Two electron mechanism may involve the direct transport of two electrons from a mononuclear transition complex to a substrate. The probability of two electron processes, however, increases sharply if two electrons are transferred together with a proton as a hydride or the change in electric charge is compensated by the simultaneous shift of the electron cloud from neighboring groups. Hydrogenases are a diverse group of metalloenzymes that catalyze the conversion of dihydrogen into protons and electrons and the reverse reaction, the generation of dihydrogen. The [NiFe] hydrogenase and the [FeFe] hydrogenase contain sulfur bridged bimetallic centers. The concept of four-electron mechanism of N2 reduction at mild conditions was first introduced by Likhtenshtein and Shilov in 1970. It was suggested that the four-electron process can occur in a multinuclear cluster of transition metals. The multielectron nature of the energetically favorable processes in clusters does not evidently impose any new additional restrictions. The need for a synchronous four-electronic elementary act with the formation of a hydrazine-like intermediate is dictated by thermodynamics. The central enzyme of biological nitrogen fixation catalyzes in the nitrogen-fixing bacteria the reduction of molecular nitrogen to ammonia by a reductant with the assistance of ATP hydrolysis: The nitrogenase active has iron-sulfur F cluster (F4S4), P-clusters (F8S7) and FeMo cofactor (F7S9Mo). After the accumulation of four and six reduction equivalents in, the reduction of u occurs first to the hydrozine derivative and then to ammonia, respectively, the reduction of u occurs first to the hydrozine derivative and then to ammonia, respectively. The role of ATP in the nitrogenase reaction was discussed. Subsequent absorption of four light quanta by Photosystem I (PS)I and photosystem II (PS II) results in evaluation of dioxygen from a two-water molecule. Photosynthetic water oxidation occurs at the oxygen-evolving complex (OEC) of (PSII) which contains a Mn4CaO5 inorganic cluster ligated by oxides, waters and amino acid residues. In 1976, Semenov, Shilov, and Likhtenstein suggested that in plant photosynthesis, the splitting of a water molecule with the release of molecular oxygen can occur only according the thermodynamically allowed four-electron mechanism.