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Journal of Experimental Biology, Vol 196, Issue 1 1-4, Copyright © 1994 by Company of Biologists

JOURNAL ARTICLES

COUPLING AS A WAY OF LIFE

W. R. Harvey and C. L. Slayman

More than three decades years ago, Aharon Katchalsky and Ora Kedem (1958, 1962) pointed out that the essence of biological transport is coupling. This insight was not so much a deduction as a concise summary of observations. At an elementary level, movement of a solute in the simplest aqueous solution implies movement of a hydration shell, and therefore movement of water, thence coupling of water movement to solute movement. In biological systems, for our present purposes specifically in biomembranes, more complex coupling mechanisms were clearly perceived, albeit largely without mechanistic understanding. The necessary response of these and other physically oriented biologists was to seek an understanding of coupling processes, along with formal descriptions, in the familiar language of thermodynamics. Their effort produced an elegant systematic statement of transport coupling, in the so-called Kedem equation (Kedem, 1961; see Gerencser and Stevens, 1994): JS = (l/RS)(deltamuS + zSFdelta psi + RSWJW + RSAJA + RSBJB + RSRJR) , stating that any species (S) of ion or molecule capable of flow (J) across a biological membrane may do so under the influence of several categories of driving force: that due to thermal agitation acting on a concentration difference deltamuS, that due to an electric field (delta psi), that due to water flow (JW) and drag, that due to chemical/physical interaction with other transiting ions or molecules (JA, JB, ...) and that due to chemical coupling with metabolic reactions (JR). This formalism neatly solved a major conceptual problem of the time, i.e. how to define (primary) active transport in an unequivocal and still useful way: as endergonic transport due to coupling with JR alone, where the coupled exergonic reaction would provide the driving energy. [In this version of the Kedem equation, Rs is the resistance to movement of the solute, S; and RSW, RSA, RSB..., are the operational resistances of the membrane to the coupled flows.] A logical spin-off of this exercise was another useful definition, that of secondary active transport: i.e. transport of S coupled to JW, JA, JB, etc.), wherein S moves up its electrochemical gradient (usually concentrated into cytoplasm) at the expense of downhill movement of W, A, B, etc. Passive transport was taken by default as being due to deltamuS, or deltapsi, with both of these resultant flows being exergonic and requiring no other input of energy.

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