Fig 5.

Synthesis of sym-homospermine and of hypusine. sym-Homospermine can be formed from two molecules of putrescine or from putrescine and spermidine as shown in (a). In both cases, NAD is needed for hydrogen extraction, but is regenerated in the second half of the reaction. The reaction using spermidine and putrescine, which also generates 1,3-diaminopropane, is similar to that used for the hypusine modification of eIF5A shown in (b). In this case, a lysine residue in eIF5A is used instead of putrescine forming deoxyhypusine in eIF5A and free 1,3-diaminopropane. A second enzyme hydroxylates the deoxyhypusine to form the complete hypusine modification (see Chapters 12 and 13 for more details of the reactions forming hypusine).