Spermidine synthase is an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
() that
catalyzes the transfer of the propylamine group from
''S''-adenosylmethioninamine to
putrescine
Putrescine is an organic compound with the formula (CH2)4(NH2)2. It is a colorless solid that melts near room temperature. It is classified as a diamine. Together with cadaverine, it is largely responsible for the foul odor of putrefying flesh, b ...
in the biosynthesis of
spermidine
Spermidine is a polyamine compound () found in ribosomes and living tissues and having various metabolic functions within organisms. It was originally isolated from semen.
Function
Spermidine is an aliphatic polyamine. Spermidine synthase (SPDS) ...
. The systematic name is S-adenosyl 3-(methylthio)propylamine:putrescine 3-aminopropyltransferase and it belongs to the group of aminopropyl transferases. It does not need any cofactors. Most spermidine synthases exist in solution as dimers.
Specificity
With exception of the spermidine synthases from ''Thermotoga maritimum'' and from ''Escherichia coli'', which accept different kinds of polyamines, all enzymes are highly specific for putrescine. No known spermidine synthase can use
''S''-adenosyl methionine. This is prevented by a conserved
aspartatyl residue in the active site, which is thought to repel the carboxyl moiety of ''S''-adenosyl methionine. The
putrescine-N-methyl transferase whose substrates are putrescine and ''S''-adenosyl methionine and which is evolutionary related to the spermidine synthases lacks this aspartyl residue. It is even possible to convert the spermidine synthase by some mutations to a functional putrescine-N-methyltransferase.
Mechanism
It is assumed that the synthesis of spermidine follows the
Sn2 mechanism. There is some uncertainty if the reaction occurs via a
ping-pong
Table tennis, also known as ping-pong and whiff-whaff, is a sport in which two or four players hit a lightweight ball, also known as the ping-pong ball, back and forth across a table using small solid rackets. It takes place on a hard table div ...
or via a
ternary-complex mechanism. Some kinetic data, but not all, suggest a ping-pong mechanism, while the investigation of the stereochemical path of the reaction argues for a ternary-complex mechanism.
Prior to the nucleophilic attack of the putrescine onto the ''S''-adenosylmethioninamine the putrescine has to be deprotonated rendering the nitrogen nucleophilic since the putrescine is protonated at physiological pH and is therefore inactive.
Inhibitors
The spermidine synthase can be inhibited by a wide variety of analogues of putrescine, ''S''-adenosyl methioninamine and transition state analogues as Adodato (for further information se
here
See also
*
Spermine synthase
Spermine synthase (, ''spermidine aminopropyltransferase'', ''spermine synthetase'') is an enzyme that converts spermidine into spermine. This enzyme catalyses the following chemical reaction
: S-adenosylmethioninamine + spermidine \rightlefthar ...
*
Adenosylmethionine decarboxylase
The enzyme adenosylmethionine decarboxylase () catalyzes the conversion of ''S''-adenosyl methionine to ''S''-adenosylmethioninamine.
Polyamines such as spermidine and spermine are essential for cellular growth under most conditions, being imp ...
References
External links
Spermidine synthaseat BRENDA
Spermidine synthaseat ExPASy
*
EC 2.5.1
{{2.5-enzyme-stub