28. Note:
Cystine: is a dicysteine
In proteins, the -SH groups of two cysteines can become
oxidized to form a dimer cystine, which contains a covalent
cross-link called a disulfide bond (-S-S-).
Selenocysteine: Selenocysteine is an L-α-amino acid found in
proteins Humans contain approximately two dozen
selenoproteins that include certain peroxidases and
reductases, which circulates in the plasma, and the
iodothyronine deiodinases responsible for converting
thyroxine (T4) to the thyroid hormone 3,3'5-triiodothyronine
(T3)
29. A selenium atom replaces the sulfur of its elemental
analog, cysteine.
Selenocysteine is not the product of a posttranslational
modification, but is inserted directly into a growing
polypeptide during translation. Selenocysteine thus is
commonly termed the “21st amino acid.”
Unusual tRNA called tRNASec which utilizes the UGA
anticodon that normally signals STOP. However, the
protein synthetic apparatus can identify a
selenocysteine-specific UGA codon by the presence of
an accompanying stem-loop structure, called the
selenocysteine insertion element, in the untranslated
region of the mRNA
30. D-Amino Acids
D-Amino acids that occur naturally include free
D-serine and D-aspartate in brain tissue, D-
alanine and D-glutamate in the cell walls of
gram-positive bacteria, and D-amino acids in
certain peptides and antibiotics produced by
bacteria and fungi