ATPases catalytic cycle

Mendlein, J., and Sachs, G. (1989). The substitution of calcium for magnesium in ATPase catalytic cycle, f. Bioi. Chem. 264,18512 18519. 

The catalytic cycle of the Na+/K+-ATPase can be described by juxtaposition of distinct reaction sequences that are associated with two different conformational states termed Ei and E2 [1]. In the first step, the Ei conformation is that the enzyme binds Na+ and ATP with very high affinity (KD values of 0.19-0.26 mM and 0.1-0.2 pM, respectively) (Fig. 1A, Step 1). After autophosphorylation by ATP at the aspartic acid within the sequence DKTGS/T the enzyme occludes the 3 Na+ ions (Ei-P(3Na+) Fig. la, Step 2) and releases them into the extracellular space after attaining the E2-P 3Na+ conformation characterized by low affinity for Na+ (Kq5 = 14 mM) (Fig. la, Step 3). The following E2-P conformation binds 2 K+ ions with high affinity (KD approx. 0.1 mM Fig. la, Step 4). The binding of K+ to the enzyme induces a spontaneous dephosphorylation of the E2-P conformation and leads to the occlusion of 2 K+ ions (E2(2K+) Fig. la, Step 5). Intracellular ATP increases the extent of the release of K+ from the E2(2K+) conformation (Fig. la, Step 6) and thereby also the return of the E2(2K+) conformation to the EiATPNa conformation. The affinity ofthe E2(2K+) conformation for ATP, with a K0.5 value of 0.45 mM, is very low. 

P-Type ATPases Undergo Phosphorylation during Their Catalytic Cycles... 

In the course of the catalytic cycle of the Na+-K+ ATPase, the terminal phosphate group of ATP is transferred to the enzyme, where it forms a carboxyl ic-phosphoryl anhydride with an aspartyl residue ... 

Suzuki, H., Obara, M., Kubo, K., Kanazawa, T. (1989). Changes in the steady-state fluorescence anisotropy of N-iodoacetyl-N -(5-sulfo-l-naphthyl)ethylenediamine attached to the specific thiol of sarcoplasmic reticulum Ca2+-ATPase throughout the catalytic cycle. J. Biol. Chem. 264, 920-927. 

Fig. 2. General concept of the catalytic cycle of P-type ATPases. cation to be...

P-type ATPase. Methanococcus voltae, a marine organism growing on H2/CO2, contains high activities of a membrane-bound ATPase, which was sensitive towards vanadate rather than to DCCD [147], suggesting the presence of a P-type ATPase (see ref [141]). Accordingly the purified enzyme, composed of one 74 kD subunit, could be phosphorylated in a vanadate-sensitive fashion [148], a characteristic property for P-type ATPases, which involve a phosphoprotein as intermediate in the catalytic cycle. 

The Na K ATPase of the plasma membrane and the Ca " transporters of the sarcoplasmic and endoplasmic reticulums (the SERCA pumps) are examples of P-type ATPases they undergo reversible phosphorylation during their catalytic cycle and are inhibited by the phosphate analog vanadate. F-type ATPase proton pumps (ATP synthases) are central to energy-conserving mechanisms in mitochondria and chloroplasts. V-type ATPases produce gradients of protons across some intracellular membranes, including plant vacuolar membranes. 

The stimulatory effect of flubendiamide on the Ca pump was also diminished in the calcium buffers comprised of calcium chelators with high and low calcium affinity [3]. The low affinity calcium chelator, diBr-BAPTA (Kd = 3.7 pM), evidently accelerated the catalytic cycles of Ca pump as in the case with A23187. The result also implies importance of luminal calcium, since the low affinity of this chelator could not interrupt the calcium association with high affinity binding sites (cytoplasmic site) on Ca -ATPase. In addition, the results also demonstrate... 

Ion-motive ATPases membrane-bound enzymes which, as part of their catalytic cycle, couple the transport of one or more ionic species across the membrane in which they are located, to either the hydrolysis of ATP to ADP and P or to the synthesis of ATP from ADP and Pj. They comprise 3 major classes P-type ATPases (see), F-type ATPases (see) and V-type ATPases (see). 

Kinetic studies on the H,K ATPase have defined the reaction steps shown in the figure illustrating the H and K transport steps. Understanding of the overall catalytic cycle of this ion pump has facilitated a mechanistic description of the process of acid secretion by the stomach. From this derives our understanding of stimulation of add secretion and the processes inhibited by the PPIs. 

Wallmark B, Stewart HB, Rabon E, Saccomani G, Sachs G (1980) The catalytic cycle of gastric H K -ATPase. JBiol Chem 255 5313-5319... 

Jones, P.M George, A.M (2009). Opening of the ADP-bound active site in the ABC transporter ATPase dimer evidence for a constant contact, alternating sites model for the catalytic cycle. Proteins, Vol.75, No.2, p>p. 387-96... 

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