Why do we have oxygen in the air?

[shkrobius]

We have oxygen, because our plants, alga and other obligate phototrophs split water to O2 releasing four electrons and four protons in the process. The protons are used to make ATP to power the cell, while the electrons are used to recharge the porphyrin in the reaction center where light separates charges and to fix CO2; oxygen is waste. It is a

Comments

[combinator30]

I see a weak point of this theory in the statement that Chl replaced BChl in the RCs. There are at least two independent hints that it was vice versa...

[shkrobius]

True, but it is a secondary point that does not invalidate the general logic. The main point is that Fe(II) oxidation provides at least three preadaptations required for water splitting photosynthesis. Observe also that sea water having Fe(II) absorbs light and so the porphyrins and other pigments would be optimized to different absorption bands before and after Fe(II) removal from the oceans. I have my own pet theory that would require something along these lines. NAD+/NADH is a surprising choice because it poorly works in an oxygented world. If you look at NAD, it has ortho -C(O)NH2 substitution in the pyridinium unit. A chemist realizes that this group is the product of hydrolysis of -CN group. I have an idea that the first "NAD" was, actually, o-carbonitrile; that would considerably change energetics, increasing its reductive power. Perhaps the first phototrophs were more efficient in CO2 fixation than today's phototrophs operating in oxygenic world, which stuck to the machinery developed for a completely different situation and

[combinator30]

Well I believe that NAD+/NADH (and NADP+/NADPH) was used in the metabolic processes long time before chlorophyll photosynthesis, probably already in the RNA world...

[shkrobius]

It breaks the C-N motif of nucleobase heterocycles. It is hard to see why would you use pyridinium rather than something more similar, like imidazolium. However if there is cyanide in o-positions then it would follow the pattern.