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- Belogurov, G A, et al.
(författare)
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Catalytically important ionizations along the reaction pathway of yeast pyrophosphatase
- 2000
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 39:45, s. 13931-13938
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Tidskriftsartikel (refereegranskat)abstract
- Five catalytic functions of yeast inorganic pyrophosphatase were measured over wide pH ranges: steady-state PP(i) hydrolysis (pH 4. 8-10) and synthesis (6.3-9.3), phosphate-water oxygen exchange (pH 4. 8-9.3), equilibrium formation of enzyme-bound PP(i) (pH 4.8-9.3), and Mg(2+) binding (pH 5.5-9.3). These data confirmed that enzyme-PP(i) intermediate undergoes isomerization in the reaction cycle and allowed estimation of the microscopic rate constant for chemical bond breakage and the macroscopic rate constant for PP(i) release. The isomerization was found to decrease the pK(a) of the essential group in the enzyme-PP(i) intermediate, presumably nucleophilic water, from >7 to 5.85. Protonation of the isomerized enzyme-PP(i) intermediate decelerates PP(i) hydrolysis but accelerates PP(i) release by affecting the back isomerization. The binding of two Mg(2+) ions to free enzyme requires about five basic groups with a mean pK(a) of 6.3. An acidic group with a pK(a) approximately 9 is modulatory in PP(i) hydrolysis and metal ion binding, suggesting that this group maintains overall enzyme structure rather than being directly involved in catalysis.
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| 4. |
- Dubrovinsky, L., et al.
(författare)
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Iron-silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle
- 2003
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 422:6927, s. 58-61
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Tidskriftsartikel (refereegranskat)abstract
- The boundary between the Earth's metallic core and its silicate mantle is characterized by strong lateral heterogeneity and sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition(1-7). To investigate the composition and properties of the lowermost mantle, an understanding of the chemical reactions that take place between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earth's lower mantle is first required(8-15). Here we present a study of the interaction between iron and silica (SiO2) in electrically and laser-heated diamond anvil cells. In a multianvil apparatus at pressures up to 140 GPa and temperatures over 3,800 K we simulate conditions down to the core-mantle boundary. At high temperature and pressures below 40 GPa, iron and silica react to form iron oxide and an iron-silicon alloy, with up to 5 wt% silicon. At pressures of 85-140 GPa, however, iron and SiO2 do not react and iron-silicon alloys dissociate into almost pure iron and a CsCl-structured (B2) FeSi compound. Our experiments suggest that a metallic silicon-rich B2 phase, produced at the core-mantle boundary (owing to reactions between iron and silicate(2,9,10,13)), could accumulate at the boundary between the mantle and core and explain the anomalously high electrical conductivity of this region(6).
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