13.2: Membrane Transporters
Transporters are essential membrane transport proteins with functions related to cell nutrition, homeostasis, communication, etc. Approximately 7% of all genes in the human genome code for transporters or transporter-related proteins.
Transporters are mainly composed of alpha-helices, built from bundles of ten or more helices traversing the plasma membrane. The solute-binding sites are located midway, where some of the helices are broken or distorted, making space for the binding site through the membrane. The two major superfamilies of transporters include the ATP binding cassette (ABC) and solute carrier (SLC) transporters. ABC transporters utilize energy from ATP hydrolysis and function as efflux transporters, whereas SLC transporters are essentially involved in the uptake of small molecules into cells.
Coupled transporters like symporters and antiporters are also called secondary active transporters. For example, when we eat a chocolate bar, the sodium-glucose symporter helps absorb the glucose from the intestinal lumen into the small intestine's epithelial cells before passing it on to the bloodstream. The glucose concentration is low in the epithelial cells and high in the intestinal lumen. Therefore, glucose is transported against its concentration gradient. The energy for the transport comes from the potential energy of the sodium’s electrochemical gradient. The concentration of sodium ions inside epithelial cells is kept very low through the sodium-potassium pump's constant action. Therefore, the influx of sodium down the concentration gradient pumps the glucose against its concentration gradient into the epithelial cells. Thus, during the mechanism of cotransport, transport of one type of solute down the concentration gradient propels the transport of another solute against its gradient concentration.
Transporters seen distributed in the hepatic, intestinal, and renal epithelia often pump the drugs out of the cell, rendering several drugs ineffective. Modulating the activity of such transporters is one of the therapeutic ways to manage drug resistance.
