70 Perspectives in Microbiology 



oxidizes gluconate to 2-ketogluconate, which is then further 

 oxidized to 2,5-diketogluconate (Figure 1). The last com- 

 pound is chemically unstable and is converted, in part, into 

 the characteristic brown pigment of this species. 2-keto- 

 gluconate also is a major product of glucose oxidation 

 by several Pseudomonas species. 



The unusual feature of this type of glucose oxidation is 

 that it involves only nonphosphorylated compounds. Al- 

 most no information is available at present about the 

 further steps in the oxidation of the various ketogluconates 

 or about the role of these compounds in the energy metab- 

 olism of the bacteria. Enzymes are known that phosphory- 

 late gluconate and 2-ketogluconate, but it is not known 

 whether such phosphorylation is an obligatory require- 

 ment for the complete oxidation of these compounds. 



Rihulose Phosphate Pathway. The existence of another 

 nonglycolytic mechanism for the direct oxidation of carbo- 

 hydrate via 6-phosphogluconate by animals and yeasts was 

 established first by the work of Warburg, Lipmann, and 

 Dickens in 1935-36. The specific reactions involved in this 

 process (Figure 2) have recently been worked out by 

 Horecker, Racker, Cohen, and others (10). 



Glucose is first phosphorylated and the resulting glucose- 

 6-phosphate is oxidized to 6-P-gluconate. This in turn un- 

 dergoes an oxidative decarboxylation to carbon dioxide and 

 ribulose-5-phosphate. Although it has not been identified, 

 3-keto, 6-phosphogluconate has been postulated as an inter- 

 mediate in the latter reaction. The ribulose-5-phosphate 

 isomerizes to give ribose-5-phosphate. The upper two car- 

 bons of ribulose-5-phosphate are next transferred to carbon 

 atom 1 of ribose-5-phosphate to give one molecule each of 

 glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate 

 (Figure 3). These compounds then react in such a way that 

 the first three carbon atoms of the sedoheptulose-phosphate 

 are transferred to triose phosphate, giving fructose-6-phos- 

 phate and a tetrose phosphate. There is now some evidence 



