The following Figure superimposes the PCO cycle (blue arrows), and alternative pathways of Ser synthesis potentially operating under non-photorespiratory conditions (green arrows), upon the 1-C metabolism, adenosine salvage cycle, the SMM cycle (red arrows) and the "methionine salvage" cycle: methylthioadenosine (MTA) ---> methylthioribose (MTR) ----> methylthioribose-phosphate (MTR-P) ---> MTOB (methylthiooxobutyrate, or 2-oxo-4-methylthiobutyrate, or alpha-ketomethylthiobutyrate [KMTB]).

1. Ribulose-bisphosphate carboxylase/oxygenase (Rubisco) [EC 4.1.1.39]

2. Phosphoglycolate phosphatase [EC 3.1.3.18]

3. Glycolate oxidase [EC 1.1.3.15]

4. Glyoxylate reductase [EC 1.1.1.79] (see also #10, NADH-hydroxypyruvate reductase, and glycerate dehydrogenase)

5. Catalase [EC 1.11.1.6]

6. Glutamate(alanine):glyoxylate aminotransferase [EC 2.6.1.44]

7. Serine(asparagine):glyoxylate aminotransferase [EC 2.6.1.45]

8. Glycine decarboxylase (glycine cleavage system P-protein) [EC 1.4.4.2], Glycine decarboxylase (glycine cleavage system T-protein) [EC 2.1.2.10], Glycine cleavage system L-protein (lipoamide dehydrogenase) [EC 1.8.1.4], Glycine cleavage system H-Protein

9. Serine hydroxymethyltransferase [EC 2.1.2.1]

10. Glycerate dehydrogenase [EC 1.1.1.29] or Hydroxypyruvate reductase [EC 1.1.1.81]

11. Glycerate kinase [EC 2.7.1.31]

12. 3-Phosphoglycerate phosphatase [EC 3.1.3.38]

13. Serine:pyruvate aminotransferase [EC 2.6.1.51]

14. 3-Phosphoglycerate dehydrogenase [EC 1.1.1.95]

15. Phosphoserine aminotransferase (Glutamate:phosphohydroxypyruvate aminotransferase) [EC 2.6.1.52]

16. Phosphoserine phosphatase [EC 3.1.3.3]

17. Glutamine synthetase [EC 6.3.1.2]

18. Ferredoxin-dependent glutamate synthase [EC 1.4.7.1]

19. Formate dehydrogenase (NAD+) [EC 1.2.1.2] or Formate dehydrogenase (NADP+) [EC 1.2.1.43]

20. 10-Formyl-THF synthetase (SYN) [EC 6.3.4.3]

21. 5,10-Methenyl-THF cyclohydrolase (CYC) [EC 3.5.4.9]

22. 5,10-Methylene-THF dehydrogenase (NADP+) (DHY) [EC 1.5.1.5] or 5,10-Methylene-THF dehydrogenase (NAD+) (DHY) [EC 1.5.1.15]

23. 5,10-Methylene tetrahydrofolate reductase (NADPH) [EC 1.5.1.20] or 5,10-Methylene tetrahydrofolate reductase (FADH2) [EC 1.7.99.5]

24. Methionine synthase [EC 2.1.1.14]

25. Methionine adenosyltransferase (AdoMet synthetase) [EC 2.5.1.6]

26. Methyltransferases [miscellaneous EC numbers]

27. Adenosylhomocysteinase (AdoHcy hydrolase) [EC 3.3.1.1]

28. Methionine S-methyltransferase [EC 2.1.1.12]

29. Homocysteine S-methyltransferase [EC 2.1.1.10]

30. ACC synthase [EC 4.4.1.14], or AdoMet decarboxylase [EC 4.1.1.50] + Spermidine synthase (putrescine aminopropyltransferase) [EC 2.5.1.16] or Spermine synthase (spermidine aminopropyltransferase) [EC 2.5.1.22]

31. Methylthioadenosine phosphorylase [EC 2.4.2.28] or Methylthioadenosine nucleosidase [EC 3.2.2.16] plus Methylthioribose kinase

32. see: Myers and Abeles (1990) and Furfine and Abeles (1988).

33. Methionine: glyoxylate aminotransferase [EC 2.6.1.73]. Note that in Klebsiella pneumoniae a tyrosine aminotransferase [EC 2.6.1.5] catalyzes the final step of methionine recycling (Heilbronn et al, 1999).

34. Adenosine kinase (adenosine 5'-phosphotransferase) [EC 2.7.1.20]

35. Adenylate kinase [EC 2.7.4.3]

Furfine ES, Abeles RH 1988 Intermediates in the conversion of 5'-S-methylthioadenosine to methionine in Klebsiella pneumoniae. J. Biol. Chem. 263: 9598-9606.

Heilbronn J, Wilson J, Berger BJ 1999 Tyrosine aminotransferase catalyzes the final step of methionine recycling in Klebsiella pneumoniae. J. Bacteriol. 181: 1739-1747.

Myers RW, Abeles RH 1990 Conversion of 5-S-methyl-5-thio-D-ribose to methionine in Klebsiella pneumoniae. Stable isotope incorporation studies of the terminal enzyme reactions in the pathway. J. Biol. Chem. 265: 16913-16921.