Figure 5.

Chicken-and-egg synthesis and chirality loss. (a) Schematic of the circumstance that in some micro-organisms TPP (thiamine pyrophosphate) and PLP are required for their own synthesis (see text), but also positively feedback into their own synthesis in the sense of a chemical hypercycle (Hordijk & Steel 2004). (b) Disappearance of chiral centres through aromatization in PLP biosynthesis by PdxA and PdxJ according to the mechanism proposed by Laber et al. (1999), see also Eubanks & Poulter (2003). The chiral carbon atoms a–d in substrates and products are labelled. GA3P, glyceraldehyde-3-phosphate; DX5P, 1-deoxy-xylulose-5-phosphate; 4PH-Thr, 4-(phosphohydroxy)-threonine; glu, glutamate; αkg, α-ketoglutarate; E4P, erythrose-4-phosphate; PNP, pyridoxine phosphate.