Characterization of the peptide substrate specificity of glutathionylspermidine synthetase from Crithidia fasciculata

Trypanothione, a metabolite specific to trypanosomatid parasites, is enzymatically synthesized from spermidine and glutathione by the consecutive action of the ATP-dependent carbon-nitrogen ligases, glutathionylspermidine synthetase and trypanothione synthetase. As part of our programme aimed at developing inhibitors of these enzymes, we have synthesized a series of analogues of glutathione (γ-L-Glu-L-Cys-Gly) and tested them as substrates or inhibitors of glutathionylspermidine synthetase. Recognition at the γ-glutamyl moiety appears to be essential, as any modification of this part of glutathione results in a total loss of activity as a substrate. Alkylation of the thiol side chain of cysteine wilh methyl, ethyl or propyl groups yields analogues with catalytic efficiencies (k_cat/K_m) as substrates equivalent to or better than glutathione. In contrast, the bulkier S-butyl analogue was a much poorer substrate. Substitution of L-Cys by amino acids with an alkyl side-chain is also well tolerated giving relative catalytic efficiencies of 1.1 and 1.5 for peptide analogues containing L-Val and L-Ile respectively. Other analogues, where the bulk of the alkyl chain is increased further (as in L-Leu or L-Phe) or where the glycine moiety is replaced with L-Ala, are inhibitors rather than substrates.