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 Thermus thermophilus 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.