Acceleration rate of hydration of CO2 by carbonic anhydrase

Range ~14,000 fold
Organism Human Homo sapiens
Reference Forster RE, Gros G, Lin L, Ono Y, Wunder M. The effect of 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate on CO2 permeability of the red blood cell membrane. Proc Natl Acad Sci U S A. 1998 Dec 22 95(26):15815-20. p.15816 left column bottom paragraph & p.15819 left column 2nd paragraphPubMed ID9861053
Method "The reactions take place in an airtight 3-ml glass water-jacketed and stirred chamber connected to the ion source of a mass spectrometer through a thin 0.012-mm-thick Teflon membrane supported by a sintered glass disc, through which gases are dissolved in the solution diffuse. The chamber has a glass pH electrode and a removable stopper for the addition of reactants."
Comments "The initial rapid phase is produced by 12C18O16O diffusing into the cells, where the 18O is exchanged with HOH at a rate accelerated some 14,000 times by intracellular CA [carbonic anhydrase]. This rapid intracellular consumption of 12C18O16O in relation to its production at the uncatalyzed rate from extracellular HC18O16O3 lowers 12C18O16O in the extracellular solution...The rate of O2 uptake by red cells is about 1/30th of that in hemolysate, owing to the effect of simultaneous chemical reaction and diffusion, in distinct contrast to CO2, a fact that deserves explanation. This difference results simply because the velocity constant k in Eq. 3 for O2 is 3,500mM^1Xsec^1X18mM hemoglobin, equal to 63,000 sec^-1, 1,400 times larger than the exchange velocity constant for 18O in CO2, making average cell [O2] about 1/30 of the surface concentration according to Eq. 3."
Entered by Uri M
ID 110584