Dear Milon,

Your question is quite complicated as the mechanisms of biochemical reactions are very complex and in many processes the biochemical pathways are only hypotheses or assumptions but the intermediate reactions and products are not fully known. The reactions in the image are not complete, they are expressed globally, without highlighting all the reactants and products. I have some doubt that the reactions are fully correct; the first reaction seems pretty strange, I don't see where the carboxylic carbon disappeared, it is probably eliminated in the form of CO2 but even in this case the transformation of the amino acid into imine (C-N double bond) seems questionable to me. Returning to your question, I think that at any stage in the biochemical chain, the double bond, which is very reactive, may be affected by temperature, water, CO2, as well as by free radicals in the environment. This may be done at least in the following ways: 1. At elevated temperature and/or in the presence of radicals, radical substitution can occur, but also radical addition of water; 2. Addition reactions to carbon-nitrogen double bond, resulting carbonic acid ester; 3. Electrophilic addition of water to the double bond that creates two new sigma bonds, resulting an alcohol; 4. Rearrangement, transposition and isomerization involving double bond, but also the allyl radical and the glucose cycle. The increased reactivity of the double bond makes it very susceptible to environmental factors. Hope my answer helps a bit.