Kinetic studies on electron transfer and interaction between nitrogenase components from Azotobacter vinelandii

Abstract

Kinetic properties of electron transfer by nitrogenase of Azotobacter vinelandii are dependent on the concentration of the two components of nitrogenase. An excess of the MoFe protein inhibits electron transfer in a distinctive manner, and the inhibition is reversed by increasing levels of reductant. The saturation curve for Fe protein is hyperbolic, indicating that only one Fe protein molecule per MoFe protein is required for full activity in ATP hydrolysis and electron transfer. These results can be interpreted on the basis of a complex between the Fe protein and the MoFe protein that dissociates rapidly during turnover. Both 2:1 and 1:1 complexes (Fe-MoFe) are active. Dithionite appears to be a relatively poor reductant for nitrogenase from Azotobacter vinelandii, whereas azotobacter flavodoxin is much better. In the presence of the flavodoxin it is possible to increase the specific activity of the Fe protein more than 50% relative to its activity with dithionite alone as a reductant; specific activities greater than 3000 nmol of C2H4 formed min(-1) (mg of Fe protein)(-1) have been observed.