Cotransporter

A cotransporter is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient.

It is sometimes equated with symporter, a word made up of a conjunction of the Greek syn- or sym- for "together, with" (cf. symphony, synonym) and -porter. Symporter is also sometimes misspelled simporter because of the simultaneous transport of molecules (and the phonetic resemblance to symporter).

In order for any protein to do work, it must harness energy from some source. In particular, symporters do not require the splitting of ATP because they derive the necessary energy for the movement of one molecule from the movement of the another. Overall, the movement of the two molecules still acts to increase entropy.

Proton-sucrose cotransporters are common in plant cell membranes. An ATP molecule in the cell phosphorylates a carrier protein, causing a conformational change that shuttles a proton across the membrane. The proton binds with sucrose in the extracellular fluid, then undergoes passive transport down its concentration gradient (i.e. up the concentration gradient of sucrose).

In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.^[1] Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.^[2][3]

See also

• Na-K-2Cl symporter
• K-Cl cotransporter
• Sodium/phosphate cotransporter
• Sodium-glucose transport proteins
• Glucose transporter
• Co-transport

References

1. ^ Robert K. Crane, D. Miller and I. Bihler. “The restrictions on possible mechanisms of intestinal transport of sugars”. In: Membrane Transport and Metabolism. Proceedings of a Symposium held in Prague, August 22–27, 1960. Edited by A. Kleinzeller and A. Kotyk. Czech Academy of Sciences, Prague, 1961, pp. 439-449.
2. ^ Ernest M. Wright and Eric Turk. “The sodium glucose cotransport family SLC5”. Pflügers Arch 447, 2004, p. 510. “Crane in 1961 was the first to formulate the cotransport concept to explain active transport [7]. Specifically, he proposed that the accumulation of glucose in the intestinal epithelium across the brush border membrane was coupled to downhill Na+ transport cross the brush border. This hypothesis was rapidly tested, refined and extended [to] encompass the active transport of a diverse range of molecules and ions into virtually every cell type.”
3. ^ Boyd, C A R. “Facts, fantasies and fun in epithelial physiology”. Experimental Physiology, Vol. 93, Issue 3, 2008, p. 304. “the insight from this time that remains in all current text books is the notion of Robert Crane published originally as an appendix to a symposium paper published in 1960 (Crane et al. 1960). The key point here was 'flux coupling', the cotransport of sodium and glucose in the apical membrane of the small intestinal epithelial cell. Half a century later this idea has turned into one of the most studied of all transporter proteins (SGLT1), the sodium–glucose cotransporter.”

External links

• symport at eMedicine Dictionary
• cotransport at eMedicine Dictionary