Calcium Pump with a bound ATP analogue
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 consists mainly of alpha-helices. Located above this region are 3 cytoplasmic domains-A, N and P, separate from each other in the absence of ATP. In the structure here, the 3 domains are brought into close proximity by an ATP analog that bridges the N and P domains and causes the A-domain to tilt. This causes one of the transmembrane helices to move into the cytoplasmic channel and capture the Ca2+ ions before releasing them into the lumen of the sarcoplasmic reticulum.
Protein Data Bank (PDB)
author: Ashwini Patil