OXYGEN AND HYDROGEN IN BIOLOGICAL NITROGEN FIXATION

INTRODUCTION . . . . . . . . . . . . . . 1 84 INVOLVEMENT OF OXYGEN . . . . . . ...... . . . .. . . ... . . . . .. . .. . . . .. . . . 1 84 Effects of Oxygen on Nitrogen-Fixing Systems . ...... ...... ..... 184 Protection of the Nitrogen-Fixing System From Oxygen in Vivo 1 86 Mechanisms of Protection From Oxygen in Azotobacter 1 86 Respiratory protection " •... 1 86 Conformational protection 188 Ox?gen Re1J.Ulat!on of N�tr0K.enase Synthesis 190 Mlcroaeroblc Nitrogen FIXatIon . 191 Xanlhobacter (Corynebacterium) aUlolrophicus and X. (Mycobacterium) flovus 191 Azospirillaceae ... . . . . . . ........ .. . ....... ... . . . .. 191 Rhizobiaceae ...... 192 Methanotrophs 192 Phototrophs 192 Other bacteria 192 Relative oxygen tolerance 192 Conformational Protection in Microaerophilic Diazotrophs 193 Role of Gum 193 The Nodule as an Oxygen-Restricting Compartment . . . . . . . . . . ........ 193 Heterocysts as Oxygen-Restricting Compartments 194 Other Protective Strategies 194 INVOLVEMENT OF HYDROGEN 194 Hydrogen as a Substrate for Nitrogen Fixation 195 Hydrogen as an Inhibitor of Nitrogenase ..... 195 The Hydrogen-Deuterium "Exchange" Reaction 195 Hydrogen Evo/ution by Purified Nitrogenase 197 Hydrogenase in Nitrogen-Fixing Bacteria 199 Hydrogenase and the Recycling of "Waste" Hydrogen 201 Removal of inhibitory hydrogen 201 Enhancement of respiratory protection ........ . . ........ . ... ... . ...... . ...... . ...... ...... ...... ....... . .. .. 201 Provision of ATP and/or reducing power 202 CONCLUSIONS 202