Discovery of the Sodium World allowed the filling of a significant gap in our knowledge concerning cellular energetics based on the Mitchellian H+ cycle scheme. This is why I have decided to formulate some general bioenergetic principles of universal applicability to living cells. In 1992, I published a paper in Eur. J. Biochem. called "The laws of cell energetics'' .
The first law was presented as follows: "The living cell avoids direct utilization of external energy sources in the performance of useful work. It transforms energy of these sources to a convertible currency, i.e. ATP, A/x_h+, or A/xh+, which is then spent to support various types ofenergy-consuming processes.'' In other words, the cell prefers to deal with energy in a money-type circulation rather than with barter. In fact, this law extended a principle put forward in 1941 by F. Lipman when he assumed that ATP is the biological energy currency . It was now postulated that similar role can be performed in some cases also by protonic or sodium potentials. The validity of the postulate was proved by numerous cases when A/xH+ or A/xNa+ were generated by some energy-releasing enzymes to be consumed by other proteins to perform all types of the useful work in the cell with no ATP involved.
The second law proved to be shorter: "Any living cell always possesses at least two energy currencies, one water-soluble (ATP) and the other membrane-linked (A/h+ and/or A/xNa+).'' Continuing with the analogy between cell bioenergetics and everyday life, this law states that the cell always has some currency in cash and some in checks. Really, there is not a single well documented and reproducible observation that a living cell uses ATP but not A/xh+ or A/xNa+ and vice versa.
The third law was also rather short: 'All the energy requirements of the living cell can be satisfied if at least one of three energy currencies is produced at the expense of external energy sources... This law can be paraphrased as it does not matter how an income is received, in cash or in checks, as long as they are interconvertible. Frequently, there are both ATP- and A/x_h+ (A/xNa+)-producing mechanisms coexisting in one and the same cell. However, some bacteria produce only one type of the currency by primary energy-releasing mechanisms. There are anaerobes using ATP-producing fermentation as the primary energy source. In this case, A/xH+ and/or A/xNa+ are generated by H+-(Na+)-ATPases. In certain respiring bacteria, respiratory substrates and O2 are the primary energy source, and A/h+ is formed to be converted to ATP by H+-ATP-synthase and to A/xNa+ by the Na+/H+-antiporter. In other bacteria, like Propioni-genium modestum, A/xNa+ is primarily formed by, say, Na+-motive decarboxylase. As to ATP, it is produced by Na+-ATP-synthase.
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