Reductant-Independent ATP Hydrolysis Catalyzed by Homologous Nitrogenase Proteins fromAzotobacter vinelandiiand Heterologous Crosses withClostridium pasteuranium

Abstract

Reductant-independent ATPase activity was initiated and studied forAzotobacter vinelandiiandClostridium pasteuraniumnitrogenase proteins (Av1, Cp1 and Av2, Cp2, 1 designating the iron molybdenum protein and 2 the iron protein) and their heterologous crosses by two methods: (1) allowing dithionite to be depleted from a normal assay in the presence of substrate levels of MgATP and (2) using reduced but reductant-free nitrogenase proteins in the presence of substrate levels of MgATP. In both cases, at a 1:1 protein ratio, MgATP is converted initially to MgADP with a specific activity of 400–500 nmol MgATP hydrolyzed/min·mg Av1, but in slower steps the MgADP is converted to AMP and, after 12 h, AMP is ultimately converted to adenosine. This reactivity requires the presence of both proteins, increases with increasing Av2/Av1 ratio, and is not a result of unique redox states of either protein. For Av1–Av2, ATP hydrolysis in the absence of Mg2+occurred at nearly the same rate as reductant-dependent MgATP hydrolysis. Reductant-independent ATPase activity also occurred for the Av1–Cp2 and Cp1–Av2 heterologous crosses and was 2-fold and 18-fold slower than the Av1–Av2 or Cp1–Cp2 combinations. In both cases further hydrolysis of MgADP to AMP and AMP to adenosine occurred. A unique nucleotide hydrolysis system is apparently operating in the complex formed between the two nitrogenase proteins in the absence of reductant. The relationship between the reductant-independent and reductant-dependent activities of nitrogenase catalysis is explored.