Biosynthesis of Homocysteine -A complex biochemical process indeed !!!! Homocysteine is biosynthesized from methionine via a multistep process. In the presence of S- adenosylmethionine synthetase (B 12 coenzyme) methionine receives an adenosine group from ATP . This produces S- adenosylmethionine . Activated SAM is the key methyl donor for numerous biological reactions such as those involving 1) neurotransmitter molecules 2) hormones 3) phospholipids , 4) DNA and 5) proteins
- Adenosylmethionine transfers the methyl group to an acceptor molecule for instance, norepinephrine as an acceptor during epinephrine synthesis ; DNA methyltransferase as an intermediate acceptor in the process of DNA methylation gets converted to S- adenosyl homocysteine which further gets hydrolyzed to L- homocysteine. This methionine / Hcy cycle works in synchrony with the folate pathway , which is regulated by methyltetrahydrofolate reductase gene.
- Homocysteine may convert back to L- methionine by remethylation using N5- methyl tetrahydrofolate as the methyl donor and cobalamin related enzymes or convert to L- cysteine. Alternately , hcy can also be remethylated into methionine by betaine – homocysteine methyltransferase . Here the methyl group is provided by betaine that is transformed into dimethyl glycine. This alternative pathway seems to be limited to the liver.
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