The amino acid, homocysteine, is basically a cysteine with an added methylene bridge. Methionine, an essential amino acid, gives rise to homocysteine, a sulfur-containing amino acid, through the methionine cycle. Methionine is converted to S-adenosyl methionine (SAM) when an adenosine group from ATP is added on to activate methionine’s methyl group. SAM is a methyl donor that is a substrate used in many reactions. When it donates the methyl group, SAM is converted to S-adenosyl homocysteine (SAH). SAH is rapidly converted to homocysteine by removal of the adenosine group. Homocysteine is an important intermediate and can take a couple of pathways. It can either be converted back to methionine in a transmethylation reaction or be used to make other sulfur-containing amino acids.
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Firstly, a methyl group from methyltetrahydrofolate (MTHF) can be taken by methionine synthase and the cofactor vitamin B12 and be added on to homocysteine giving back methionine and tetrahydrofolate (THF). Secondly, the intermediate betaine could add a methyl group onto homocysteine to make methionine. This occurs in the liver, and the enzyme that catalyzes this reaction is betaine-homocysteine methyltransferase. Betaine is converted to N,N-dimethyl glycine as a result. Homocysteine can go into the transsulfuration pathway and ultimately become cysteine in the liver. This pathway is done in two steps. In the first step, serine is also used as a reactant, and the reaction is catalyzed by the enzyme cystathionine β-synthase and its cofactor Vitamin B6. The intermediate formed is cystathionine, and water is also released. In the second step, the reaction is catalyzed by cystathionine γ-synthase and B6. The water released in the first step is used up in this
Firstly, a methyl group from methyltetrahydrofolate (MTHF) can be taken by methionine synthase and the cofactor vitamin B12 and be added on to homocysteine giving back methionine and tetrahydrofolate (THF). Secondly, the intermediate betaine could add a methyl group onto homocysteine to make methionine. This occurs in the liver, and the enzyme that catalyzes this reaction is betaine-homocysteine methyltransferase. Betaine is converted to N,N-dimethyl glycine as a result. Homocysteine can go into the transsulfuration pathway and ultimately become cysteine in the liver. This pathway is done in two steps. In the first step, serine is also used as a reactant, and the reaction is catalyzed by the enzyme cystathionine β-synthase and its cofactor Vitamin B6. The intermediate formed is cystathionine, and water is also released. In the second step, the reaction is catalyzed by cystathionine γ-synthase and B6. The water released in the first step is used up in this