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Figure 2: (a) Cobalamin metabolism and homocysteine remethylation pathway: Dietary cobalamin (Co) binds to transcobalamin (TC) and enters cells by receptor-mediated endocytosis. Certain proteins help in transport of cobalamin across the lysosomal membrane into the cytosol. Cobalamin is then processed by MMACHC and MMADHC proteins. Cellular cobalamin can then enter into two major pathways, cytosolic or mitochondrial. This sorting is done by MMADHC protein. In the cytosolic pathway, cobalamin is changed to methylcobalamin (MeCbl) after donation of a methyl group by 5-methyl tetrahydrofolate. Methylcobalamin then participates in the conversion of homocysteine (hcy) to methionine (meth). In the mitochondrial pathway, adenosylcobalamin (AdoCbl) is formed and helps in the conversion of methylmalonyl-CoA to succinyl-CoA (cobalamin remethylation defects mentioned in [] or bold headings; genes involved written in block letters). (b) Differential diagnosis of hyperhomocysteinemia. Other causes are severe B6 deficiency, renal failure, and hypothyroidism

Figure 2: (a) <i>Cobalamin metabolism and homocysteine remethylation pathway</i>: Dietary cobalamin (Co) binds to transcobalamin (TC) and enters cells by receptor-mediated endocytosis. Certain proteins help in transport of cobalamin across the lysosomal membrane into the cytosol. Cobalamin is then processed by MMACHC and MMADHC proteins. Cellular cobalamin can then enter into two major pathways, cytosolic or mitochondrial. This sorting is done by MMADHC protein. In the cytosolic pathway, cobalamin is changed to methylcobalamin (MeCbl) after donation of a methyl group by 5-methyl tetrahydrofolate. Methylcobalamin then participates in the conversion of homocysteine (hcy) to methionine (meth). In the mitochondrial pathway, adenosylcobalamin (AdoCbl) is formed and helps in the conversion of methylmalonyl-CoA to succinyl-CoA (cobalamin remethylation defects mentioned in [] or bold headings; genes involved written in block letters). (b) <i>Differential diagnosis of hyperhomocysteinemia</i>. Other causes are severe B6 deficiency, renal failure, and hypothyroidism