A cycle of deprotonation and reprotonation energizing amino-acid transport?

Abstract

Although lowering the pK2 of neutral amino acids only weakens their concentrative uptake by Ehrlich cells, the same change greatly enhances uptake of diamino acids. This effect does not arise merely from putting the distal amino group in its uncharged form, but depends on an enhanced deprotonation of the a-amino group. Parallel effects are seen for the transport system for basic amino acids, for which the assignment of pK values within the membrane is less ambiguous. To explain the paradoxical advantages of having the a-amino group protonated yet readily deprotonated, we propose that a proton withdrawn from that group is pumped over an intramembrane interval to energize amino-acid transport.Although a driving of amino-acid uptake by cotransport with Na+ has long been known (1-4), this has recently been shown not to be the obligatory mode of energization (5). Furthermore, Na+-independent systems can unambiguously produce uphill transport (5). Therefore, we have long given attention to suggestive effects of modifying the H+-dissociation of the amino acids as clues to a more fundamental mode of energization (ref. 5; ref. 6, pp. 91-92; refs. 7-9). Table 1 illustrates that a systematic lowering of pK2' of neutral amino acids (in this case by introducing one, two, or three fluorine atoms) decreases both the initial rate of uptake and the steady-state distribution attained with Ehrlich ascites tumor cells. Fig. 1 shows a corresponding effect of the presence of two fluorine atoms in the amino-acid molecule: the pH optimum for uptake is lowered sharply. These results indicate that the amino acid is accepted for uptake by the system in question with the amino group in the form RNH3+ rather than RNH2. This result did not prove, however, that the recognition site at the interior surface of the membrane has the same preference. In 1958 we considered, using Fig. 2, whether a difference in the state of protonation preferred at the two surfaces might produce co-or countertransport of H+ with the amino acid (3). The figure shows a hydrogen ion left behind when the amino acid combines with the external receptor site, and a new hydrogen ion taken up by the amino acid on entering the cytoplasm. This scheme produces a countertransport between H+ and the amino acid, which was in 1958 an arbitrary choice over cotransport. Possibilities of this kind have subsequently become more important through the development by Mitchell of his chemiosmotic hypothesis for the intermediation of hydrogen ion gradients in the process of energy transduction by the mitochondrial and chloroplast membrane (11,12). The observation of cotransport of H+ with amino acids (see for example refs. 8 and 13) and with sugars (14-16) has accelerated the extension of these ideas to the plasma membrane (see ref. 17).Uptake of the lower homologs of lysine and ornithine By chance we encountered in 1952 behavior revealing that the data of Table 1 and Fig. 1 do not tell the whole story of the role of dissociation in amino-acid transport (18). The structural feature...

Na+/L+ selectivity in transport systemA: Effects of substrate structure

“…Some diamino acids with pK~ < 8.5 show paradoxical intensity of transport by two neutral systems [5,6]. 2,4-Diaminobutyric acid and thialysine show extreme positions for their rate responses to Na + (Figs.…”