Pathway: cysteine biosynthesis III (mammalia)

In humans, |FRAME: CYS| is synthesized from |FRAME: MET|, an essential amino acid that must be provided in food |CITS: [3309559]|. More accurately, the cysteine sulfur is derived from methionine whereas the carbon and nitrogen of cysteine are derived from |FRAME: SER| |CITS: [3309559]|. L-Methionine is converted to |FRAME: HOMO-CYS| by the actions of |FRAME: CPLX-5142| (EC 2.5.1.6), various S-adenosylmethionine methyltransferases, (EC 2.1.1.-) and |FRAME: CPLX-5161| (EC 3.3.1.1). |FRAME: HOMO-CYS| occupies a branch point in methionine metabolism. In human, about half of the homocysteine formed is irreversibily converted by transsulfuration to |FRAME: CYS|, |FRAME: 2-OXOBUTANOATE|, and |FRAME: AMMONIA| as desribed in this pathway, whereas the remainder is remethylated to |FRAME: MET| |CITS: [4593752][1128236]|. |FRAME: CPLX-5162| (EC 4.2.1.22) is a PLP-dependent enzyme that catalyzes the condensation of |FRAME: HOMO-CYS| and |FRAME: SER| to form |FRAME: L-CYSTATHIONINE|. Finally, |FRAME: L-CYSTATHIONINE| is converted to |FRAME: CYS| by |FRAME: CPLX-5163| (EC 4.4.1.1).