Methionine metabolism. The first step in (L)-Methionine metabolism, the synthesis ofS-Adenosyl-(L)-methionine, is catalyzed by the Methionine adenosyltransferase I,alpha ( MAT1A ) , ,  and Methionineadenosyltransferase II, alpha ( MAT2A ) , , , , . S-Adenosyl-(L)-methionine (AdoMet) is the primary methyl group donor invirtually all mammalian systems. DNA (cytosine-5-)-methyltransferase 1 ( D nmt 1 ), , DNA (cytosine-5-)-methyltransferase 3 alpha (DNMT3A )  and DNA (cytosine-5-)-methyltransferase 3 beta (DNMT3B )  naturally catalyze the transfer of the activated methylgroup from AdoMet to C5 atom of DNA cytosine with formation ofS-Adenosyl-(L)-homocysteine ( AdoHcy ). S-adenosylhomocysteine hydrolase ( SAHH ) , ,, S-adenosylhomocysteine hydrolase-like 1 ( SAHH2 )  and S-Adenosylhomocysteine hydrolase-like 2 ( SAHH3 ) catalyze the hydrolysis of AdoHcy to adenosine and (L)-Homocysteine. In turn,Cystathionine beta-synthase ( CBS ) catalyzes the condensation of(L)-Serine and (L)-Homocysteine to form (L)-Cystathionine , . Cystathionase (cystathionine gamma-lyase) ( CTH )catalyzes the second step in the reverse trans-sulfuration pathway (the cleavage of the(L)-Cystathionine C-gamma-S bond) yielding (L)-Cysteine, 2-Oxobutanoicacid, and ammonia . (L)-Methionine, as another amino acid involved in protein biosynthesis, thatforms aminoacyl-tRNA conjugated with corresponding tRNA by Methionyl-tRNA synthetase (MARS) , . Mitochondrial (L)-Methionine*(tRNA)then undergoes formylation to N-Formyl-(L)-methionine*(tRNA) by Mitochondrialmethionyl-tRNA formyltransferase ( FMT) .