The Ca2+-ATPase

The first of these consists of a transmembrane domain. This area contains two high affinity Ca2+ stick localises and a Ca2+ pathway. Second, the enzyme possesses a walloping pear-shaped, cytosolic head (5:303-309). This extramembranal head accommodates both the ATP medical dressing site and an aspartyl residue phosphorylation site (Asp-351). A "P-type" ionic meat, the Ca2+-ATPase forms a phosphorylated intermediate during its catalytic cycle (1:299). Finally, a narrow pale yellow region links the membranous region to the cytosolic head.

Electron micrographs of Ca2+-ATPase typically show an outline of a globule structure attached to the plasma bilayer by a narrow stalk. About two-thirds of the enzyme's mass is uncovered on the cy expirelasmic surface of the plasma bilayer; most of the remain one-third is contained within the lipid phase (6:147-167). On freeze out fracture the transmembrane helices show up as intramembranous particles. Researchers have suggested that " terce extramembranous globular domains form a headpiece" which "lies on top of a stalk comprised of five helices which are, in turn, adjoined to five transmembrane helices (3:696-700)." These transmembrane helices indeed combine with five additional helices at the molecule's carboxy-terminal end; together, the go helices form the ATPase's basepiece.

The SR Ca2+ pump differs from the plasma membrane Ca2+ pump by the fact that it does not have a calmodulin binding site. In the plasma memb

knowledge may also be obtained about Ca2+-ATPase through analytic thinking of the enzyme's crystalline structure. In the presence of Ca2+, the ATPase molecules may be crystallized in the E1 state. Furthermore, in the absence of Ca2+, ATPase molecules may also be crystallized by the addition of either lanthanides, vanadate, or inorganic phosphate. The resulting enzymatic states may be characterized as follows: (1) E-La3+; (2) E-V; and (3) E-Pi, respectively. Within much(prenominal) crystals different types of interactions may be observed. These various contacts can stick out researchers with important information with regard to the ATPase molecules (4:365-370).

Various analyses have been employed in an attempt to locate the NCD4 binding site.

For example, reaction aim analyses indicate that the process preferentially involves "aquaphobic carbodiimide derivatives and occurs with four orders of order of magnitude faster rates than the corresponding reaction in aqueous media (9:12682-12688)." Hence, the carbodiimide derivatization which inhibits calcium binding probably occurs within a hydrophobic region of the enzyme. In fact, the NCD4 label has been associated with the intramembranous portion of Ca2+-ATPase's A1 proteolytic fragment. Therefore, it must attach close to the tryptophan residues either within or near the enzyme's transmembrane region. Obviously, this is a relatively large distance from the ATPase's extramembranous catalytic site (9:12682-12688).

It has been found that incubation of SR vesicles with increasing concentrations of NCD4 causes a progressive inhibition of ATPase activity. The NCD4 probe is a rather large moiety. It thus introduces a structural perturbation into the ATPase. This perturbation is sufficiently large to inhibit the binding of calcium ion at both the enzyme's activating and transporting sites (9:12682-12688). Carbodiimide derivatization of SR ATPase, however, does not interfere with the phosphorylation of the enzyme's active site by Pi
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