Sarcoplasmic reticulum calcium pump
Oryctolagus cuniculus (rabbit)
The lipid bilayer, which forms the cell membrane, is very hydrophobic. As a result, polar or charged molecules are not efficiently transported across the cell membrane. However, such polar molecules, which include ions, sugars, amino acids, nucleotides and various cell metabolites, are essential for the survival and functioning of the cell. The transport of such molecules is facilitated by membrane transport proteins of two types - carrier proteins and channel proteins. Carrier proteins bind to a specific molecule and undergo a change in their structure (conformational change) to transfer the bound solute across the membrane. Channel proteins form charged pores across the membrane and allow specific solutes to pass through them when they are open. For every charged solute, such as K+, Na+, Ca2+, H+ and Mg2+, its concentration across the cell membrane and its charge with respect to that of the cell membrane establishes a driving force for its motion in the form of its electrochemical gradient. Solutes move across the membrane in the direction of their electrochemical gradient. However, maintenance of the gradient often requires movement against it. Ion pumps are carrier proteins that actively perform such transport to establish ion concentration gradients across cell and organelle membranes by transferring ions across them. P-type ATPases are ion pumps that transfer cations (positively charged ions) across membranes and utilize ATP as a source of energy in the process.
The structure of one such P-type ATPase that pumps Ca2+ across the membrane of skeletal muscle sarcoplasmic reticulum is shown here. The transmembrane part (part embedded in the membrane) consists mainly of 10 alpha-helices. Located above this region are 3 cytoplasmic domains - A, N and P, separate from each other in the absence of ATP. The 3 domains are brought into close proximity by ATP that bridges the N and P domains and causes the A-domain to tilt and close the cytoplasmic gate of the Ca2+ binding sites to capture the Ca2+ ions. The structure here shows the conformational change in the transmembrane alpha-helices that results from the transfer of a phosphate from the ATP to the P domain. This causes the release of ADP, the opening of the N- and P-domain interface and the rotation of the A domain to open the luminal gate of the protein and release the Ca2+ ions into the lumen of the sarcoplasmic reticulum.
Protein Data Bank (PDB)
author: Ashwini Patil