Methioninsyklusen

The Methionine cycle, also known as the transmethylation cycle, is a fundamental metabolic pathway in most organisms. It is closely linked to the folate cycle and plays a crucial role in methylation reactions. The cycle begins with the activation of methionine to S-adenosylmethionine (SAM). SAM is a high-energy compound that serves as the primary methyl group donor for a wide array of biological methylation reactions. These reactions are essential for numerous cellular processes, including DNA and RNA synthesis, protein synthesis and modification, neurotransmitter synthesis, and the metabolism of various compounds.

Following the transfer of its methyl group, SAM is converted into S-adenosylhomocysteine (SAH). SAH is then hydrolyzed by the enzyme S-adenosylhomocysteine hydrolase to homocysteine and adenosine. Homocysteine is a key intermediate in the cycle. It can either be remethylated back to methionine, thus regenerating the starting compound of the cycle, or it can enter the transsulfuration pathway. The remethylation of homocysteine to methionine is primarily carried out by methionine synthase, a reaction that requires vitamin B12 as a cofactor and a methyl group typically donated from 5-methyltetrahydrofolate, a product of the folate cycle. This interdependency highlights the tight regulation between the methionine and folate cycles. The overall purpose of the methionine cycle is to provide methyl groups for essential cellular functions while also managing homocysteine levels, which can be toxic if allowed to accumulate.