How does homocysteine convert to methionine?

Homocysteine can be converted to methionine through a process called remethylation. This process involves the transfer of a methyl group from a methyl donor, such as 5-methyltetrahydrofolate or betaine, to homocysteine. This reaction is catalyzed by the enzyme methionine synthase, which requires vitamin B12 as a cofactor. The methyl group is transferred to homocysteine to form methionine, which is an essential amino acid that plays a crucial role in various biochemical processes in the body.

Homocysteine converts to methionine through a process called remethylation, involving two steps:

Step 1: Conversion of Homocysteine to S-Adenosylhomocysteine (SAH)

- Homocysteine reacts with adenosine triphosphate (ATP) and a methyl group donor, such as folate (vitamin B9), to form S-adenosylhomocysteine (SAH).
- The reaction is catalyzed by the enzyme methionine synthase reductase (MTRR).

Step 2: Conversion of SAH to Methionine

- SAH reacts with another methyl group donor, such as methylcobalamin (vitamin B12), to form methionine.
- The reaction is catalyzed by the enzyme methionine synthase (MS).

Overall Reaction:

```
Homocysteine + Folate + B12 → Methionine + SAH
```

Cofactors Required:

- Folate (methyl donor)
- Vitamin B12 (methyl donor)
- SAM (coenzyme for MS)