Interactions among substrates and inhibitors of nitrogenase

Abstract

Examination of interactions among various substrates and inhibitors reacting with a partially purified nitrogenase from Azotobacter vinelandii has shown that: nitrous oxide is competitive with N2; carbon monixide and acetylene are noncompetitive with N2; carbon monoxide, cyanide, and nitrous oxide are noncompetitive with acetylene, whereas N2 is competitive with acetylene; carbon monoxide is noncompetitive with cyanide, whereas azide is competitive with cyanide; acetylene and nitrous oxide increase the rate of reduction of cyanide. The results are understandable if nitrogenase serves as an electron sink and substrates and inhibitors bind at multiple modified sites on reduced nitrogenase. It is suggested that substrates such as acetylene may be reduced by a less completely reduced electron sink than is required for the six-electron transfer necessary to reduce N2.

Effect of sulfur dioxide (SO2) on growth and physiological activity in Alnus sieboldiana at Miyakejima Island in Japan

“…Nitrogenfixers in the sample trees were collected by taking 10 cm of the roots. They were placed in a 120-ml vial inside which 10 % of the air was replaced with acetylene (99.9 %) (Hardy et al 1968;Rivera-Ortiz and Burris 1975;Shah et al 1975). The vial was incubated underground for 1 h, and the ethylene gas that had been emitted was analyzed by gas chromatography (G-3000, Hitachi, Tokyo, Japan).…”

Hydrogen metabolism and energy costs of nitrogen fixation

“…Nitrogen fixation is always linked to proton reduction, and consequently to hydrogen production [5]. The hydrogen production by nitrogenase can vary (see below) but a minimal stoicheiometry of 1 mol hydrogen produced per mol nitrogen fixed is generally accepted [6][7][8][9], which gives the overall reaction given by Eqn. 1 (1) In addition to nitrogen and protons, nitrogenase catalyses the reduction of a number of alternative substrates, e.g., acetylene, cyanide, azide, nitrous oxide, alkyl isocyanides and hydrazine.…”

“…As mentioned above, non-physiological substrates can also be reduced by nitrogenase, and are often used to study nitrogenase reactivity [6,[38][39][40]. Azide [40], cyanide [39] and acetylene [24] bind to the MoFe-protein when the enzyme is in a less reduced state than necessary for binding of the nitrogen molecule (before E 3 in Fig.…”

Relationship between nitrogenase and hydrogenase activity in a rhizosphere diazotroph

“…where n and n', h and h', are the respective electron fractions used in nitrogenase or hydrogenase activity under N 2 and C2H2. Such linear relations result from Lineweaver and Burk's transformation of Michaelis-Menten equations: it ensues that the reduction rate of N 2 (or C2H2), as well as the inhibition rate of H 2 evolution are fist-order functions of N 2 (or C2H2) pressure and respond to one single enzymic kinetics [2].…”

“…It has long been known that enzyme nitrogenase also acts as an irreversible ATP-dependant hydrogenase reducing H ÷ to H2 [1 ]. This hydrogenase activity is diversely inhibited by the enzyme substrates [2], and for example by N 2 and C2H2 which would then bind with different sites. The picture is further complicated by the occurrence of a conventional ATP-independant hydrogenase which is either unidirectional, as in different aerobes [3][4][5] where it reoxidizes H 2 and recovers wasted reducing power, or reversible, as in Clostridium, but mainly directed towards H2 production [6].…”