AQUAPORINS: FUNCTIONS, STRUCTURE AND TYPES - BIOLOGY - 2025

The aquaporins, also known as water channels are proteinaceous molecules that go through biological membranes. They are responsible for mediating the rapid and efficient flow of water into and out of cells, preventing water from interacting with the typical hydrophobic portions of the phospholipid bilayers.

These proteins resemble a barrel and have a very particular molecular structure, made up mainly of helices. They are widely distributed in different lineages, including from small microorganisms to animals and plants, where they are abundant.

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Historical perspective

With a basic knowledge of physiology and the mechanisms that solutes move through membranes (active and passive), we could guess that water transport does not pose any problem, entering and leaving the cell by simple diffusion.

This idea has been around for many years. However, some researchers detected the existence of some water transport channel, since in certain cell types with high permeabilities to water (such as the kidney, for example), diffusion would not be a sufficient mechanism to explain transport of water.

The doctor and researcher Peter Agre discovered these protein channels in 1992, while working with the membrane of erythrocytes. Thanks to this discovery, he won (together with his colleagues), the Nobel Prize in 2003. This first aquaporin was called "aquaporin 1".

Structure

The shape of the aquaporin is reminiscent of an hourglass, with two symmetrical halves oriented opposite each other. This structure crosses the double lipid membrane of the cell.

It should be mentioned that the shape of aquaporin is very particular and does not resemble any other type of membrane-spanning proteins.

The amino acid sequences are predominantly polar. Transmembrane proteins are characterized by having a segment rich in alpha helical segments. However, aquaporins lack such regions.

Thanks to the use of current technologies, the structure of the porin has been elucidated in detail: they are monomers from 24 to 30 KDa that consist of six helical segments with two small segments that surround the cytoplasm and are connected by a small pore.

These monomers are assembled in a group of four units, although each one can function independently. In small helices, there are some conserved motifs, including NPA.

In some aquaporins found in mammals (AQP4) higher aggregations occur that form supramolequal crystal arrangements.

In order to transport water, the inside of the protein is polar and the outside is nonpolar, as opposed to common globular proteins.

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Features

The role of aquaporins is to mediate the transport of water into the cell in response to an osmotic gradient. It does not need any additional force or pumping: the water enters and leaves the cell by osmosis, mediated by aquaporin. Some variants also carry glycerol molecules.

To carry out this transport and to increase the permeability to water substantially, the cell membrane is packed with aquaporin molecules, in a density order of 10,000 square micrometers.

Functions in animals

Water transport is vital for organisms. Let's take the specific example of the kidneys: they must filter huge amounts of water every day. If this process does not occur properly, the consequences would be fatal.

In addition to urine concentration, aquaporins are involved in general body fluid homeostasis, brain function, gland secretion, skin hydration, male fertility, vision, hearing - just to mention a few processes biological.

In experiments carried out in mice, it was concluded that they also participate in cell migration, a role that is far removed from water transport.

Functions in plants

Aquaporins are mostly diverse in the plant kingdom. Crucial processes such as perspiration, reproduction, metabolism mediate in these organisms.

In addition, they play an important role as an adaptive mechanism in environments whose environmental conditions are not optimal.

Functions in microorganisms

Although aquaporins are present in microorganisms, a specific function has not yet been found.

Mainly for two reasons: the high surface-to-volume ratio of the microbes implies a rapid osmotic equilibrium (making aquaporins unnecessary) and studies of deletions in microbes have not yielded a clear phenotype.

However, it is speculated that aquaporins may offer some protection against successive freezing and thawing events, maintaining the permeability of water in the membranes at low temperatures.

Types

Aquaporin molecules are known from various lineages, both in plants and animals and in less complex organisms, and these are very similar to each other - we assume then that they appeared early in evolution.

Some 50 different molecules have been found in plants, while mammals have only 13, distributed throughout various tissues, such as the epithelial and endothelial tissue of the kidney, lung, exocrine glands and organs related to digestion.

However, aquaporins can also be expressed in tissues that do not have an obvious and direct relationship with fluid transport in the body, such as in astrocytes of the central nervous system and in certain regions of the eye, such as the cornea and ciliary epithelium.

There are aquaporins even in the membrane of fungi, bacteria (such as E. coli) and in the membranes of organelles, such as chloroplasts and mitochondria.

Medical pathologies associated with aquaporins

In patients who have a defect in the sequence of aquaporin 2 present in kidney cells, they must drink more than 20 liters of water to stay hydrated. In these medical cases, there is no adequate urine concentration.

The opposite case also results in an interesting clinical case: the production of aquaporin 2 in excess leads to the retention of excessive fluid in the patient.

During pregnancy, there is an increase in the synthesis of aquaporins. This fact explains the common fluid retention in expectant mothers. Similarly, the absence of aquaporin 2 has been linked to the development of a certain type of diabetes.

References

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