PubMed · 13378413
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W HUFSCHMID. 1956-09-07. Natrium bicarbonicum.. https://pubmed.ncbi.nlm.nih.gov/13378413/
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A theoretical density functional theory (DFT, B3LYP) investigation has been carried out on the catalytic cycle of the carbonic anhydrase. A model system including the Glu106 and Thr199 residues and the "deep" water molecule has been used. It has been found that the nucleophilic attack of the zinc-bound OH on the CO(2) molecule has a negligible barrier (only 1.2 kcal mol(-1)). This small value is due to a hydrogen-bond network involving Glu106, Thr199, and the deep water molecule. The two usually proposed mechanisms for the internal bicarbonate rearrangement have been carefully examined. In the presence of the two Glu106 and Thr199 residues, the direct proton transfer (Lipscomb mechanism) is a two-step process, which proceeds via a proton relay network characterized by two activation barriers of 4.4 and 9.0 kcal mol(-1). This pathway can effectively compete with a rotational mechanism (Lindskog mechanism), which has a barrier of 13.2 kcal mol(-1). The fast proton transfer found here is basically due to the effect of the Glu106 residue, which stabilizes an intermediate situation where the Glu106 fragment is protonated. In the absence of Glu106, the barrier for the proton transfer is much larger (32.3 kcal mol(-1)) and the Lindskog mechanism becomes favored.
The high concentration of HCO(3)(-) ions (150 mM) in the human pancreatic ducts raises the question of the membrane proteins responsible for their secretion in addition to the Cl(-)/HCO(3)(-) exchanger. In this study, we investigated the expression of carbonic anhydrase IV (CA IV), a possible candidate. Experiments were carried out on specimens of normal human pancreas obtained from brain-dead donors ( n=9) as well as on isolated human ductal cells. Two antibodies were generated: CA IV NH(2) antibody directed against the NH(2) terminal of human glycosyl phosphatidylinositol (GPI)-anchored CA IV and CA IV COOH antibody directed against the COOH terminal of the same protein before its association with a GPI in the rough endoplasmic reticulum. A 35-kDa CA IV was detected in the homogenates of human pancreas. Immunocytochemistry demonstrated the expression of CA IV in centroacinar cells and in intercalated, intralobular, and interlobular ductal cells. The immunoreactivity observed with the CA IV COOH antibody was mainly localized on luminal membranes of ductal cells. Treatment of purified plasma membranes with phosphatidylinositol-phospholipase C indicated that the CA IV expressed in pancreatic ducts was not GPI-anchored. Its detection in the same extracts by the CA IV COOH antibody indicated that it was anchored by a hydrophobic segment at the carboxy terminal. Taken together, these results suggest that normal human pancreatic ductal cells express a 35-kDa CA IV anchored in their luminal plasma membrane by a hydrophobic segment of the COOH terminus. In view of its localization and its mode of anchorage in luminal plasma membranes, this CA IV may participate in the maintenance of luminal pH.
The Cu,Zn superoxide dismutase catalyzes HCO(3)(-) -dependent oxidations by H(2)O(2). This activity has been shown to depend on the creation of a bound oxidant at the Cu(II) by interactions with H(2)O(2). The bound oxidant was then thought to oxidize HCO(3)(-) to CO(3)(.-), which diffuses into the bulk solution and there oxidizes diverse substrates. We now find that CO(2) rather than HCO(3)(-) facilitates the peroxidations catalyzed by Cu,Zn superoxide dismutase. This fact was shown by a lag in the rate of peroxidation of NADPH when NaHCO(3)(-) was added last and by a burst in the rate when aqueous CO(2) was added last. Both the lag and the burst were eliminated by carbonic anhydrase.