The plasma membrane ATPase of Neurospora: a proton-pumping electroenzyme.

摘要: Probably the best marker enzyme for plasma membranes of eukaryotic cells is a magnesium-dependent, vanadate-inhibited ATPase whose primary function transmembrane transport cations. In animal cells, different species cations: sodium ions released in unequal exchange potassium ions, calcium extruded alone (perhaps), or protons secreted equal ions. But plants and fungi only proton secretion has been clearly demonstrated. A useful model cell studying proton-secreting ascomycete fungusNeurospora, which drives an outward current that can exceed 50 µA/cm2 support membrane potentials greater than 300 mV. Both thermodynamic kinetic studies have shown proton-pumping ofNeurospora normally transports single each ATP molecule split; modelling suggested (contrary to conventional assumptions) fast steps overall reaction are transit its dissociation following transport, while slow binding and/or ATP. The structure theNeurospora enzyme, recently deduced by gene sequencing, very close yeast (Saccharomyces) hydropathic patterns both closely resemble those animal-cell plasma-membrane ATPases. All these enzymes appear 6–10 membrane-spanning α-helices, plus large cytoplasmic headgroup bears catalytic nucleotide-binding site. Structural data, taken together with electrical-kinetic behavior, suggest functions as energized gate protons. From geometric point view, action such would transfer field across “transported” ion, rather vice versa.