membranes with the synthesis of ATP. F-type ATPases are found in mitochondria and bacteria.
ABC transporters couple the hydrolysis of ATP with the transport of small molecules across
The Ca2+ Pump is the Best Understood P-Type ATPase
Ca2+-ATPases are found in the plasma membrane of most living cells where they pump
Ca2+ out of the cell to maintain a low Ca2+ concentration in the cytosol. We will consider the role
of Ca2+ in cell signaling later in the course.
Among all P-type ATPases the structure of the calcium pump is best understood. Its structure is
shown in Figure 11-13. It contains ten transmembrane segments. Two calcium ion-binding sites
are located within the transmembrane region. The ATP binding site and phosphorylation site are
not in the transmembrane region. Rather, they are found in the part of the protein that projects
into the cytoplasm. Somehow, phosphorylation transmits a conformational change that alters
calcium ion accessibility across the membrane. In this regard, it is interesting to note that
transmembrane segment 5 comes out of the membrane and comes close to the phosphorylation
site. This is a critical region that transmits the conformational change from the phosphorylation
site to the transmembrane region.
The Plasma Membrane P-type Na+-K+ Pump Establishes the Na+ Gradient Across the
All living cells maintain an ion electrochemical gradient across their plasma membrane. In animal
cells, a strong Na+ gradient is typically present. This gradient is generated by a transporter called
the Na+, K+-ATPase or simply the sodium pump. For each ATP it hydrolyzes, three sodium ions
are pumped out and two potassium ions are pumped into the cell (Figure 11-14).
Figure 11-15 is a description of the reaction mechanism for the sodium pump. You should be
familiar with this figure. The reaction mechanism describes the series of steps that occur for this
transporter to work. First, three sodium ions bind to the protein from within the cell. ATP is
hydrolyzed, and a phosphate group is temporarily attached to the protein covalently. The protein
then undergoes a conformational change such that the sodium ions are exposed to the outside of
the cell. The sodium ions are released and then two potassium ions bind. This promotes the
dephosphorylation of the protein, which then causes a conformational change so that the
potassium ions are exposed to the inside of the cell. The potassium ions are released and then the
cycle can start over again.
There are many functional roles of the sodium pump in animal cells. One role is to drive sodium-
coupled symporters as described previously in Figure 11-8. A second role is to maintain cell
volume by controlling the amount of ions across the membrane. The sodium pump transports
three ions out and two ions in. If the cell is swelling due to osmosis, the sodium pump will speed
up to remove excess intracellular ions. If the cell is shrinking, it will slow down.
ABC Transporters Constitute the Largest Family of Membrane Transport Proteins
ABC transporters are members of a gene family that transport a diverse array of solutes
including ions, sugars, and amino acids. Several examples are described in your text (Figure 11-
19). ABC transporters have a modular structure. They have two transmembrane regions, each
composed of six transmembrane segments, and two ATP binding domains that project into the
ION CHANNELS AND THE ELECTRICAL PROPERTIES OF MEMBRANES
Let’s now turn our attention to the other type of transport protein, namely channels. In this
section, we will focus primarily on ion channels and the properties of ion electrochemical
gradients. Later in the course, we will survey a few other types of channels.