Enzymatic HCO 3 Mechanism for All Fixation" Enzymes

" C 0 2 " exists in solution in the cytoplasm of cells predominantly as the hydrated and deprotonated form, HCO~, and also (albeit in smaller proportion) as the unhydrated, dissolved gas form, CO2 (Fig. 1). "CO2" fixation denotes the process of incorporation into organic matter of any form of the inorganic "CO2" compounds, and involves the establishment of a covalent bond between the carbon atom of "CO2" and an atom of C or N in other compounds. It normally requires nucleophilic attack on the carbon atom of "COz" and ends with the formation of a new carboxylic or carboxyliclike group in the resulting compound. "CO2" fixation is a key process for life. It is at the beginning of the chain leading to formation of new living matter from inorganic compounds in autotrophic organisms, and even in heterotrophic organisms "CO2" is incorporated into various biological molecules. Biological "CO2" fixation is catalyzed by enzymes and there are reasons why either CO z or HCO~ might be favoured as the substrate for these enzymatic reactions. Thus, CO2 is more susceptible than HCO~ (Fig. 1) to nucleophilic attack, and therefore is more suitable for carboxylation, it also crosses membranes readily, ensuring its access to cellular sites of enzymatic fixation. On the other hand, HCO~ is more abundant than CO2 at physiological pH (HCO~) = 20 x (CO2) at pH 7.4) and its negative charge might facilitate binding at the catalytic site of the enzyme. In fact, some enzymes use CO 2 and others use HCO~ (Table 1). We shall not discuss here the mechanisms of the former: the nucleophilic susceptibility of CO 2