pH-dependent activity of cysteine on the formation of different products from 6-phosphogluconate.

(A) Concentrations of sugar phosphate products detected by LC/MS after 6-hour incubation of 100 μM 6PG at 70°C in 50 mM phosphate solution at different pHs, either in the absence (purple lines) or presence (green lines) of 400 μM cysteine. The optimum pH range where the reaction enhancement by cysteine was larger is highlighted as a shaded, gray region where applicable. A control consisting of 100 μM 6PG in unbuffered (aqueous) conditions, as well as a negative control without 6PG are shown for comparison. Data shown as mean ± SD (N ≥ 3 in all conditions) (see S1 Data). (B) Same protocol was followed but in the presence of a metal ion, 200 μM FeCl₂, either without (purple lines) or with 400 μM cysteine co-present (green lines). Data shown as mean ± SD (N ≥ 3 in all conditions) (see S1 Data and Supporting information Note in S1 Text). 6PG, 6-phosphogluconic acid; LC/MS, liquid chromatography–mass spectrometry.

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