The catalytic effect of buffers on the reaction CO2+H2O⇌H2CO3

Abstract

When CO2 is taken up by buffer solns. the buffer substance itself has some direct effect on the reaction besides "instantaneous" removal of H+ concerned in the ionization of H2CO3 to H+ + HCO3-. Exptl. proof is offered that the effect cannot be due to impurities in the solns. or to "neutral salt" action, but is proportional to the conc. of the more negative constituent of the buffer. Thus in the case of phosphate buffer, the overall velocity constant, V[mu], is given by V[mu] = 0.0021 [CO2]{1+1[mu][HPO4=]} l[mu], the catalytic coefficient, being 8, and for cacodylate 9. The figure 0.0021 represents the true velocity constant of the reaction CO2 + H2O[forward arrow]H2CO3 and is about 25% lower than the erroneous value previously accepted. All other oxy-acids, so far tested, which buffer in the pH range 6-9 show similar effects, whereas salts of stronger acids have much smaller effects (l[mu]< 1.5). The effects with sulphite, selenite and tellurate are much larger (l[mu] of order of 1000). Straight chain N bases, e.g. NH2OH, show no appreciable catalytic activity, though they combine readily with CO2 to form carbamino compounds. Cyclic N bases, e.g. glyoxaline, on the other hand do not form carbamino compounds but do act catalytically. The effect is proportional again to the conc. of the more negative constituent[long dash]in this case the unionized N base. Values for 1[mu] of the order of 10 are found for several cyclic bases of pK> 7.0. With weaker bases the effect tends to disappear. Mixed compounds, e.g. histidine, show both catalytic action and carbamino formation. Similar effects are observed on the rate of CO2 from bicarbonate soln. suddenly mixed with buffer (output), for the oxy-acid buffers phosphate, cacodylate, chromate, selenite, and for the N base buffers pilocarpine and glyoxaline being equal to l[mu]. The effect must therefore be a catalysis. The buffers only catalyse the CO2 + H2O H2CO3 reaction and not the CO2 + OHHCO3- reaction. A mechanism is suggested in which CO2 (or H2CO3) combines reversibly with the more negative constituent of the buffer. The effects of phosphate and cacodylate are additive, but of the enzyme and phosphate are probably multiplicative. Some biochemical implications of these results are discussed. They must be allowed for in work involving velocities of CO2 reactions. Selenite is suggested as a possible inorganic model of carbonic anhydrase.