Radical SAM enzymes Ī large number of enzymes cleave SAM reductively to produce radicals: 5′-deoxyadenosyl 5′-radical, methyl radical, and others. In the rate-limiting step of the SAM cycle, MTHFR (methylenetetrahydrofolate reductase) irreversibly reduces 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. This methionine can then be converted back to SAM, completing the cycle. This is hydrolysed to homocysteine and adenosine by S-adenosylhomocysteine hydrolase EC 3.3.1.1 and the homocysteine recycled back to methionine through transfer of a methyl group from 5-methyltetrahydrofolate, by one of the two classes of methionine synthases (i.e. S-Adenosyl homocysteine is a strong negative regulator of nearly all SAM-dependent methylases despite their biological diversity. In the first step of this cycle, the SAM-dependent methylases (EC 2.1.1) that use SAM as a substrate produce S-adenosyl homocysteine as a product. The reactions that produce, consume, and regenerate SAM are called the SAM cycle. Only the SAM cofactor and cytosine base are shown for simplicity. methylthioadenosine (SMT), homoalanine radicalīiochemistry SAM cycle The S N2-like methyl transfer reaction.S-adenosyl homocysteine, releasing methyl radical.adenosyl radical, which converts to deoxyadenosine (AdO): classic rSAM reaction, also cogenerates methionine.Depending on the enzyme, S-adenosyl methionine can be converted into one of three products: The sulfonium functional group present in S-adenosyl methionine is the center of its peculiar reactivity. It is synthesized from ATP and methionine by S-Adenosylmethionine synthetase enzyme through the following reaction:ĪTP + L-methionine + H 2O ⇌ \rightleftharpoons phosphate + diphosphate + S-adenosyl- L-methionine S-Adenosyl methionine consists of the adenosyl cation attached to the sulfur of methionine. In plants, SAM is crucial to the biosynthesis of ethylene, an important plant hormone and signaling molecule. In eukaryotic cells, SAM serves as a regulator of a variety of processes including DNA, tRNA, and rRNA methylation immune response amino acid metabolism transsulfuration and more. In bacteria, SAM is bound by the SAM riboswitch, which regulates genes involved in methionine or cysteine biosynthesis. SAM was first discovered by Giulio Cantoni in 1952.
It is made from adenosine triphosphate (ATP) and methionine by methionine adenosyltransferase.
More than 40 methyl transfers from SAM are known, to various substrates such as nucleic acids, proteins, lipids and secondary metabolites. Although these anabolic reactions occur throughout the body, most SAM is produced and consumed in the liver. S-Adenosyl methionine ( SAM), also known under the commercial names of SAMe, SAM-e, or AdoMet, is a common cosubstrate involved in methyl group transfers, transsulfuration, and aminopropylation.
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