Intrinsic Proton-Donating Power of Zinc-Bound Water in a Carbonic Anhydrase Active Site Model Estimated by NMR

Abstract

Using liquid state NMR spectroscopy we have estimated the proton donating ability of Zn-bound water in organometallic complexes designed as models for the active site of the metalloenzyme carbonic anhydrase (CA). This ability is important for the understanding of the enzyme reaction mechanism. The desired information was obtained by 1H and 15N NMR at 180 K of solutions of [TpPh,MeZnOH] (1, TpPh,Me = tris(2-methyl, 4-phenyl pyrazolyl) hydroborate) in CD2Cl2, in the absence and presence of the proton donors (C6F5)3BOH2 (aquatris(pentafluorophenyl)boron) and Col-H+ (2,4,6-trimethylpyridine-H+). Col-H+ forms a strong OHN hydrogen bond with 1, where the proton is located closer to nitrogen than to oxygen. (C6F5)3BOH2 - which exhibits a pKa value of 1 in water - also forms a strong hydrogen bond with 1, where the proton is shifted slightly across the hydrogen bond center towards the Zn-bound oxygen. Finally, a complex between Col and (C6F5)3BOH2 was identified, exhibiting a zwitterionic OHN hydrogen bond where H is entirely shifted to nitrogen. The comparison with complexes of Col with carboxylic acids studied previously suggests that, surprisingly, the Zn-bound water exhibits in an aprotic environment a similar proton donating ability as a carboxylic acid characterized in water by a pKa of 2.2 ± 0.6. This value is much smaller than the value of 9 found for [Zn(OH2)6]2+ in water, and those between 5 and 8 reported for different forms of CA. Implications for the biological function of CA are discussed.

Kinetic study of catalytic CO 2 hydration by metal-substituted biomimetic carbonic anhydrase model complexes

“…This supports the hypothesis that the lower the pK a of the water ligand, the faster the conversion rate since deprotonation would occur more easily and bicarbonate would be more rapidly produced from CO 2 [24,25]. In previous studies, the development of model catalysts has been aligned mainly with the modulation of the intrinsic proton-donating ability of the water ligand in Zn catalysts to accelerate CO 2 hydration [26]. The synthesis of model catalysts has been promoted in this direction because it was believed that the proton-donating ability can be tuned by adding an electron-withdrawing group to the ligand or stabilizing OH − through hydrogen bonding [20,[26][27][28].…”

Intrinsic electronic conductivity of individual atomically resolved amyloid crystals reveals micrometer-long hole hopping via tyrosines

“…In addition to hydrogen bond network changes from thermal fluctuations, electronic effects that modulate the pK a of any residues surrounding the charge transfer region could also regulate PCET (45). Although Zn 2+ is not redox active and cannot directly take part in electron transport, it could potentially lower the pK a of residues by up to 4 pH units which would facilitate PCET (49). However, our density functional theory (DFT) calculations showed that Zn 2+ does not significantly change the pK a of tyrosine in X1 microcrystals (SI Appendix, Table S3), confirming that it must play a primarily structural role.…”

NMR Study of Solvation Effect on the Geometry of Proton-Bound Homodimers of Increasing Size

“…[2][3][4][5] The presence of competing H-bonds and solvent complicates the analysis. [6][7][8] Sophisticated theoretical tools are required in order to simulate enzymatic reactions and transporters and receptors activity at the atomic level. [9][10][11] At the current stage of knowledge, it is possible to predict the effect of a mutation on the fate of a particular biosystem, to prove this prediction experimentally, and to explain it theoretically.…”