Active transport of peptides in bacteria.

Energy-generating sugar efJlux during the steady state The net uptake of a non-metabolized glucose analogue is linear for a few minutes and then slowly starts to level off to a plateau value because the influx of sugar is balanced by sugar efflux which is mainly catalysed by the sugar transport system. The rate of sugar exchange is very high and can be twice that of the net uptake rate. Despite this high sugar transport rate there is apparently a lowered energy demand for transport, since the respiratory increase per sugar transported is only half of the value during net influx conditions (Decker & Tanner, 1972). It was concluded that efflux of sugar partly generates ‘energy’, to 50% in aerobic conditions and up to 90% in anaerobic conditions (Komor et af., 1973). The mechanism, explained in present terms, is that a proton is transported out of the cells, at 50-90% probability, together with sugar during the steady-state situation. The exit of a proton against the adversely oriented protonmotive potential difference at a high rate can be explained as a failure of the proton to dissociate fast enough on the internal side. As a result the exchange of sugar might occur and a back-shuttle of the proton-sugar complex over the small intra-membrane distance of the gate (Fig. 2) could occur without energetic problems. The slow association and dissociation of proton on the binding site or its access through the pore is a prerequisite for this interpretation.