Figure 2.

Reactions catalyzed by HSS (A) and DHS (B) in their metabolic background. Both enzymes transfer an aminobutyl moiety of spermidine to their individual substrates in an NAD⁺-dependent reaction. (A) HSS catalyzes the first step in PA biosynthesis, generating homospermidine as the precursor of the necine base, the typical backbone structure of PAs. (B) DHS commences the posttranslational activation of eIF5A by modifying a protein-bound lysine residue to deoxyhypusine, which is hydroxylated in a second step to hypusine by deoxyhypusine monooxygenase.