Homospermidine synthase of Rhodopseudomonas viridis: Substrate specificity and effects of the heterologously expressed enzyme on polyamine metabolism of Escherichia coli.

Abstract

Homospermidine synthase (HSS) catalyses the formation of the polyamine homospermidine from 2 mol of putrescine. The general and kinetic properties of purified HSS from Rhodopseudomonas viridis are given and compared with those of the respective enzymes from other sources. The R. viridis enzyme is shown to catalyse a number of side reactions: (I) In the presence of putrescine or spermidine as donors of the 4-aminobutyl moiety, various homologous diamines are transformed into the respective N-(4-aminobutyl)derivatives. (II) In the absence of putrescine, spermidine as a substrate yields homospermidine, putrescine and diaminopropane as reaction products. The mechanism of the reactions catalysed by HSS and its role in the formation of uncommon bacterial polyamines are discussed. Overexpression of the homospermidine synthase (hss) gene in Escherichia coli revealed the formation of two HSS-products, homospermidine and N-(4-aminobutyl)-cadaverine, which are absent from wild-type E. coli. Expression of the hss gene in E. coli does not dramatically affect the pool concentrations of the cellular polyamines.