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Related Concept Videos

Aquaporins01:25

Aquaporins

Aquaporins or AQPs are a family of integral membrane proteins whose primary function is to transport water, while some called aquaglyceroporins also transport glycerol. In addition, aquaporins have also been suspected to be involved in transporting volatile substances, such as carbon dioxide and ammonia, across membranes. Such AQPs that act as gas channels are often highly expressed in cells involved in the gaseous exchange, such as red blood cells, epithelial cells, and pulmonary capillaries.
Ligand Binding Sites02:40

Ligand Binding Sites

Proteins are dynamic macromolecules that carry out a wide variety of essential processes; however, the activities of most proteins depend on their interactions with other molecules or ions, known as ligands.
Protein-ligand interactions are quite specific; even though numerous potential ligands surround a cellular protein at any given time, only a particular ligand can bind to that protein. Moreover, a ligand binds only to a dedicated area on the surface of the protein, known as the...
Reabsorption and Secretion in the DCT and Collecting Duct01:26

Reabsorption and Secretion in the DCT and Collecting Duct

The early phase of the DCT manages the reabsorption of approximately 10-15% of filtered water, 5–10% of filtered sodium, and 5–10% of filtered chloride. This process is facilitated by Na+–Cl− symporters in apical membranes and sodium-potassium pumps, as well as Cl− leakage channels in basolateral membranes. The early DCT also stands out as a site where parathyroid hormone (PTH) stimulates calcium reabsorption, depending on the body's requirements.
The distal part of the DCT, along with the...
Pore Transport and Ion-Pair Transport01:17

Pore Transport and Ion-Pair Transport

Pore transport and ion-pair formation are critical mechanisms for the absorption and distribution of drugs in the body.
Pore transport, also known as convective transport, is a process where small molecules like urea, water, and sugars rapidly cross cell membranes as though there were channels or pores in the membrane. Although direct microscopic evidence is limited  but the concept of pores or channels is widely accepted based on physiological evidence. Despite the lack of direct microscopic...
Facilitated Diffusion01:16

Facilitated Diffusion

The plasma membrane, a critical structure in cellular biology, houses an array of transporters, or carrier proteins, interspersed within its lipid bilayer. These proteins play a crucial role in solute transport through facilitated diffusion, a form of passive diffusion that uses transporters to move the molecules across the membrane.
In this process, substrates such as organic compounds and ions interact with a transporter on one side, triggering conformational changes in proteins that enable...
Physiology of the Genitourinary System II: Tubular Reabsorption and Secretion01:22

Physiology of the Genitourinary System II: Tubular Reabsorption and Secretion

The kidneys maintain homeostasis through filtration, reabsorption, and secretion. Tubular reabsorption and secretion are crucial in forming urine and regulating electrolytes, water balance, and waste elimination.Tubular Reabsorption and Secretion ProcessesTubular reabsorption is the process that reclaims essential substances such as electrolytes, glucose, amino acids, and water from the glomerular filtrate back into the bloodstream. This is achieved through passive and active transport...

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Tracking Single Proteins in Lipid Bilayers Using Fluorescence Microscopy
08:39

Tracking Single Proteins in Lipid Bilayers Using Fluorescence Microscopy

Published on: December 12, 2025

Structural context shapes the aquaporin selectivity filter.

David F Savage1, Joseph D O'Connell, Larry J W Miercke

  • 1Graduate Group in Biophysics, Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94158-2517, USA.

Proceedings of the National Academy of Sciences of the United States of America
|September 22, 2010
PubMed
Summary
This summary is machine-generated.

Aquaporins selectively transport water or glycerol. Engineering specific amino acids in aquaporin Z (AqpZ) altered water and glycerol transport, revealing that channel size and hydrophilicity determine substrate selectivity.

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Area of Science:

  • Membrane Biology
  • Structural Biology
  • Biochemistry

Background:

  • Aquaporins are protein channels crucial for transmembrane water and small molecule transport.
  • Distinct aquaporin subfamilies facilitate either pure water or water and glycerol conduction.
  • The structural basis for aquaporin substrate selectivity remains incompletely understood.

Purpose of the Study:

  • To investigate the role of specific amino acid residues in determining aquaporin substrate selectivity.
  • To understand how structural modifications affect water and glycerol permeability in aquaporins.

Main Methods:

  • Site-directed mutagenesis of aquaporin Z (AqpZ) to engineer glycerol-conducting subfamily residues.
  • Functional assays to measure water and glycerol permeability of mutant channels.
  • X-ray crystallography to determine the high-resolution structures of mutant AqpZ channels.

Main Results:

  • Engineered AqpZ mutants showed reduced water permeability without increased glycerol conduction.
  • X-ray structures revealed pore sizes similar to wild-type AqpZ, indicating size alone is insufficient for selectivity.
  • Channel hydrophilicity dominated water transport energetics, while cross-section size governed selectivity for glycerol.

Conclusions:

  • Aquaporin selectivity is a complex interplay of factors, including hydrophilicity and pore dimensions.
  • While the selectivity filter is key, accessory structural elements also significantly contribute to substrate discrimination.
  • Understanding these determinants is vital for characterizing aquaporin function and engineering novel transport properties.