44 THE PLANT CELL WALL 



promising mechanism which has been put forth to account for 

 the close interrelationships among polysaccharide precursors is 

 described in the nucleotide theory of polysaccharide synthesis. 

 According to this concept, uridine disphosphate-glucose (UDPG) 

 and similar nucleotide sugars are the pivotal intermediates in 

 a pool of glucosyl donors for polysaccharide synthesis. 



UDPG and UDP-D-galactose are readily interconverted. UDPG 

 can be oxidized to its glucuronic acid. UDP-N-acetylglucosamine 

 is known, and has been proposed as the chitin precursor in fungi. 

 Similarly, guanosine diphosphate mannose is known in yeast 

 where it is presumed to serve as the mannan precursor. UDPG 

 itself is believed to be involved in the synthesis of cellulose in 

 Acetobacter. Uridine diphosphate glycosides in vascular plants 

 include the D-glucose, D-galactose, L-arabinose, D-xylose, and 

 D-glucuronic acid. 



Thus, interconversions among the free sugar pool, the phosphate 

 ester pool and the nucleotide glycoside pool, and the mechanisms 

 for transfer of glycosyl groups offer the outlines for a fuller under- 

 standing of wall polysaccharide synthesis in the near future. 



The chemical uniqueness of lignins among wall polymers allows 

 them to be distinguished from the much intergraded wall polysac- 

 charides, an advantageous property. Nevertheless, although the 

 outlines of lignin synthesis are clear, the broad physiological and 

 biochemical details of synthesis remain to be established. 



In the early days of biopolymer chemistry, a great deal of 

 interest was focused upon the hydrolytic enzymes which were 

 obtained from many sources with comparative ease. These enzymes 

 were recognized as catalysts for the degradation of proteins and 

 polysaccharides, but were also viewed as agents of significance 

 in synthesis as well. Although equilibria in proteinase and carbo- 

 hydrase systems are far over toward hydrolysis, some of these 

 enzymes could be used for synthesis under special conditions. 

 Accordingly, the early view that polymer synthesis was a reversed 

 hydrolysis was difficulty sustained, but seemingly necessary. 

 As we now realize, the synthesis of biopolymers can be accomplished 

 in general only with the driving force of phosphorylative mechanisms 

 and the mediation of nucleotides and other carriers. 



