BIOSYNTHESIS OF PENTOSES 



269 



Cs »► COOH ► CO2, etc. 



I 

 C4 +C4 



+ c 



*► c. 



CsH" ^2 



■*Ce 



■♦ Cs+C, 



■♦Cs+Cj 



L C1+C2 



Fig. 8. Methods of pentose metabolism in microorganisms (after Lampen'"^). 



port for Dickens' hypothesis. The failure, however, to obtain oxidation of 5-phos- 

 phopentonate by partially purified liver enzymes" or by leaf extracts'"' probably 

 indicates that oxidative decarboxylation does not proceed beyond the stage of pen- 

 tose phosphate in plant and animal tissues. 



h. Fermentation of Pentose 



The general mechanisms which hav-e been proposed for pentose metabolism in 

 microorganisms are shown in Fig. 8.'"^ The most generally accepted mechanism is 

 the cleavage into C2 and C3 units. A variation of this is the secondary condensation 

 of the C2 unit either with Ci to form a C3 or with other Co fragments to give Ce , 

 which could split to C3 compounds. Addition of Ci to the C^ chain to jield Ce , with 

 subsequent cleavage to two C3 units, has also been suggested. 



Considerable evidence exists in support of cleavage into C2 and C3 units. Fred 

 et aL'"* first reported the formation of equivalent amounts of acetic and lactic acids 

 during fermentation of pentoses bj^ lactic acid bacteria. Dickens'* found that in the 

 presence of DPN, D-ribose-5-phosphate was fermented by yeast extracts producing 

 ethanol, CO2 , inorganic phosphate, and an unknown C2 compound, indicating pre- 

 liminary formation of triose phosphate and pyruvate. '"' This is supported by Rack- 

 er,'* who showed that extracts of E. coli split ribose-5-phosphate into triose phos- 

 phate and a C2 fragment. Many other examples of C2-C3 cleavage exist. ^^ ■"-**•'" 



The mechanism of fermentation of pentose bj' lactic acid bacteria has recently 

 been studied with Lactobacillus -pentosus using o-xylose-l-C" '"* and with Lactobacil- 



i"M. Gibbs, Federation Proc. 12, 208 (1953). 



1"^ J. 0. Lampen, /. Cellular Comp. Phijsiol. 41, 183 (1953). 



lo^E. B. Fred, W. H. Peterson and J. A. Anderson, /. Biol. Chem. 48, 385 (1921); 



E. B. Fred, W. H. Peterson, and A. Davenport, J. Biol. Chem. 39, 347 (1919). 

 iM F. Dickens, Brit. Med. Bull. 9, 105 (1953). 



"' R. Kaushal, P. Jowett, and T. K. Walker, Nature 167, 949 (1951); J. Marmur and 



F. Schlenk, Federation Proc. 10, 221 (1951). 



"8 J. O. Lampen, H. Gest, and J. C. Sowden, /. Bacterial. 61, 97, (1951) ; H. Gest and 

 J. O. Lampen, /. Biol. Chem. 194, 555 (1952). 



