472 FURTHER EVOLUTION 



flavoproteins act as true oxidases, i.e. they transfer the hydro- 

 gen which they receive directly to molecular oxygen. 



("other Qccepror) 



>> 



r 



pyridine nucleotide— > FLAVOPROTEIN 



D 



Substrate »► FLAVOPROTEIN — »-cytochromes->02 



FLAVOPROTEIN 



Fig. 43. Role of the flavoproteins in 



electron transport (after Mahler). 



A. DPNH-oxidase ; DPNH-cytochrome reductase ; 

 TPNH-cytochrome reductase ; xanthine oxidase ; 

 nitrate, nitrite, hydroxylamine reductases ; dia- 

 phorase ; old, new yellow enzymes ; quinone 

 reductases. B. Lactic oxidase ; aldehyde oxidase ; 

 xanthine oxidase ; butyryl-CoA dehydrogenase ; 

 hydrogenase ; succinic dehydrogenase ; sulphite 

 oxidase. C. Amino-acid oxidases ; glucose oxidase ; 

 amine oxidases (?). 



During the transfer of hydrogen from one link of the 

 respiratory chain to another high-energy bonds are formed. 

 According to F. Lipmann^^^ this is brought about by phos- 

 phorylation which proceeds in accordance with the following 

 scheme : 



XH2 + Y->X + YH2 

 YH2 -I- H3P04-^YH2 — H2PO3 

 YH2 — H0PO3 4- Z^Y — H2PO3 + ZH2 

 Y — H0PO3 -f ADP->Y -t- ATP 



where XH2 and z are successive members of the chain of 

 respiratory reactions and y is a third substance which acts 

 as an intermediary in the transfer of hydrogen from xHj to 

 z (xHo -f z-^x + ZH2). As we can see, this leads to the forma- 

 tion of one high-energy bond by the formation of atp from 



ADP. 



On the basis of his work with isolated mitochondria E. 

 Slater^^' puts forward a somewhat different scheme of oxida- 

 tive phosphorylation 



XH2 + Z + Y^=^X f^ Y + ZH2 

 X v*Y + H3PO4 + ADP^^^X + Y + ATP 



