Active transport of nonpolar amino acids in Chromatium vinosum

Abstract

The photosynthetic purple sulfur bacterium, Chromatium vinosum, takes up the amino acids, l-phenylalanine and l-leucine, via two apparently different electrogenic, H +/amino acid symports. Na + serves as an allosteric modulator for leucine transport, lowering the K m for leucine from 66 to 15 μ m. C. vinosum cells also contain a system that transports both isoleucine and valine. The isoleucine/valine system has the attributes of a H +/amino acid symport at pH < 7.5 but appears to function as a H +/Na + (Li +)/amino acid symport at pH ⩾ 7.5. Na + gradients produce an allosteric lowering of the K m values for both isoleucine and valine, from 14 to 7 μ m and from 34 to 17 μ m, respectively. C. vinosum also accumulates d-alanine in an energy-dependent reaction. The transport process appears to involve the electrogenic cotransport of d-alanine and Na +. The K m value for d-alanine was determined to be 9 μ m. Unlike the previously characterized C. vinosum l-alanine/Na + symport, Na + gradients did not affect the K m for d-alanine transport. l-Alanine and glycine, but not α-aminoisobutyric acid, act as competitive inhibitors for d-alanine transport.