Introduction
Metabolism is very central to evolution. To understand evolution it is necessary to understand metabolism. It is today performed by enzymes that are either free or bound to membranes. The latter is common for energetic metabolism in aerobic organisms. Free enzymes have a large benefit. They are not dependent upon membrane area. This limitation of membranous metabolism is often overcome by having another metabolic system as reserve when large effects are needed. Therefore e.g. humans use anaerobic metabolism when performing strenuous exercises. Anaeobic respiration may however also be maintained to survive long terms of anoxic conditions. An example is yeast, which under anoxic conditions produce alcohol and CO2 instead of water and CO2 as waste products.

Metabolism in aerobic organisms consists of two main branches connected by a ring. The branches are glycolysis and the electron transport chain. The ring is a closed reaction chain, called Krebs cycle or citric acid cycle. One of these must have been the original. To me it seems obvious that the first energy source must have been glycolysis. Nick Lane has come to another conclusion. He argues that Krebs cycle is the original. One reason glycolysis must be the original is that it is the only path that exists in all organisms. It is exactly the same for all eukaryotes and eubacteria, but (some?) archaebacteria have another variant, as if they have constructed their own metabolism.

Another reason glycolysis is primitive is that synthesis of all genetic components branch from it. And if we take a look at the reactions taking place and the components, we see similarities to the genetic components. The latter consist of phosphates and purines or pyrimidines connected by a sugar group. Glycolysis consists of reactions on a sugar group connected to phosphates (SIMPLE co-enzymes/helpers??). Krebs cycle, on the other hand, is based on the end product of glycolysis, pyruvate, which is further processed, using more complex helper? molecules, like CoA (SJEKK).

Anaerobic organisms do not have any electron transport chain, and even though they have many of the reactions that constitute Krebs cycle, they do not necessarily use them in a cyclic manner.

CoA is used in Krebs cycle. That is a quite complex RNA-phosphate based enzyme. (?)

I will show how organelles improved metabolism, that two types of organelles were created, one accepting electrons, the other accepting hydrogen. The former organelle type also gave rise to organelles that donated electrons.

Glycolysis
The initial metabolism was glycolysis. It produces energy and electrons. The figure below shows how glycolysis converts one glucose molecule to two pyruvate molecules, producing two energy units (E) and four electrons. Initially glycolysis was probably performed by RNA based catalysts, ribozymes. The coenzymes are still to a large degree based on RNA and phosphates:

Extended glycolysis
More energy and electrons may be achieved by extending the reaction chain so that two acetate molecules are the waste:

Fermentation
The metabolism shown above is not electrically neutral, so it cannot be used unless it is combined with other reactions that bind electrons. There is a reaction chain that produces succinate via oxaloacetate:

The problem is that the succinate pathway consumes energy, and only each third pyruvate molecule could now use the efficient acetate pathway. Due to the energy used to produce oxaloacetate the net energy from one glucose molecule in this case is only 1.33 E.

A better way is to consume the electrons while proucing produce one ethanol and one carbon dioxide molecule per pyruvate molecule. This is shown in the figure below:

A way to get rid of electrons: Fe-S respiration
As we have seen, release of electrons may consume a lot of energy. I will however in the following show how electrons can be released without any energy consumption. Actually the release can instead increase the energy yield. The clue is to find a suitable electron acceptor. The biosphere was slowly becoming less reductive. Among the first electron acceptors that became available was FeS2. Used as an oxidant it could be reduced to FeS + H2S. FeS2 could be available in the environments, but a commuting organelle could reach them, as shown in this figure:

In the figure below the FeS complex that performs the electron export is shown in an excerpt of the inner membrane of the organelle:

Fe-S structures work as electron transport chains in this organelle. It is here shown as commuting. But I will show that later variants of this organelle, that I will call organelle type “B” (blue), can also work as stationary.

Hydrogenosome (proton respiration)
Two electrons can however become a hydrogen molecule, H2, if two protons (H+) are supplied. They will always be available in sufficient amounts the interior fluid, more the lower the pH. As H2 is a small, neutral molecule, it will readily pass through the plasma membrane to the environments. The commuting organelle with the Fe-S structures was a good starter to create the hydrogen producing entity. It was therefore built as a B organelle, as shown here together with the acetate pathway:

Methanosome
Even though hydrogen passes through the membrane, hydrogen concentration inside the cell can become so high that reaction ceases. A way around this problem is hydrogen consumption. Hydrogen could e.g. react with CO2 to become CH4. A new organelle was invented to perform this reaction, the “methanosome”. In the same way as hydrogenosomes produce hydrogen, the new, “A” type organelle (red) produces methane by reducing carbon dioxide. If CH4 concentration is not too high, this reaction may even yield energy. In combination with the acetate pathway the reaction would be as seen in this figure:

As we shall see later, the two organelles at different stages of evolution were commuting to the environments, and sometimes they were given enough autonomy that they could survive for long periods, in some cases even without their host. And in many cases their host became extinct, and the organelle continued to reproduce. As the autonomous organelle could not evolve so much when they had no host to update their genes, they became largely non-evolving, free-living organisms. Today, we know the A organelles as archaebacteria (named Archaea by Woese). The free-living variants of the B organelles became the eubacteria (named Bacteria by Woese).

There is an alternative that releases ten instead of eight electrons:

Extra electrons now give rise to extra energy. Used with the methanosomes and hydrogenosomes the yield is as shown here:

But as we see, the extra energy from the electrons is by far not enough to weigh up for the low energy efficiency of this pathway. It was maintained mostly due to its anabolic use. Intermediates in this pathway are used to synthesize important molecules, e.g. several amino acids. But this pathway may be extended with further reduction reactions. The succinate pathway that was shown above, which consumed electrons, can be run in the opposite direction if there is a red-ox reaction that could consume electrons from a succinate-fumarate reduction. Sulphur has this ability, and a special organelle could be used in much the same way as the FeS organelle above:

The two other electrons could be consumed with the hydrogenosome/methanosome cooperation. But this pathway has a much greater potential. If combined with the succinate pathway above, glucose could be completely broken down to CO2 and electrons after two rounds in the cycle, and in combination with use of the organelles this would be the result:

This cycle is known as the Krebs cycle. The hydrogenase activity could be internal to the organelle, but it could also be incorporated in the membrane, as shown in the membrane excerpt, with a hydrogenase (H2) and a sulfur (S) complex in the inner membrane:

In this figure an alternative metabolism is also shown. Ethanol fermentation (dotted) has been maintained as a reserve pathway to be used when there is no access to sulfur. Krebs cycle has even greater potential. As will be shown in continuation of the evolution of metabolism, in the next post: “Metabolism scenario continuation”, much more energy can be released by each electron consumed.