Aquaporins

Synonym(s)

AQP

Definition
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Protein family whose members function as "water channels" in animal and plant cells. In mammals, aquaporins (AQP1) regulate the water balance of e.g. erythrocytes, kidney cells, keratinocytes and other cells. An erythrocyte, for example, has about 200,000 water channels per cell. In the alveoli, such "channels" provide the fluid film necessary for gas exchange. Malfunctions of the aquaporins are responsible for diseases such as diabetes insipidus, atopic eczema, cataracts and glaucoma. Aquaporin-3 is expressed more frequently in atopic eczema. This would be among others an explanation for the disturbance of the barrier function in atopics. Retinoids are also able to upregulate AQPs.

Classification
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Aquaporins are divided into:

so-called common aquaporins
Aquaglyceroporins.

Aquaglyceroporins conduct water as well as small organic molecules such as glycerol or urea.

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General information
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Cell membranes are hydrophobic in their interior. Therefore water can only diffuse through the cell membrane to a limited extent. Cells with very high water permeability, such as renal tubule cells or erythrocytes, require larger connecting channels, so-called "water channels", for rapid water exchange. Through these water channels the water can migrate almost unhindered in the direction of the osmotic gradient. The water transport of an aquaporin channel is up to 3 billion molecules per second.

Note(s)
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All known aquaporins have a similar structure and amino acid sequence with 268 amino acids. Aquaporins span the cell membrane (integral membrane protein) and structurally form an hourglass-like channel structure. AQP1 is inhibited by mercury, gold or silver ions. The ion binds to a cysteine in the pore entrance and thus blocks the water flow. The search for specific aquaporin inhibitors is the subject of current research.