| Popis: |
The E1E2- or P-ATPases are a large and physiologically important family of cation pumps, widely distributed in both prokaryotic and eukaryotic cells (Table 1). In eubacteria, they play a series of specialized roles: scavenging potassium ions from the medium under conditions of K+ starvation, accumulating Mg2+, regulating the cytoplasmic Ca2+ and Cu2+ concentrations, and ridding the cell of toxic heavy metals such as Cd2+ and Zn2+. In fungi, as in other eukaryotic cells, the PATPases have taken on a more central function, creating the primary ion gradient that underlies virtually all nutrient uptake by an array of secondary, cation-coupled cotransporters. The fungal plasma-membrane H+-ATPase uses as much as a fourth of cellular ATP to extrude protons electrogenically (Gradmann et al. 1978); the resulting gradient (largely a membrane potential) distributes energy to H+-dependent cotransporters for sugars (Seaston et al. 1973; Slayman and Slayman 1974), amino acids (Eddy and Nowacki 1971; Seaston et al. 1973; Sanders et al. 1983), and inorganic ions (Rodriguez-Navarro et al. 1986). Plant cells resemble fungi in relying upon a proton gradient formed by a plasma-membrane H+-ATPase, while animal cells use a sodium gradient, produced by the Na+,K+-ATPase, to regulate cell volume and drive Na+-dependent cotransport of ions and nutrients. |
