In eukaryotic cells, the steady-state pH of intracellular compartments varies greatly, is tightly controlled and is an important determinant of their function.
In general, the cytoplasm tends to acidify as a result of catabolism and a negative membrane potential that drives the accumulation of H+ through cation channels and the loss of basic HCO3− through anion channels. Countering this acidification are intrinsic buffers (ionizable groups on amino acids, phosphates and other molecules) and HCO3−, which have a finite capacity, as well as distinct plasma membrane pH-regulatory transporters (for example, Na+–H+ exchangers and bicarbonate transporters) that finely control pH to keep it close to neutral — a level that is optimal for many protein interactions and cellular processes.
Some organelles, including the nucleus, endoplasmic reticulum and peroxisomes, seem to lack intrinsic pH-regulatory systems and instead seem highly permeable to H+ (or acid equivalents). Hence, these compartments readily equilibrate their luminal pH to levels found in the cytoplasm.
Organelles of the secretory and endocytic pathways are distinguished by their luminal acidity (pH 6.7–4.7), which is attained through the concerted actions of vacuolar H+–ATPases, 2 Cl−/1 H+ and Na+–H+ or K+–H+ exchangers. Progressive acidification of organelles along the secretory pathway is important for proper post-translational processing, sorting and transport of newly synthesized proteins. Likewise, graded acidification of vesicles along the endocytic pathway is essential for the recycling and/or degradation of internalized membrane proteins, fluid-phase solutes and entry of various microbial organisms.
By contrast, the mitochondrial matrix is quite alkaline (pH 8.0) owing to H+ extrusion across the inner membrane by components of the electron transport chain. Together with the electrical potential (inside negative) generated by the electrogenic proton extrusion process, the transmembrane pH gradient constitutes a proton-motive force that is harnessed by the inner membrane H+-ATP synthase (F1F0-ATPase) to generate ATP from ADP and inorganic phosphate.
Many cellular processes are exquisitely sensitive to changes in the surrounding pH. Fluctuations in the H+ concentration in some cases act as a general permissive factor, whereas in other cases they act directly as a regulatory signal.
Protons dictate the charge and structure of macromolecules and are used as energy currency by eukaryotic cells. The unique function of individual organelles therefore depends on the establishment and stringent maintenance of a distinct pH. This, in turn, requires a means to sense the prevailing pH and to respond to deviations from the norm with effective mechanisms to transport, produce or consume proton equivalents. A dynamic, finely tuned balance between proton-extruding and proton-importing processes underlies pH homeostasis not only in the cytosol, but in other cellular compartments as well.
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Original work from the authors' laboratories is supported by the Heart and Stroke Foundation of Canada, the Kidney Foundation and the Canadian Institutes of Health Research. S.G holds the Pitblado Chair in Cell Biology. J.R.C is a scientist of the Alberta Heritage Foundation for Medical Research.
The authors declare no competing financial interests.
- Proton-motive force
(ψH+). The driving force for proton (or equivalent) movement, consisting of the proton concentration gradient and the transmembrane electrical potential.
- pH buffering capacity
A measure of the ability of a solution to withstand changes in pH. It is defined as β = dn/dpH, where n is the number of acid or base equivalents that need to be added to alter pH.
The acid dissociation constant. A quantitative measure of the tendency of an acid to dissociate in solution. It is calculated as pKa = −log10Ka, where Ka = [A−][H+]/[HA] and [A−], [H+] and [HA] are the concentration of the dissociated acid, protons and the undissociated (protonated) acid, respectively.
A ubiquitous plasmalemmal enzyme that uses ATP to extrude 3 Na+ ions in exchange for 2 K+ ions. Also known as the Na+–K+-pump or simply the Na-pump.
- Hill coefficient
A measure of the cooperativity of a binding process. It is calculated by applying the Hill equation, which relates the fraction of filled ligand-binding sites to the ligand concentration.
- Aquaporin water channel
One of a family of proteins that facilitate the passage of water across biological membranes.
A vacuole that forms inside cells following the engulfment of large (≥0.5 μm) particles by a receptor-mediated, actin-driven process.
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Casey, J., Grinstein, S. & Orlowski, J. Sensors and regulators of intracellular pH. Nat Rev Mol Cell Biol 11, 50–61 (2010). https://doi.org/10.1038/nrm2820
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