Common patterns and unique features of P-type ATPases:: a comparative view on the KdpFABC complex from Escherichia coli

P- type ATPases are ubiquitously abundant primary ion pumps, which are capable of transporting cations across the cell membrane at the expense of ATP. Since these ions comprise a large variety of vital biochemical functions, nature has developed rather sophisticated transport machineries in all kingdoms of life. Due to the importance of these enzymes, representatives of both eu- and prokaryotic as well as archaeal P- type ATPases have been studied intensively, resulting in detailed structural and functional information on their mode of action. During catalysis, P- type ATPases cycle between the so- called E1 and E2 states, each of which comprising different structural properties together with different binding affinities for both ATP and the transport substrate. Crucial for catalysis is the reversible phosphorylation of a conserved aspartate, which is the main trigger for the conformational changes within the protein. In contrast to the well- studied and closely related eukaryotic P- type ATPases, much less is known about their homologues in Bacteria. Whereas in Eukarya there is predominantly only one subunit, which builds up the transport system, in Bacteria there are multiple polypeptides involved in the formation of the active enzyme. Such a rather unusal prokaryotic P- type ATPase is the KdpFABC complex of the enterobacterium Escherichia coli, which serves as a highly specific K (+) transporter. A unique feature of this member of P-type ATPases is that catalytic activity and substrate transport are located on two different polypeptides. This review compares generic features of P- type ATPases with the rather unique KdpFABC complex and gives a comprehensive overview of common principles of catalysis as well as of special aspects connected to distinct enzyme functions.