kotransport

Kotransport, more commonly referred to as cotransport, is the process by which two or more chemical species cross a cellular membrane simultaneously through the same transport protein or protein complex. This mechanism is a form of secondary active transport: energy is not supplied by ATP hydrolysis at the transporter itself, but by an existing electrochemical gradient of one substrate that drives the transport of another. Cotransport can operate in two main modes: symport, where substrates move in the same direction, and antiport, where they move in opposite directions.

Examples include the sodium-glucose cotransporter SGLT in intestinal and renal epithelium, which uses the inward sodium gradient to move glucose against its gradient often with a 2 Na+ to 1 glucose stoichiometry. In bacteria, lactose permease LacY couples the inward proton motive force to uptake lactose. Cotransporters are commonly grouped in the solute carrier (SLC) families, such as SLC5 (SGLTs). Kotransport is essential for nutrient absorption, ion homeostasis, and cell signaling, and its efficiency depends on gradients, membrane potential, and transporter regulation.

Because cotransport couples substrate movement to energy stored in gradients, it is a target for drugs and therapeutic strategies. Inhibitors of SGLT, such as phlorizin and clinically used SGLT2 inhibitors, modulate glucose reabsorption in the kidney. Studying cotransport mechanisms helps in understanding epithelial physiology, membrane energetics, and the design of drug delivery systems. The term cotransport is sometimes used interchangeably with ko-transport in some literature, though cotransport is the more common form.