Eukaryotic cells maintain very low concentrations of free Ca2+ in their cytosol (~10–7 M) in the face of a very much higher extracellular Ca2+ concentration (~10–3 M). Therefore, even a small influx of Ca2+ significantly increases the concentration of free Ca2+ in the cytosol, and the flow of Ca2+ down its steep concentration gradient in response to extracellular signals is one means of transmitting these signals rapidly across the plasma membrane (discussed in Chapter 15). It is thus
Figure 11–12 Three types of aTP-driven pumps. Like any enzyme, all ATP-driven pumps can work in either direction, depending on the electrochemical gradients of their solutes and the ATP/ADP ratio. When the ATP/ADP ratio is high, they hydrolyze ATP; when the ATP/ADP ratio is low, they can synthesize ATP. The F-type ATPase in mitochondria normally works in this “reverse” mode to make most of the cell’s ATP.
important that the cell maintains a steep Ca2+ gradient across its plasma membrane. Ca2+ transporters that actively pump Ca2+ out of the cell help maintain the gradient. One of these is a P-type Ca2+ ATPase; the other is an antiporter (called a Na+–Ca2+ exchanger) that is driven by the Na+ electrochemical gradient (discussed in Chapter 15).
The Ca2+ pump, or Ca2+ ATPase, in the sarcoplasmic reticulum (SR) membrane of skeletal muscle cells is a well-understood P-type transport ATPase. The SR is a specialized type of endoplasmic reticulum that forms a network of tubular sacs in the muscle cell cytoplasm, and it serves as an intracellular store of Ca2+. When an action potential depolarizes the muscle cell plasma membrane, Ca2+ is released into the cytosol from the SR through Ca2+-release channels, stimulating the muscle to contract (discussed in Chapters 15 and 16). The Ca2+ pump, which accounts for about 90% of the membrane protein of the SR, moves Ca2+ from the cytosol back into the SR. The endoplasmic reticulum of nonmuscle cells contains a similar Ca2+ pump, but in smaller quantities.
Enzymatic studies and analyses of the three-dimensional structures of transport intermediates of the SR Ca2+ pump and related pumps have revealed the molecular mechanism of P-type transport ATPases in great detail. They all have similar structures, containing 10 transmembrane α helices connected to three cytosolic domains (Figure 11–13). In the Ca2+ pump, amino acid side chains protruding from the transmembrane helices form two centrally positioned binding sites for Ca2+. As shown in Figure 11–14, in the pump’s ATP-bound nonphosphorylated state, these binding sites are accessible only from the cytosolic side of the SR membrane. Ca2+ binding triggers a series of conformational changes that close the passageway to the cytosol and activate a phosphotransfer reaction in which the terminal phosphate of the ATP is transferred to an aspartate that is highly conserved among all P-type ATPases. The ADP then dissociates and is replaced with a fresh ATP, causing another conformational change that opens a passageway to the SR lumen through which the two Ca2+ ions exit. They are replaced by two H+ ions and a water molecule that stabilize the empty Ca2+-binding sites and close the passageway to the SR lumen. Hydrolysis of the labile phosphoryl-aspartate bond returns the pump to the initial conformation, and the cycle starts again. The transient self-phosphorylation of the pump during its cycle is an essential characteristic of all P-type pumps.
The Plasma Membrane Na+-K+ Pump Establishes Na+ and K+ Gradients Across the Plasma Membrane