ATPase model

Except for the two additional membrane-spanning stretches in the second membrane-embedded segment, the proposed membrane-embedded stretches indicated in Fig. 1 are similar to those proposed for the closely related Ca -ATPase [53]. In fact, the overall topography proposed in Fig. 1 is quite similar to the Ca -ATPase model, lending additional credibility to each. 

Photoprotonic signals can be generated by light induced proton release, this might happen for the quaternary N-R+ derivatives of 117, for instance [8.233]. The photoproduction of proton gradients across membranes could serve as a light-powered proton pump for inducing vectorial processes such as the transport of protons [8.233, 8.234] or H+-ATPase model reactions [8.274]. 

Fig. 1. Working models of the F-, V-, and A-ATPases. Model of the subunit arrange-mentin the (A) FjFo-ATP synthase from Escherichia colt, (B) vacuolar ATPase from bovine brain clathrin- coated vesicles, and (C) A A0-ATPase from Thermoplasma acidophilum. The catalytic domain is in blue, the rotor domain is in green, and the stator domain is in orange.

FIGURE 10.11 A mechanism for Na, K -ATPase. The model assumes two principal conformations, Ei and E9. Binding of Na ions to Ei is followed by phosphorylation and release of ADP. Na ions are transported and released and ions are bound before dephosphorylation of the enzyme. Transport and release of ions complete the cycle. 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

Myosin as an ATPase Activation of Myosin ATPase by Actin Lymn and Taylor Model 1971 Eisenberg and Hill Model 1985... 

Low resolution models (20-30 A) based on diffraction analysis of membrane crystals of Na,K-ATPase [34,35,39] and Ca-ATPase [40,41] show that the cytoplasmic protrusions of the proteins are remarkably similar. A notable difference is a 10-20 A... 

Fig. 3. (A) Disposition of afi unit in the membrane, based on sequence information [14,15], selective proteolytic digestion of the a subunit [5,6] and hydrophobic labelling (Table 1). The model for the (S subunit is based on sequencing of surface peptides and identification of S-S bridges [64,65]. T, T2 and C3 show location of proteolytic splits. CHO are glycosylated asparagines in the P subunit. (B) Peptide fragments remaining in the membrane after extensive tryptic digestion of membrane-bound Na,K-ATPase from outer medulla of pig kidney as described by Karlish et al. [7,58].

Information about the putative folding of the H,K-ATPase catalytic subunit through the membrane has been obtained by the combined use of hydropathy analysis according to the criteria of Kyte and Doolittle [51], identification of sites sensitive to chemical modification [46,48,50,52-55], and localization of epitopes of monoclonal antibodies [56]. The model of the H,K-ATPase catalytic subunit (Fig. 1) which has emerged from these studies shows ten transmembrane segments and contains cytosolic N- and C-termini [53]. This secondary structure of the catalytic subunit is probably a common feature of the catalytic subunits of P-type ATPases, since evidence supporting a ten a-helical model with cytosolic N- and C-termini has also been published recently for both Ca-ATPase of the sarcoplasmic reticulum and Na,K-ATPase [57-59]. 

The widely accepted model, of how the class of P-type ATPases transports ions... 

Combining structural and biochemical information, MacLennan and his colleagues [8,11,42,45,48,87] constructed a hypothetical model of the tertiary structure of Ca " -ATPase that has interesting mechanistic implications (Fig. 2). The structure was divided into three major parts, designated as the cytoplasmic headpiece, the stalk domain and the transmembrane domain each was assigned distinct functional... 

The intramembranous domain of Ca -ATPase contains of the mass of the ATPase molecule based on electron microscopy of Ca -ATPase crystals [90,91] and X-ray diffraction analysis of oriented multilayers of sarcoplasmic reticulum [140]. Although in speculative models developed from these reconstructed structures the intramembranous domain was pictured as containing ten transmembrane helices [141,142], at the resolution attainable so far, several alternative transmembrane arrangements would be equally possible. 

The mechanism of the coupling between ATP hydrolysis and Ca transport is determined by the spatial relationship of the phosphorylation and ATP binding domains of the Ca -ATPase to the Ca channel involved in the translocation of calcium. Two alternative coupling mechanisms have been proposed, based on two rather different hypothetical models of the structure of the Ca -ATPase. In the conformational coupling mechanism the energy transfer between ATP hydrolysis and transport involves a mechanical coupling over long distances between... 

In another study, ATPase reconstituted into liposomes was analyzed by infrared attenuated total reflection spectroscopy and the secondary-structure elements of the molecule were determined from the spectra obtained by Fourier self-deconvolution [42]. Gratifyingly, essentially identical secondary-structure estimates for the ATPase were obtained by this entirely different approach, suggesting quite strongly that these secondary-structure estimates are reasonably accurate. Thus, any future models for the structure of the H -ATPase must take this information into account. 

The results of all of these topography studies are summarized in Fig. 1. The open eircles indicate residues of the H -ATPase shown in one way or another to be located on the cytoplasmic side of the membrane and the closed circles indicate residues in membrane-embedded segments. The lines in the sequence indicate minor regions with locations as yet not established. Thus, the topographical locations of nearly all of the 919 residues in the molecule have been established. It should be emphasized that the exact points of entry and exit of the polypeptide chain into and out of the membrane are not implied in the model. 

A first-generation model for the tertiary structure of the -ATPase... 

Fig. 1. Model for the transmembrane topography of the H -ATPase. OUT and IN indicate points of reference outside and inside an intact cell, respectively. See text for additional details.

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