Electron Transfer in Nitrate and Sulphate Respiration 



401 



The properties of E. coli nitrate reductase of respiration type are sum- 

 marized in Table 10 in comparison with nitrate reductase of assimilation 

 type from Neurospora. The electron transport sequence of these two 

 nitrate reductase systems is shown in Fig. 4. 



(a) RespirotKxi type 



Ecoli 



2 - heptyl - 4 - hydroxy - quinoline- N - oxide 

 NaR 



cyt Z», 



Fe (40 atoms) Mo (I atom) 



FADHj . MVH 

 phenosafronine - Hg 



no; 



(b) Assinmlofion type in Neurospora 



Cyt c 



TPNH- 



NoR- 



FAD (or FMN) Mo 



•no; 



2,3,6 - trichlorandophenol 

 Fig. 4. Electron transport system leading to nitrate. 



SULPHATE RESPIRATION 



Pathway of Sulphate Reduction 



Although extracts obtained from cells of Desulphovibrio by sonic or alumina 

 destruction had ability to reduce sulphite or thiosulphate to sulphide by 

 hydrogen, they did not reduce sulphate. Recently, Ishimoto (1959) and Peck 

 (1959) found independently that sulphate was reduced to sulphide in the 

 presence of ATP in the extracts. Hydrogen uptake by the extracts, fortified 

 with methylviologen as an intermediary electron carrier, was measured with 

 Warburg manometers in a hydrogen atmosphere and the amounts of hydro- 

 gen sulphide developed and absorbed in alkaU in the centre wells were 

 determined colorimetrically (Table 11). The ratio of amounts of the added 

 sulphate, the adsorbed hydrogen and the formed sulphide was 1:3-5:1-02, 

 which indicates the complete reduction of sulphate to sulphide : 



SO4— + 4H2 = s— + 4H2O 



The formation of adenosine-5'-phosphosulphate (APS) from sulphate and 

 ATP by the extracts in the absence of hydrogen was demonstrated by radio- 

 autography. For this purpose, paper electrophoresis (ammonium acetate 

 buffer, pH 5, 3 hr, 12 V/cm) and paper chromatography (solvent: wobutyric 



