INHIBITION OF NITROGEN FIXATION BY OTHER GASES 291 



group is necessary for substrate activity. The most reactive substrates 

 are dimethyl or trimethyl compounds, indicating that an exact fit of the 

 cationic head is necessary to place the hydroxyl group in position for 

 oxidation. Substitution of groups on the C-1 atom reduces the binding 

 whereas substitution on the C-2 atom increases the affinity even though 

 the groups are fairly bulky. The simplest interpretation is that the cationic 

 head anchors the molecules in position so that the CH2OH group can react 

 with an enzyme group on the opposite side of a hole or slit in the protein. 

 When the analogs are too long they do not readily fit into this region, 

 whereas groups protruding from C-2 interact by van der Waals' forces 

 with the walls of the cavity. A three-point attachment of the cationic head 

 is suggested by the reduction in affinity brought about by altering only 

 one of the R groups, this perhaps tilting the molecules so that the hydro- 

 carbon chain is not in the normal direction. The differences in binding 

 energies between these analogs are rather small and this might indicate 

 that dispersion forces are mainly involved, but it may also be that changes 

 in the electrostatic interactions (resulting from the different volumes of 

 the R groups, for example) are offset by opposite changes in the dispersion 

 energy. Since choline must be oxidized to betaine before it can serve as 

 a methyl donor, it is interesting that Wells demonstrated the inhibition 

 of methionine synthesis in liver homogenates by 2-amino-2-methyl-l- 

 propanol and its triethyl derivative. Niemer and Kohler (1957) studied 

 analogs of choline in which one of the methyl groups is substituted by var- 

 ious radicals (e.g., — CHoCH.^OH, — CHaCH^Br, — CH2CH=CH2, — CH2= 

 =CH2, and —CH2COO-) and found 10-20% inhibition of liver choline 

 oxidase at concentrations approximately equimolar with choline (11.5 mM). 

 None of these analogs is a potent inhibitor, confirming the importance of 

 fit at the cationic head. 



INHIBITION OF NITROGEN FIXATION BY OTHER GASES 



Some simple instances of competitive interference between gases in 

 nitrogen fixation and hydrogen evolution have been observed, and are 

 reminiscent of the suppression of hemoglobin oxygenation by carbon mon- 

 oxide, nitric oxide, and other gases. The primary product of nitrogen 

 fixation in microorganisms is probably ammonia, and the enzyme system 

 responsible for this is generally termed nitrogenase. A few examples of 

 inhibition are given in Table 2-4. Most of these have been shown to be 

 strictly competitive. CO and NO are the most potent inhibitors while H2 

 and NgO are relatively weak. O2 is a special case in that as pOg is increased 

 from zero the nitrogen fixation accelerates, but above a certain value, 

 depending on the organism, the rate falls off (Burris, 1956). Ethane, neon, 

 argon, and helium have no significant effects (Molnar et al., 1948). 



