Substrate Specificity of an Aminopropyltransferase and the Biosynthesis Pathway of Polyamines in the Hyperthermophilic Crenarchaeon Pyrobaculum calidifontis

The facultative anaerobic hyperthermophilic crenarchaeon Pyrobaculum calidifontis possesses norspermine (333), norspermidine (33), and spermidine (34) as intracellular polyamines (where the number in parentheses represents the number of methylene CH2 chain units between NH2, or NH). In this study, the polyamine biosynthesis pathway of P. calidifontis was predicted on the basis of the enzymatic properties and crystal structures of an aminopropyltransferase from P. calidifontis (Pc-SpeE). Pc-SpeE shared 75% amino acid identity with the thermospermine synthase from Pyrobaculum aerophilum, and recombinant Pc-SpeE could synthesize both thermospermine (334) and spermine (343) from spermidine and decarboxylated S-adenosyl methionine (dcSAM). Recombinant Pc-SpeE showed high enzymatic activity when aminopropylagmatine and norspermidine were used as substrates. By comparison, Pc-SpeE showed low affinity toward putrescine, and putrescine was not stably bound in its active site. Norspermidine was produced from thermospermine by oxidative degradation using a cell-free extract of P. calidifontis, whereas 1,3-diaminopropane (3) formation was not detected. These results suggest that thermospermine was mainly produced from arginine via agmatine, aminopropylagmatine, and spermidine. Norspermidine was produced from thermospermine by an unknown polyamine oxidase/dehydrogenase followed by norspermine formation by Pc-SpeE.