The Growth of Saccharide Macromolecules 



Shlomo Hestrin 



Department of Biological Chemistry, The Hebrew University, 

 Jerusalem, Israel 



In the context of a symposium on " Biological structure and function", 

 it is surely appropriate to consider processes of polysaccharide synthesis 

 which directly underlie striking morphological change in living organisms. 

 Levan and dextran synthesis from sucrose and bacterial cellulose synthesis 

 afford instructive examples of reactions in this class. The syntheses of 

 these polymers proceed often to levels of product concentration at which 

 a polymer-rich aqueous phase separates out in the surround of the cells. 

 The ability for extracellular polymer synthesis appears to be so firmly 

 established in the genetic make-up of widely different species, that one 

 cannot but wonder whether some important biological function is not 

 fulfilled by this property. Experiments reported in this symposium by P. A. 

 Albertsson [i] are of interest in this connection. He has discovered that 

 when phases arise in a mixed aqueous solution of appropriate species of 

 macromolecules, particles dispersed in the system are often partitioned 

 between the phases in a highly selective manner, and that particle size 

 and shape affect this distribution very markedly. When phases separate 

 out in a living system in the wake of polysaccharide synthesis, a similar 

 specific partition pattern must occur. Thus a means may be afforded in 

 the evolutionary process whereby potent macromolecular agents could be 

 selectively concentrated or excluded from any separated phase. Such a 

 phenomenon might have particular importance in the coacervate systems 

 to which a prominent role has been assigned in a recent hypothesis 

 concerning the nature of the primaeval milieu in which biological structure 

 originated [2]. 



Size of the radical transferred from donor to the growing polymer 

 chain in syntheses of levan and dextran 



Reactions catalyzed by a range of carbohydrases are now^ known to 

 consist in the transfer of a glycosyl rather of a glvcosido group from the 

 donor to an oxygen atom in the acceptor. If a change in substrate structure 

 at an atom position close to that at which the bond breakage is to occur 



