Synthetic Study of 8- and 9-Carbon Sugars by Transaldolase

In nature, higher carbon sugars composed of 7 or more carbons exist in limited quantities. Since some higher carbon sugars have attracted attention due to their biological activities, it is necessary to develop a strategy to synthesize them. Transaldolase catalyzes the transfer of three-carbon units from d-fructose-6-phosphate (donor) to d-erythrulose-4-phosphate (acceptor) to produce d-sedoheptulose-7-phosphate. If transaldolase can recognize nonphosphorylated monosaccharides as substrates, it can synthesize 8-carbon octuloses and 9-carbon nonuloses using nonphosphorylated pentoses and hexoses as acceptors, respectively. We performed biochemical and structural characterization of thermophilic <i>Thermus thermophilus</i> HB8 transaldolase and successfully synthesized octuloses and nonuloses using nonphosphorylated aldoses as acceptors: d-ribose (conversion rate of 74%), d-xylose (55%), l-arabinose (49%), l-lyxose (84%), d-allose (13%), d-galactose (56%), and l-altrose (71%). Products were identified by LC/MS and NMR spectroscopic analyses. X-ray structure of the enzyme showed that the wide and hydrophilic catalytic site facilitates the binding of nonphosphorylated aldoses as acceptors.