254 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Another mode of formation is the condensation of C^ and Cg fragments to 

 form phosphoric esters of the pentoses and desoxypentoses, particularly 

 of desoxyribose. 



This reaction 



glyceraldehyde-3-P + acetaldehyde ^ desoxyribose-5-P 



is a reversible aldol condensation, catalysed by desoxyribose-phosphate 

 aldolase. This enz3nne has been prepared from E. colt and numerous animal 

 tissues; the desoxyribose-5-P formed in the reaction is incorporated into 

 nucleosides. 



As far as the metabolism of the pentose phosphates is concerned, the 

 principal reactions are those of transaldolization and transketolization. 



A transketolase has been isolated from plant tissues and from animal 

 tissues, it catalyses the follow^ing reaction : 



ribulose-5-P + ribose-5-P ^ sedoheptulose -7-P + glyceraldehyde-3-P 



The enzyme is of low^ specificity and it also acts on ribulose-5-P, sedo- 

 heptulose-7-P, L-erythrulose, hydroxypyruvate and fructose-6-P. A rupture 

 of the ketol bond occurs and the "active glycolaldehyde" formed is 

 condensed w^ith an acceptor aldehyde. When the acceptor is glyceralde- 

 hyde 3-P, ribulose-5-P is formed. 



(d) Biosynthesis of Oside Linkages 



Although, theoretically, the enzymes catalysing the hydrolysis of oside 

 linkages should be capable of catalysing the reverse reaction of synthesis, 

 this is not considered to occur in practice. 



In the case of sucrose, for example, the action of invertase causes the 

 reaction to go almost to completion from left to right. One of the substances 

 participating in the reaction, water, being present in overwhelming con- 

 centration, the hydrolysis of sucrose is in practice irreversible. 



The synthesis of the osides operates, in fact, through their phosphory- 

 lated derivatives. 



In plants, sucrose results from the condensation and the simultaneous 

 dephosphorylation of a molecule of phosphorylated glucose and a molecule 

 of phosphorylated fructose, in the presence of sucrose phosphorylase. A 

 system exists, and has been isolated from various bacteria, which, in the 

 presence of G — 1 — P and D-fructose, condenses these two molecules to 

 form sucrose vnth the elimination of phosphoric acid. This sucrose phos- 

 phorylase can combine glucose with, say, an aldose such as L-arabinose with 

 formation of a 1-3 oside linkage. Its specificity for the second half of the 

 molecule is hence seen to be of a low order. On the other hand, the sucrose 

 phosphorylase can utilize other sources than G— 1 — P as a donor of glucose 

 for the formation of sucrose. It is therefore also a transglucosidase capable of 

 transferring glucose derived from various donors, to a diversity of acceptors. 



