In nature, carbon fixation is done by green plants using the enzyme RuBisCO as a part of the Calvin cycle. RuBisCO is a rather slow catalyst compared to the vast majority of other enzymes, incorporating only a few molecules of carbon dioxide into ribulose-1,5-bisphosphate per minute, but does so at atmospheric pressure and in mild, biological conditions. The resulting product is further reduced and eventually used in the synthesis of glucose, which in turn is a precursor to more complex carbohydrates, such as cellulose and starch. The process consumes energy in the form of ATP and NADPH.
Artificial CO reduction for fuel production aims mostly at producing reduced carbon compounds from atmospheric CO. Some transition metal polyphosphine complexes have been developed for this end; however, they usually require previous concentration of CO before use, and carriers (molecules that would fixate CO) that are both stable in aerobic conditions and able to concentrate CO at atmospheric concentrations haven't been yet developed. The simplest product from CO reduction is carbon monoxide (CO), but for fuel development, further reduction is needed (for example, to multi-carbon products), and a key step also needing further development is the transfer of hydride anions to CO.