Which enzyme is probably the most abundant protein in the world that fixes atmospheric co2 into organic molecules?

RubisCO.

Explanation: RubisCO which is an enzyme with the chemical formular

Ribulose-1,5-bisphosphate carboxylase/oxygenase,it is the most abundant usually referred to as the most abundant Enzyme and protein molecule in the universe,its Activities cut across everywhere photosynthesis takes place,it helps to fix atmospheric carbon dioxide in the green plants during photosynthesis leading to the production of glucose etc.

hypothesising or postulating