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In an answer to a recent question on Worldbuilding, I suggested that an organism convert $\ce{H2O}$ into $\ce{H2O2}$. I suggested a few processes that yielded the desired final result ($\ce{2H2O \rightarrow H2O2 + H2}$ and $\ce{H2O + O3 \rightarrow H2O2 + O2}$). I noted that both were mere guesses. The feasibility of it was debated because hydrogen peroxide is a rocket fuel and contains a lot of energy (I heavily modified the answer and set it on an entirely different track after another mistake was pointed out).

Could some process similar to this ever happen in an organism?

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Hydrogen peroxide as a reactive oxygen species (ROS) is a highly problematic molecule for the processes inside of cells (it can spontaneously oxidize and damage cellular components). With Catalase, organisms have a very effective enzyme which degrades the peroxide into water and oxygen.

However, although the molecule is problematic, it is still produced by organisms in traces. For its production by the NADPH oxidase, first NADPH is oxidized and superoxide is formed. The reaction looks like the following:

$NADPH + O_2 \rightarrow NADP^+ + H^+ + O_2^-$

In a second step the superoxide dismutase uses two molecules of superoxide and H+ to generate H2O2, the total reaction (which is divided into several steps) looks like this:

$2O_2^- + 2 H^+ \rightarrow H_2O_2 + O_2$

H2O2 is then usually degraded by the catalase (or used in immunological processes like the respiratory burst).

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Actually, there's a lot of animals, plants and microbes can do it. However, it is just an Intermediate process in general. Here's a strong example. In the condition with the absence of catalase, glucose oxidase can catalyze glucose + H2O + O2 become gluconic acid + H2O2. Another process can also generate H2O2 catalyzed by Ero1.

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    $\begingroup$ Would it be possible to add a source to your answer? It helps with people finding material to pursue more reading about it. $\endgroup$
    – anongoodnurse
    Nov 25, 2014 at 6:28
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    $\begingroup$ Shan, C., Yang, H., Song, J., Han, D., & Niu, L. (2009). Direct electrochemistry of glucose oxidase and biosensing for glucose based on graphene. Analytical Chemistry, 81(6), 2378-2382. Shu, S., Ju, G., & Fan, L. (1988). The glucose oxidase-DAB-nickel method in peroxidase histochemistry of the nervous system. Neuroscience Letters, 85(2), 169 Umana, M., & Waller, J. (1986). Protein-modified electrodes. the glucose oxidase/polypyrrole system. Analytical Chemistry, 58(14), 2979 Wilson, R., & Turner, A. (1992). Glucose oxidase: An ideal enzyme. Biosensors and Bioelectronics, 7(3), 165-185. $\endgroup$
    – meateatingbunny
    Dec 2, 2014 at 5:43

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