Methionine spermidine synthesis

In addition to its role in the synthesis of proteins and the polyamines spermidine and spermine, the main metabolic role of methionine is as a methyl donor in a wide variety of biosynthetic reactions. 

Putrescine is the precursor for synthesis of spermine and spermidine by reaction with decarboxylated S-adenosyl-methionine. 

Plants, in common with microorganisms and animals, require methionine chiefly for three roles, (a) As a component of protein, a role which accounts for most of the methionine in the cell, (b) As a component of methionyl tRNA (in eukaryotes) and formylmethionyl tRNA (in chloroplasts, mitochondria, and prokaryotes), factors required for initiation of protein synthesis. (c) As a component of AdoMet, the chief biological methyl donor, the obligatory precursor of spermidine and spermine, and an effector of certain enzymes. In addition to these chief roles, a major pathway for the metabolism of methionine in certain plant tissues is its conversion to ethylene (see Yang and Adams, this series, Vol. 4, Chapter 6). Only plants and microorganisms can synthesize the homocysteine moiety of methionine novo, and the importance of this synthesis in the sulfur cycle has been noted in the introduction. 

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