Ca2+-translocating ATPase

In parallel, activity of adenylate cyclase is inhibited by Ca2+, and the ion activates the membrane-embedded CAM to close the cAMP-dependent channels. In addition, CAM-PDE is induced to hydrolyze the messenger molecules rapidly. 4) Finally, Ca2+ activates the cytoplasmic CAM to enhance the activity of Ca2+-translocating ATPase, causing the cells to return to the resting state. 

Some microsomal plant membranes have Ca2+-translocating ATPases which are modulated by calmodulin, as described in Section 62.1.3.3.2 for animal cells. 

The biochemical basis of CAM-induced stimulation of Ca2+-ATPase activity in carrot cells was studied further by determining the parameters of the Ca2+-translocating reaction of the enzyme in the presence and absence of exogenous CAM, using EGTA-treated plasma membrane [45], The affinity of Ca2+-ATPase for Ca2+ was considerably increased by... 

A variety of other calcium transport systems are associated with Ca21-activated ATPases. The extraembryonic structure, the chorioallantoic membrane, of the chick embryo is responsible for the translocation of over 120 mg of eggshell calcium into (he embryo during development. The enzyme responsible for this is a (Ca2+, Mg2+)-ATPase with Km values for Ca2+ of 30 p,mol dm-3 and 0.3 mmol dm-3, and a molecular weight of 170 000. The enzyme can be crossiinked and co-isolated with a calcium-binding protein.158 Transport of Ca2+ is also associated with (Ca2+, Mg2+)-ATPases in neutrophil plasma membranes,159 transverse tubule membranes from rabbit skeletal muscle,160 rabbit myocardial membrane,161 endoplasmic reticulum,162 sar-colemma,163 brain microsomes,164 the Golgi apparatus165 and rat liver plasma membranes.166... 

Phosphatase Hydrolysis of phosphate Translocation of ATPases linked to cation Inorganic pyrophosphatase (pyrophosphate orthophosphate + P043-) Glucose-6-phosphatase Fructose-1,6-bisphosphatase Alkaline phosphatase Phosphoprotein phosphatase (Na+, K+)-ATPase (Mg2+, Ca2+)-ATPase Proton-translocating ATPase Zn2+ Zn2+ Sodium pump Calcium pump H+ pump... 

Figure 4. Simplified scheme for the reaction cycle in Ca2+ pumps. The pumps may adopt two major conformations E, and E2. The E, conformation shows high affinity for two Ca2+ (SERCA pumps) or one Ca2+ (PMCA pumps) on the cis side. Ca2+ binding greatly enhances the pumps ATPase activity, leading to the rapid formation of the h igh-energy phosphorylated intermediate E, P and occlusion (occ) of the transported Ca2+ ion(s). Ca2+ translocation across the membrane presumably occurs concomitantly with the release of energy stored as conformational constraint during the transition from the E, P to the low-energy E2-P conformation. Ca2+ affinity on the trans side is low and Ca2+ is therefore released. This is followed by hydrolysis of the phosphoenzyme and a poorly understood rearrangement step(s) from the E2 to the E, conformation.

Sarcoplasmic reticulum Ca2+-transporting ATPase (SR-ATPase) is a member of the ATPase family that couples ion translocation across cell membranes to hydrolysis. There... 

For the SR Ca2+-ATPases, it is commonly assumed that the extracytoplasmic sites on E2P from which the two translocated calcium ions have been released, either remain empty during recirculation to the cytoplasmic side or bind and countertransport 2-3 protons (Levy et al., 1990). In the former case, there would be a clear difference from the sodium pump, since the hydrolysis of the empty E2P intermediate would occur at a much higher rate in Ca2+-ATPase. It has yet to be established... 

There are, however, also problems with a symmetrical 4-state model, since studies of Ca2+-ATPase as well as Na+-K+-ATPase have made it clear that the phosphorylation of Et from ATP is a much faster reaction than the extracy toplasmic appearance of the translocated a-type ions. The latter event constitutes a major rate-limiting step in the pump cycle, and can in fact be further slowed down by... 

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... 

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