Cation transporters, P-type

Zinc efflux is mediated by a zinc exporter known as ZntA (Zn + transport or tolerance), a membrane protein which was identified through studies of bacterial strains that were hypersensitive to zinc and cadmium. Sequence inspection revealed that ZntA was a member of the family of cation transport P-type ATPases, a major family of ion-translocating membrane proteins in which ATPase activity in one portion of the protein is used to phophorylate an aspartate within a highly conserved amino acid sequence, DKTG, in another portion of the protein. The cysteine rich N-terminus of these soft metal transport proteins contains several metal-binding sites. How the chemical energy released by ATP hydrolysis results in metal ion transport is not yet known, in part because there is only partial information about the structures of these proteins. The bacterial zinc exporter also pumps cadmium and lead and is therefore also involved in protection from heavy metal toxicity (see Metal Ion Toxicity). 

Kanamaru K, Kashiwagi S, Mizuno T (1993) The cyanobacterium, Synechococcus sp. PCC7942, possesses two distinct genes encoding cation-transporting P-type ATPases. FEBS Lett 330 99-104... 

Finally we notice that in the p-type oxides CU2O and NiO, the presence of excess oxygen actually provides, through the formation of cation vacancies, a transport mechanism for the metal, while in an /i-type oxide like TiOi, the excess metal, by forming anion vacancies, provides a transport mechanism for oxygen. With /i-type oxides like ZnO and AljO, where the excess metal is accommodated interstitially, a transport mechanism is, of course, provided for the excess component itself. 

AU these features—low values of a, a strong temperature dependence, and the effect of impurities—are reminiscent of the behavior of p- and n-type semiconductors. By analogy, we can consider these compounds as ionic semiconductors with intrinsic or impurity-type conduction. As a rule (although not always), ionic semiconductors have unipolar conduction, due to ions of one sign. Thus, in compounds AgBr, PbCl2, and others, the cation transport number is close to unity. In the mixed oxide ZrOj-nYjOj, pure 0 anion conduction t = 1) is observed. 

Many types of oxide layers have a certain, not very high electrical conductivity of up to 10 to 10 S/cm. Conduction may be cationic (by ions) or anionic (by or OH ions), or of the mixed ionic and electronic type. Often, charge transport occurs by a semiconductor hole-type mechanism, hence, oxides with ionic and ionic-hole conduction are distinguished (in the same sense as p-type and n-type conduction in the case of semiconductors, but here with anions or cations instead of the electrons, and the corresponding ionic vacancies instead of the electron holes). Electronic conduction is found for the oxide layers on iron group metals and on chromium. 

P-type transporters share the same general reaction mechanism. Most pumps belonging to the P-type transporter superfamily have evolved to create cation gradients. 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

Transport ATPases transport cations—they are ion pumps. ATPases of the F type—e. g., mitochondrial ATP synthase (see p. 142)—use transport for ATP synthesis. Enzymes of the V type, using up ATP, pump protons into lyso-somes and other acidic cell compartments (see p. 234). P type transport ATPases are particularly numerous. These are ATP-driven cation transporters that undergo covalent phosphorylation during the transport cycle. 

The family of active transporters called P-type ATPases are ATP-driven cation transporters that are reversibly phosphorylated by ATP as part of the transport cycle phosphorylation forces a conformational change that is central to moving the cation across the membrane. All P-type transport ATPases have similarities in amino acid... 

The SERCA pumps belong to the P-type ATPase family, which actively transport cations across membranes at the expense of ATP hydrolysis. They show a high degree of conservation among species and their structure has recently been... 

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

Figure 1. Localization of the major types of ion-motive ATPases in the eukaryotic cell. Na+-K+, H+-K+, and Ca2+-ATPases of P-type transport the respective cations across the plasma membrane or into sarcoplasmic (SR) or endoplasmic (ER) reticulum. H+-ATPase of V-type acidifies different types of vacuoles and vesicles allowing their secondary uptake of amino acids and amines (AA+). H+-ATPase (working as ATP synthase) of F-type (FqF,) generates ATP in the mitochondria. Modified from Pedersen and Carafoli, 1987.

P-type ATPases. This is an unexpected finding as generally the cation specihcity of P-type ATPases is restricted to the amino acid composition of cation channels in transmemhrane domains (Axelsen and Palmgren, 1998 Mpller et al., 1996). It is probable that other, as yet to be identihed, intramolecular interactions may be important for the specihc cation transport by heavy metal P-type ATPases. Cysteine residues in transmembrane domain 6, which form a characteristic CPx (commonly CPC) motif, are commonly regarded as core elements of the cation channel in CuPAs. However, there is no direct evidence to date to support that assumption. 

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