268 



GERTRUDE E. GLOCK 



D-Ribose 



H 



D-Xylose ^=^ 



ATPiE 

 i 



D-Xylose-5-P 



L-Arabinose 



■ D-Xylulose 



ATP 



ATP 



ATP 



D-Ribose-5-P 

 1 



D-Ribulose-5-P 



CH3CHO 



Deoxyribose-5-P 



to 



Deoxyribose-1-P 



D-Arabinose ~ > " D-Ribulose 

 G 



Fig. 7. Interrelationships of pentoses and pentose phosphates. Key to enzymes: 



A. Phosphoribose isomerase, B. Phosphoribomutase, C. Phosphodeoxyribomutase, 



D. Ribokinase, E. Xylokinase, F. Ribulokinase, G and H. Pentoseisomerases, I. In 



presence of pentose phosphate-splitting enzyme and DR aldolase. 



These pentose and pentose phosphate interrelationships are shown in 

 Fig. 7. 



VII. Metabolism of Pentoses 

 1. In Microorganisms 

 a. Oxidation of pentoses 



A few bacteria oxidize free pentoses with the formation of the corresponding pen- 

 tonic acids. This has been reported for certain acetic acid bacteria by Bertrand,'' 

 who obtained quantitative conversion of xylose to xylonic acid, and by Herman and 

 Neuschul,'' who obtained pentonic acids from D-arabinose and rhamnose. The for- 

 mation of pentonic acid from D-arabinose and D-xylose has also been reported for 

 certain Fusaria.^'''' Higuchi et a/.'"' found that L-arabinose was oxidized by a strain 

 of Brucella melitensis first to arabonic acid and then to the keto acid, which accum- 

 ulated. With D-xylose, oxidation was more rapid and proceeded beyond the keto 

 acid stage. There is so far no indication that these oxidations involve the formation 

 of phosphorylated intermediates. 



Dickens''* investigated the oxidation and fermentation of pentoses and pentose 

 phosphates by yeast extracts and found that only the phosphorylated pentoses were 

 attacked. D-Ribose-5-phosphate was attacked much more readily than either D-arab- 

 inose-5-phosphate or D-xylose-5-phosphate, and oxidation was considered to proceed 

 by successive oxidations and decarboxylations through phosphopentonic acid and a 

 tetrose phosphate (see Fig. 1). The observation of Barker and Lipmann'"* that ery- 

 thritol is phosphorylated and oxidized by propionic acid bacteria offers some sup- 



98 G. Bertrand, Compt. reyid. 127, 124 (1898). 



99 S. Herman and P. Neuschul, Biochem. Z. 233, 129 (1931). 

 i»« A. Hayasida, Biochem. Z. 298, 169 (1938). 



>»' K. Higuchi, T. H. Sanders and C. R. Brewer, Federation Proc. 10, 197 (1951). 

 i»2 H. A. Barker and F. Lipmann, J. Biol. Chem. 179, 247 (1949). 



