Kinetic pathways and carbonic anhydrase mechanisms

Abstract

Carbonic anhydrase (CA) exists in three forms: the low-pH form (L); the high-pH form (H); and the anion-inhibited from (A). The latter includes the bicarbonate complex. All three forms have been demonstrated in CA I and, when sulfate is removed, in CA II. The L-form of CA III has not yet been seen, even at pH 5. Equilibrium among the three forms in a sample of CA can be established, in principle, by kinetic pathways connecting any two forms; which pathway dominates is as yet an open question. By invoking the usual ping-pong mechanism of CA, during which hydration of CO2 causes the enzyme to go from H to L, the kinetic pathway connecting A and H is ignored, essentially by definition. Rarely has the AH pathway been considered (cf. Koenig et al., 1980). Though there are few data to demonstrate the relative kinetics of the AL and AH pathways, it can be argued that the latter is buffer-mediated, which could distinguish the two. In this case, the lifetime of a bound anion would be buffer-dependent. We have investigated this point by measuring the nuclear relaxation rates of fluorine of trifluoroacetate in Co2+CA II solutions. The fluorine linewidth, and thus the anion exchange rate, is independent of buffer concentration up to ∼50 mM, which argues for the AL pathway predominating. © 1984.