Study Set Content:
• Found in certain peroxidases and reductases where it participates in the catalysis of electron transfer reactions
Selenocysteine
As its name implies, a (blank) atom replaces the sulphur of its structural analog, cysteine
selenium
• Unlike the 20 genetically encoded amino acids, selenocysteine is not specified by a simple
three-letter codon
22nd amino acid:
Pyrrolysine
is lysine in which a pyrroline ring is linked to the end of the lysine side chain
• Pyrrolysine
identified in a small number of methanogenic archaea and a few other microbes
Pyrrolysine
have hydroxyl side chains; found in few connective tissue proteins such as collagen
Hydroxyproline and hydroxylysine
Collagen • primary amino acid sequence
glycine-proline-X or glycine-Xhydroxyproline
Composed of 3 chains:
triple helix
Provides structural support to the extracellular space of connective tissues
Collagen
comprises 90% of the collagen in the human body
• Type I collagen
Collagen Synthesis occurs in
fibroblasts
differs from tyrosine in that it has an extra iodinecontaining aromatic group on the side chain.
Thyroxine
Thyroxine is found in the thyroid gland where it is formed from posttranslational modification of tyrosine residues in the protein
thyroglobulin
is then released as a hormone by proteolysis of thyroglobulin.
Thyroxine
Tyrosine to
Thyroxine (T4)
Thyroxine to
Triiodothyronine(T3)
Triiodothyronine(T3) to
Reverse T3 (inactive)
• There is an internal transfer of a hydrogen ion from the -COOH group to the - NH2 group to leave an ion with both a negative charge and a positive charge. This is called a
zwitterion
• Is a molecule that has a positive charge(+) on one atom and a negative charge (-) on another atom, but which has a net charge of 0.
zwitterion
