# Homocysteine Metabolism

**Homocysteine** can be metabolized down two different pathways: (1) methylation to **methionine**, (2) or transsulfuration to **cystathionine** with the eventual formation of **cysteine.**

Production of **methionine** from homocysteine is catalyzed by **methionine synthase**, which requires the [**Vitamin B9 (Folate)**](/content/view/3695/index.html)-derived **5-MTHF** and [**Vitamin B12 (Cobalamin)**](/content/view/3764/index.html). Therefore, decreases in either folate or cobalamin can lead to increases in homocysteine (see [Vitamins B9 and B12 Deficiencies](/content/view/3950/index.html)). Notably, 5-MTHF is regenerated by **MTHFR**, and defects in MTHFR can lead to excess homocysteine and homocystinuria (see Homocystinuria - coming soon).

Production of **cystathionine** and eventually **cysteine** occurs through a different set of reactions. Namely, **cystathionine synthase** combines homocysteine and **serine** to produce cystathionine, using [**Vitamin B6 (Pyridoxine)**](/content/view/3763/index.html) as a cofactor. The cystathionine is then cleaved to produce **cysteine**.

Find this Homocysteine Metabolism mnemonic and more [Biochemical Pathways](/content/browse#playlist-4028/index.html) mnemonics among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.