Characterisation and specificity of different spermidine synthases

The polyamines spermidine, spermine and thermospermine are small molecules occurring in humans, in animals and plants, and also in bacteria and fungi. Under physiological conditions they are protonated cations allowing interactions with negatively charged macromolecules such as nucleic acids and membranes. Polyamines are involved in the cell stress response, in cell differentiation and growth processes. They also have regulatory and cell protective functions.

Polyamine metabolism is a target for therapy. Tumour cells often show deviating polyamine patterns, and Plasmodium and other pathogenic protozoa depend on polyamine uptake from internal bacteriods (apicoplasts).

The in vivo formation of the different polyamines is accomplished by several enzymes. Spermidine synthase (Fig. 1) is a key enzyme for higher polyamines transfering an aminopropyl group onto putrescine forming spermidine (Fig. 2). The cosubstrate dcSAM (decarboxylated S-adenosyl-L-methionine) serves as aminopropyl group donor.

Fig. 1: Human spermidine synthase, section of the catalytic site is shown with a, putrescine and b, dcSAM created with Pymol 1.3, based on PDB 2O0L (human spermidine synthase).

In order to target spermidine synthases from particular organisms selectively, information on substrate and cosubstrate binding is necessary. Analogues of substrate and cosubstrate, partially obtained by synthesis, are applied to explore spermidine synthase binding specificity. Active site amino acids are exchanged by site-directed mutagenesis to learn about their significance in substrate and cosubstrate binding and on the catalytic mechanism.