analogs cysteine, serine and phosphoserine.
Sec has its own tRNA, referred as Sec tRNA[Ser)Sec, that has’ been characterized in
both prokaryotes (Heider and Back, 1993; Stadtman, 1996) and eukaryotes (Hatfield et al.,
1992a; 1994).