Protection of Nitrogenase against Damage by

Because nitrogenase proteins are oxygen-sensitive (Section I,A,2), mechanisms must exist to either exclude O2 from the vicinity of the enzyme or, alternatively, make the susceptible sites 02-tolerant (Dalton and Postgate, 1969a). Both these mechanisms do exist although the latter (conformational protection) has only been shown in Azotobacter. These mechanisms have been discussed exhaustively in earlier reviews (Postgate, 1971, 1974 Yates 

This is a simple concept respiration will increase as a respcmse to oxygen to maintain an O2 concentration of zero at the cell surface. Excess oxygen stops nitrogenase activity (see Fig. 11) and bacterial growth. On the other hand, bacterial growth on fixed nitrogen is not inhibited by excess O2 in 

The phenomenon of switch off in Azotobacter is probably due to autoxi-dation of Azotobacter flavodoxin hydroquinone (Yates and Jones, 1974), i.e., a simple competition for electrons between O2 and nitrogenase. Since the hydroquinone reacts very rapidly with O2 (f a = 12 ms, Yates and Jones, 1974), the 2Fe2S protein must oxidize rapidly to combine with and protect the Fe protein after sudden exposure to excess O2. 

It is not uncommon for bacteria to produce slime (polysaccharide) as a response to excess O2 (Wilkinson, 1958). Hill (1971) was first to show that the slime production associated with massive colony formation of Derxia gummosa as induced by a low oxygen regime. At 0.05 atm O2 abundant massive colonies were observed on agar but only a few were present at O2 = 0.2 atm. The massive colonies fixed N2 whereas the small colonies were inactive probably because excess O2 inhibited N2 fixation. Slime production can be shown to protect N2-fixing M. flavum against O2. When the polysaccharide was removed from the medium the O2 concentration for maximal acetylene reduction was 0.02 atm compared with 0.025 atm in the presence of slime. A gum-free strain at the same cell concentration had an optimal O2 concentration of 0.015 atm. Presumably extracellular polysaccharide diminished the 02-solution rate and capsular polysaccharide may also have impeded O2 uptake into the cell (Yates, 1977). 

Wilcockson (1977) could find no correlation between nitrogenase activity and slime production in high or low slime-producing strains of Klebsiella 

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