Role of B12 and Folate in metabolic pathways

Homocystinuria

Cystathionine beta-synthase deficiency is the enzyme defect present in classic homocystinuria.

(Cysteine becomes essential amino acid in affected patients, and homocysteine buildup leads to elevated methionine (Hypermethioninemia).

Note cofactor pyridoxine (B6) is also required in both steps of homocysteine --> cysteine pathway.

S/S

ectopia lentis, mental retardation, marfanoid habitus and osteoporosis in addition to hypercoagulability and thromboembolic occlusion

individuals with complete cystathionine synthase deficiency can develop premature acute coronary syndrome.

Affected patients cannot form cysteine from homocysteine; therefore, cysteine is essential in their diet.