This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Degradation/Utilization/Assimilation → Amino Acids Degradation → Proteinogenic Amino Acids Degradation → L-methionine Degradation|
In the degradation pathway shown here, L-methionine is first metabolized to S-adenosyl-L-methionine (SAM), a compound that mediates most biological methylations. In the second reaction, the S-methyl group is transferred to other acceptors in a transmethylation step. The product, S-adenosyl-L-homocysteine, is then hydrolyzed to L-homocysteine and adenosine.
L-homocysteine is a branch point. In rats and humans approximately half of the homocysteine formed is remethylated back to methionine via two salvage pathways, one involving methionine synthase (EC 220.127.116.11), and the other involving betaine-homocysteine S-methyltransferase (EC 18.104.22.168) (see L-methionine salvage from L-homocysteine).
The remainder of the homocysteine is degraded further. The first step is the condensation with serine to form L-cystathionine. cystathionine γ-lyase catalyzes cleavage of the C-γ-S bond of L-cystathionine, yielding L-cysteine, 2-oxobutanoate and ammonia in a transsulfuration reaction (see L-cysteine biosynthesis III (from L-homocysteine)) [Griffith87, Finkelstein90].
Brzezinski01: Brzezinski K, Janowski R, Podkowinski J, Jaskolski M (2001). "Sequence determination and analysis of S-adenosyl-L-homocysteine hydrolase from yellow lupine (Lupinus luteus)." Acta Biochim Pol 48(2);477-83. PMID: 11732617
Chattopadhyay91: Chattopadhyay MK, Ghosh AK, Sengupta S (1991). "Control of methionine biosynthesis in Escherichia coli K12: a closer study with analogue-resistant mutants." J Gen Microbiol 137(3);685-91. PMID: 2033383
Chiang77: Chiang PK, Cantoni GL (1977). "Activation of methionine for transmethylation. Purification of the S-adenosylmethionine synthetase of bakers' yeast and its separation into two forms." J Biol Chem 1977;252(13);4506-13. PMID: 194884
Gomi85: Gomi T, Ishiguro Y, Fujioka M (1985). "S-Adenosylhomocysteinase from rat liver. Evidence for structurally identical and catalytically equivalent subunits." J Biol Chem 260(5);2789-93. PMID: 3972804
Gonzalez03: Gonzalez B, Pajares MA, Hermoso JA, Guillerm D, Guillerm G, Sanz-Aparicio J (2003). "Crystal structures of methionine adenosyltransferase complexed with substrates and products reveal the methionine-ATP recognition and give insights into the catalytic mechanism." J Mol Biol 331(2);407-16. PMID: 12888348
Hu99: Hu Y, Komoto J, Huang Y, Gomi T, Ogawa H, Takata Y, Fujioka M, Takusagawa F (1999). "Crystal structure of S-adenosylhomocysteine hydrolase from rat liver." Biochemistry 38(26);8323-33. PMID: 10387078
Kamarthapu08: Kamarthapu V, Rao KV, Srinivas PN, Reddy GB, Reddy VD (2008). "Structural and kinetic properties of Bacillus subtilis S-adenosylmethionine synthetase expressed in Escherichia coli." Biochim Biophys Acta 1784(12);1949-58. PMID: 18634909
Lee97a: Lee JH, Chae HS, Lee JH, Hwang B, Hahn KW, Kang BG, Kim WT (1997). "Structure and expression of two cDNAs encoding S-adenosyl-L-methionine synthetase of rice (Oryza sativa L.)." Biochim Biophys Acta 1354(1);13-8. PMID: 9375784
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