Hypusine

Hypusine is an unusual amino acid found in all eukaryotes and in some archaebacteria, but not in eubacteria. The only known protein containing hypusine is eukaryotic translation initiation factor 5A (eIF-5A) and a similar protein found in archaebacteria. In human, two isoforms of eIF-5A have been described: eIF-5A-1 and eIF-5A-2. They are coded by two different genes. This protein is involved in protein biosynthesis and promotes the formation of the first peptide bond. The region surrounding the hypusine residue is highly conserved among the eukaryotes and is essential to the function of eIF-5A. Thus, hypusine and eIF-5A appear to be vital for the viability and proliferation of eukaryotic cells.

Hypusine is formed in eIF-5A by post-translational modification of one of the lysyl residues. There are two reactions and two enzymes involved:

• 1. Deoxyhypusine synthase catalyzes the cleavage of the polyamine spermidine and transfer of its 4-aminobutyl moiety to the ε-amino group of one specific lysine residue of the eIF-5A precursor to form deoxyhypusine and 1,3-diaminopropane.
• 2. Deoxyhypusine hydroxylase mediates the formation of hypusine by addition of a hydroxyl group to the deoxyhypusine residue.

An excess of hypusine was found in the urine of children and patients with familial hyperlysinemia.

Hypusine was first isolated from bovine brain by Japanese scientists Shiba et all. in 1971. The name hypusine indicates that the molecule comprises moieties of hydroxyputrescine and lysine.