SCHEME 1.

Proposed alditol biosynthetic pathway in U. ceramboides. Metabolic scheme showing glycolysis and the pentose phosphate pathway. Biosynthetic pathway leading to threitol is in bold. Bold numbers indicate alternative metabolism pathways of E4P discussed under “Discussion.” Question marks indicate uncertainty associated with the mechanism of conversion of E4P to T4P. Eol4P, erythritol 4-phosphate; G6P, glucose 6-phosphate; F6P, fructose 6-phosphate; GAP, glyceraldehyde phosphate; Ru5P, ribulose 5-phosphate; Xu5P, xylulose 5-phosphate; Tol4P, threitol 4-phosphate. Ribulose 5-phosphate may first be isomerized to ribose 5-phosphate or reduced directly and dephosphorylated to form ribitol.