In biology, some transmembrane enzymes in the cell membrane act as primary ion pumps to move ions across a plasma membrane against their concentration gradient involved in active transport mechanisms. These primary ion transporters could convert energy from various sources including ATP, sunlight, and other redox reactions, into potential energy stored in an electrochemical gradient. This potential energy is further utilized by secondary transporters, such as ion channels and ion carriers, to propel some dynamic cellular processes.

Classification

Ion pumps are classified as a super family of transporters containing 12 families of transporters. This family is a part of the Transport Classification system that is admitted by the International Union of Biochemistry and Molecular Biology (IUBMB), and is divided consistent with characteristics including the transport mechanism, the energy source used, the substrates being transported, and DNA sequences making up each protein. The most important unifying factor is the charged nature of the substrate, indicating the transport of an ion, not a neutral species.

Difference between Ion Pumps and Ion Channels

Ion pumps are significantly distinct from ion channels. A difference in concentration of an ion or chemical molecule in two separate areas can cause an electrochemical gradient or concentration gradient. In an equilibrium state, the ion concentrations in both areas are identical. When there is a difference in concentration, the ions will be inclined to flow "down" the concentration gradient or from a high concentration to low concentration. Ion channels enable specific ions fitting into the channel to move down their concentration gradient via passive transport, to equalize the concentrations on both sides of the cell membrane. On the contrary, ion pumps implement active transport by driving ions against their concentration gradient by using energy sources such as ATP. Energy produced in this step can then be employed by secondary transporters or other proteins as a source of energy.

Energy source

a. Primary transport

Primary transporters usually use ATP as an energy source to transport ions such as Na⁺, K⁺, and Ca²⁺ across cell membrane and thus create concentration gradients. This transport is also capable of generating ATP through various methods, such as the electron transport chain in plants.

Transporters utilizing ATP can convert the energy in ATP into potential energy in the form of a concentration gradient. During this procedure, ATP is applied to transport ions from a low concentration to a higher concentration. P-Type ATPases are a typical type of ATP consuming enzymes that transfer Na⁺, K⁺, and Ca²⁺ ions by phosphorylation, mainly including Na⁺/K⁺-ATPase which is regulated by Janus Kinase-2 and Ca²⁺ ATPase that exhibits high sensitivity to ADP and ATP concentrations. A-type ATPases transferring anions, and ABC transporters (ATP binding and cassette transporters) transporting a broad set of molecules also belong to ATP consuming enzymes. Additionally, ATP producing transporters act in the opposite way of ATP utilizing transporters by transporting ions from high to low concentration with the gradient. In the process, ATP is formed by using the potential energy in the form of the concentration gradient. In the mitochondria of animals, ATP is synthesized through F-type ATPase otherwise recognized as ATP synthase. V-Type ATPase holds a function opposite to F-type ATPase, and can hydrolyze ATP to create a proton gradient in plants. For example, during process of photosynthesis in the chloroplasts, lysosomes use V-type ATPase to acidify vesicles or plant vacuoles, which can be regulated through various methods such as pH adjustion.

b. Secondary transport

Secondary transporters also transport ions from low concentration to high concentration. However, unlike primary transporters that create a concentration gradient through energy from ATP, secondary pumps utilize the potential energy from the concentration gradient produced by primary transporters to transport ions. Symporters like sodium-chloride symporter could transport an ion through its concentration gradient, and they couple the transport of a second molecule in the same direction, while antiporters using the concentration gradient couple a molecule that is transported in the contrary direction.

Regulation of Ion Pumps

The regulation of ion transporters can be accomplished in a variety of ways such as allosteric inhibition or activation, sensitivity to ion concentration, and phosphorylation. The regulatory ligand can bind into the regulatory site though allosteric inhibition, thus either inhibiting or activating the transporter. The concentration of an ion (not necessarily the ion it transfers) in solution can also regulate ion transporters. For example, the presence of H⁺ ions in solution controls the electron transport chain. The introduction of phosphate group by protein kinases or dephosphorylation by phosphatases can change the activity of the transporter. The activation or inhibition of the transporter with the addition of the phosphate group is determined by a specific protein.