Ribose-5-phosphate yields obtained from 6PG with different cysteine analogues.

A total of 800 μM 6PG was incubated at 70°C in a 50 mM phosphate solution pH 5 containing 400 μM of cysteine analogue. (A) Representative chromatograms obtained by LC–SRM after 6-hour incubation under control conditions without amino acids (−), or in the presence of serine or cysteine, showing how serine, unlike cysteine, has no significant effect on product formation versus control conditions (N = 3 independent experiments) (see S1 Data). (B) The concentration of R5P formed after 6 hours is shown—relative to the 400 μM cysteine condition—for isomers or close homologues bearing the same functional groups (i.e., D-cysteine, DL-homocysteine and reduced glutathione (GSH)), structural analogues differing in one or several functional groups (3-mercaptopropionic acid, cysteamine, β-mercaptoethanol and L-serine), and oxidized cysteine derivatives (L-cystine, cysteine sulfonic acid and oxidized glutathione (GSSG)), evidencing the importance of the thiol group. Error bars represent mean ± SD (N = 3) (see S1 Data and Supporting information Note in S1 Text). 6PG, 6-phosphogluconate; LC–SRM, liquid chromatography–selective reaction monitoring; R5P, ribose-5-phosphate.

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