Abstract
Euglena gracilis absolutely requires vitamin B_1 for growth because it lacks a biosynthetic pathway for the formation of the pyrimidine moiety of vitamin B_1, and possesses an active transport system for exogenous vitamin B_1. Of the total amount of vitamin B_1 taken up, as much as 96% exists in the free form. Vitamin B_1 is converted to the coenzyme by the respective thiamin pyrophosphokinase located in mitochondria, chloroplasts and cytosol. For the thiamin pyrophosphate-dependent decarboxylation of 2-oxo acids, E. gracilis lacks a pyruvate dehydrogenase complex and a 2-oxoglutarate dehydrogenase complex and, instead, contains an oxygen-sensitive pyruvate : NADP^+ oxidoreductase and 2-oxoglutarate decarboxylase, which are located in mitochondria. Pyruvate : NADP^+ oxidoreductase is operative to form acetyl-CoA from pyruvate and is associated with the wax ester fermentation, an energy-generating system under anaerobic conditions. 2-Oxoglutarate decarboxylase catalyzes the conversion of 2-oxoglutarate into succinate semialdehyde and couples with NAD^+ and NADP^+-linked isozymes of succinate semialdehyde dehydrogenase to form succinate. In addition, 2-oxoglutarate decarboxylase is the key regulatinig enzyme in the alternative tricarboxylic acid cycle and is involved in the assimilation of L-glutamate.
