220 RESPIRATION 



to anticipate that for some it may be a major pathway of respiration or 

 of pentose biogenesis. It is also clear that in some organisms both the 

 oxidative and the Embden-Meyerhof systems are present and presum- 

 ably complete for glucose metabolism (64). It is to be expected that 

 cells in different physiological states will strike different balances 

 between the two systems. 



7. THE NON-PHOSPHORYLATIVE OXIDATION OF GLUCOSE 



The single reaction of this type known is the oxidation of free 

 glucose to gluconic acid (the first product is in fact glucono-8-lactone) 

 by the enzyme usually described as glucose oxidase or notatin. It is 

 debatable whether this reaction can be classed as respiratory, since 

 we know neither the subsequent metabolic history of the gluconic 

 acid nor the way in which energy from the oxidation can be used by 

 the cell. However, it seems likely that further study will reveal a 

 genuine respiratory pathway. 



The enzyme responsible for the oxidation of free glucose was dis- 

 covered by D. Muller (205, 206) in Aspergillus Jiigcr and named 

 glucose oxidase; earlier results of Maximov (193) probably involve 

 this enzyme. Interest in it was revived by the later discovery that an 

 antibiotic effect of Penicillium resticulosum and P. notatum is trace- 

 able to the hydrogen peroxide generated by the reaction: 



Glucose + <3 2 + H 2 -» gluconic acid + H 2 2 (9) 



As an antibiotic, and before its identity with the enzyme was known, 

 the material was variously named penicillin B, notatin, and penatin. 



The enzyme is a flavoprotein with two flavin adenine dinucleotide 

 groups per molecule (158). It is highly specific for /3-D-glucose; purified 

 preparations are virtually inactive toward other common sugars (160), 

 although 2-deoxyglucose is oxidized (267). The specificity has been 

 taken advantage of in a manometric assay for glucose (159) and in the 

 demonstration of the mutarotase of Penicillium notatum (161). 



Studies with isotopic oxygen show that the enzyme transfers hy- 

 drogen from glucose to oxygen (15). It is therefore a dehydrogenase 

 rather than an oxidase, and the name glucose aerodehydrogenase is 

 to be preferred. 



The enzyme differs from the glucose dehydrogenases of liver (120) 

 and of Pseudomonas fluorescens (330), neither of which is a flavo- 

 protein. Claims that other glucose dehydrogenases are present in 

 fungi (207, 224, 225) have been disputed (91), and it appears that only 

 one such enzyme is involved in the organisms studied. 



