P-type pumps

Because of the distinct roles of the two different substrate cations in its reaction mechanism, the Na+,K+ pump most conveniently illustrates the mechanism of P-type pumps. The Na+,K+ pumps are driven by a cycle of conformational transitions that is driven by phosphorylation of their catalytic sites, activated by cytoplasmic Na+, and hydrolysis of the same phosphorylated sites, activated by extracellular K+ (Fig. 5-2). 

The primary plasma-membrane Ca2+ transporter (PMCA) is a P-type pump with high affinity for Ca2+ (Km = 100-200 nmol/1) but relatively low transport capacity [19]. The stoichiometry of PMCA is one Ca2+ transported for each ATP hydrolyzed. These pumps probably do not carry out bulk movements of Ca2+ but are most effective in maintaining very low concentrations of cytosolic Ca2+ in resting cells. A distinguishing characteristic of the PMCAs is that, in addition to binding Ca2+ as a substrate, they are further activated by binding... 

Table 2. Transport Stoichiometry and Cation Substrates in Some P-Type Pumps...

In most P-type pumps the polypeptide chain that carries out ATP hydrolysis and ion translocation has a size ranging between 900 and 1,200 amino acid residues (although shorter and longer versions exist). In the Ca2+-ATPases no other peptide... 

The protein structure is not known at high-resolution for any P-type pump, as crystals of a size and quality suitable for X-ray analysis have not yet been produced. However, the discovery that ordered two-dimensional semicrystalline arrays of ATPase molecules can be formed in situ in the membrane by incubation with tightly binding inhibitors such as vanadate (Skriver et al., 1981 Dux and Martonosi, 1983) has been of great help in obtaining a relatively detailed picture of the overall shape of the protein by electron microscopy. Fourier analysis of tilt series of electron micrographs permitted 3D-reconstructions of the Na+-K+- and Ca2+- ATPase protein structures to a resolution of 20-25 A (Figure 12a). Recently, this type of reconstruction analysis has been taken a step further to a resolution of about 14 A, and detailed structural features have become visible, even for the transmembrane domain (Toyoshima et al., 1993). The model of the Ca2+-ATPase peptide shown in... 

Table 3. Nucleotide-Related Affinity Reagents Labeling P-Type Pumps Covalently...

P-Type Pumps Maintain a Low Concentration of Caicium in the Cytosoi... 

---