Effects of oxygenic condition on 5-ALA biosynthesis
5-ALA biosynthesis via the Shemin pathway requires succinyl-CoA as one of the two key precursors. Several metabolic pathways are involved in succinyl-CoA formation in E. coli , i.e., reductive TCA branch, oxidative TCA cycle, and glyoxylate shunt (Fig. 1). These metabolic pathways, along with cell growth and acetogenesis, can be sensitive to the oxygenic condition, which critically directs the dissimilated carbon flux toward succinyl-CoA for 5-ALA biosyntheses. To investigate such oxygenic effects, batch cultivation of the control strain DMH in a bioreactor was subject to three different levels of aeration, i.e., AL-I (microaerobic), AL-II (semiaerobic), and AL-III (aerobic) (Fig. 4). While cell growth and glycerol consumption were favored by oxygen exposure, 5-ALA biosynthesis was more effective under lower aeration levels, i.e., 0.53 g l-1 (3.40% yield) under AL-I, 0.39 g l-1 (2.44% yield) under AL-II, and 0.31 g l-1 (2.08% yield) under AL-III, presumably because that more carbon was directed toward the succinyl-CoA node under microaerobic conditions. In addition, the reduced 5-ALA biosynthesis under aerobic conditions occurred with less porphyrin formation, reflected by a significantly less pigmentation of the culture medium. Note that, in addition to low levels of 5-ALA biosynthesis in DMH under all investigated culture conditions, acetate was the major side metabolite with high yields up to 75.9%, implying a significant carbon spill at the acetyl-CoA node. The results suggest that 5-ALA biosynthesis in DMH was favored by lower oxygenic levels and potentially limited by acetogenesis.