Calcium-Dependent ATPase

A common eukaryotic enzyme, calcium-dependent ATPase has been extensively investigated. The ion-transport enzyme uses energy derived from the hydrolysis of adenosine triphosphate (ATP) to move Ca2+ against a concentration gradient. Innumerable techniques have been applied to Ca2+-ATPase analyses. These have included proteolytic, genetic, immunologic, and molecular approaches.

Calcium-dependent ATPase was first isolated in 1970 (3:696-700). This heterogenous family of enzymes can be broadly subdivided into two separate groups. The plasma membrane Ca2+-ATPase occurs in most eukaryotic tissues. This 140-kDa enzyme binds calmodulin and is stimulated by calcium ion (10:285-297). Although it may be derived from plants, yeasts, or, for example, pig stomach smooth muscle, perhaps the most thoroughly studied plasma membrane Ca2+-ATPase is found in erythrocytes (1:303). The sarcoplasmic reticulum (SR) is also a very good source of Ca2+-ATPase. The SR Ca2+-ATPase is derived from the SERCA gene family. Perhaps the most well-known of these enzymes comes from the SERCA 1 gene. The SERCA 2a Ca2+-ATPase isoform is found in skeletal muscle; whereas, the SERCA 2b isoform occurs in smooth muscle.

The Ca2+-ATPase molecule extends about 60 + above the surface of any given plasma bilayer. Further, the enzyme's total length is approximately 100 + (4:365-370). According to MacLennan and associates (1985), calcium-dependent ATPases g

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ion may also come from site-directed mutagenesis studies. Such methods have been used to identify the individual ATPase amino acid residues involved in calcium binding. For example, Clark et al. (1989) found that "conservative mutations of any of 6 amino acid residues in the transmembrane domain of the ATPase interfere with activation of this enzyme by Ca2+ (9:12682)." Researchers have even employed genetic approaches to the analysis of Ca2+-ATPase. For example, the ion-transport enzyme's full sequence had been elucidated using complementary genetic clones. Researchers essentially probed complimentary deoxyribonucleic acid (cDNA) libraries with radiolabelled synthetic oligomeric nucleotides. The primary structure of these nucleotide probes was based on previously determined ATPase amino acid sequences. Eventually, two cross-hybridizing clones were found that encoded for two different forms of the enzyme. In addition to revealing the ATPase's primary structure, characterization of the cDNA clones also provided a base from which researchers could predict the enzyme's secondary and tertiary structure (3:696-700). Two means of locating Ca2+-ATPase functional sites include both antibody and fluorescent probe analyses. In re
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