Enzymatic polymerization to polysaccharides

Polysaccharides are formed by repeated glycosylations between a sugar donor and a sugar acceptor. This chapter reviews the in vitro synthesis of polysaccharides by "enzymatic polymerization." As catalyst, a hydrolase enzyme is used where the monomer was designed based on the concept of a "transition-state analog substrate" (TSAS), sugar fluoride monomers for the polycondensation, and sugar oxazoline monomers for the ring-opening polyaddition. Enzymatic polymerization enabled the first in vitro synthesis of natural polysaccharides such as cellulose, xylan, chitin, hyaluronan, and chondroitin, and also of unnatural polysaccharides such as a cellulose-xylan hybrid, a regularly methylated cellulose, and derivatives of hyaluronan and chondroitin. The polymerization mechanism is discussed here. Mutant enzymes were effective for polysaccharide synthesis, where only polycondensation of monomers was induced by suppressing the hydrolysis activity. The polymerization principle was extended to a stepwise synthesis of oligosaccharides. By using characteristics of the polymerization to produce a polysaccharide by a single step, in situ observations of the polymerization reaction by TEM, POM, SEM, and SANS revealed the formation of various high-order molecular structures from the polysaccharide molecules.