Homospermidine synthase (spermidine-specific) () is an enzyme that catalyzes the chemical reaction that combines putrescine with spermidine to form sym-homospermidine and 1,3-propanediamine:
Unlike the enzyme homospermidine synthase found in bacteria, this protein from plants like Senecio vulgaris is not capable of using putrescine alone as substrate.
The enzyme is a transferase, specifically those transferring aryl or alkyl groups other than methyl groups. The systematic name of this enzyme class is spermidine:putrescine 4-aminobutyltransferase (propane-1,3-diamine-forming).
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