Methionine, homocysteine và cysteine được liên kết theo chu trình methyl hóa (Hình , bên trái một phần) và con đường xuyên lưu huỳnh (Hình , phần bên phải). Chuyển đổi methionine vào tiền thu được homocysteine qua adenosyltransferase S-methionine (enzyme 4). Điều này mang lại | 274 Chapter 21 Disorders of Sulfur Amino Acid Metabolism Metabolism of the Sulfur-Containing Amino Acids Methionine homocysteine and cysteine are linked by the methylation cycle o Fig. left part and the trans-sulfuration pathway o Fig. right part . Conversion of methionine into homocysteine proceeds via methionine S-adenosyltransferase enzyme 4 . This yields S-adenosylmethionine the methyl-group donor in a wide range of transmethylation reactions a quantitatively important one of which is glycine N-methyl-transferase enzyme 5 . These reactions also produce S-adenosylhomocysteine which is cleaved to adenosine and homocysteine by S-adenosylhomocysteine hydrolase enzyme 6 . Depending on a number of factors about 50 of available homocysteine is recycled into methionine. This involves methyl transfer from either 5-methyl-tetrahydrofolate THF catalyzed by cobal-amin-requiring 5-methyl THF-homocysteine methyltransferase enzyme 2 or betaine catalyzed by betainehomocysteine methyltransferase enzyme 3 . Homocysteine can also be condensed with serine to form cystathionine via a reaction catalyzed by pyridoxal-phosphate-requiring cystathionine P-synthase enzyme 1 . Cystathionine is cleaved to cysteine and a-keto-butyrate by another pyridoxal-phosphate-dependent enzyme y-cystathionase enzyme 7 . The last step of the trans-sulfuration pathway converts sulfite to sulfate and is catalyzed by sulfite oxidase enzyme 8 which requires a molybdenum cofactor. Protein 4-Methylthio- Cystathionine J -Ketobutyrate Cysteine so4 o Fig. . Metabolism of the sulfur-containing amino acids. 1 cystathionine p-synthase 2 5-methyltetrahydrofolate-homo-cysteine methyltransferase 3 betaine-homocysteine methyltrans ferase 4 methionine S-adenosyltransferase 5 glycine N-methyl-transferase 6 S-adenosylhomocysteine hydrolase 7 y-cystathio-nase 8 sulfite oxidase 275 21 Homocystinuria due to Cystathione p-Synthase Deficiency Several inherited defects are known in the conversion of the .
