Transport of branched-chain amino acids in Corynebacterium glutamicum

The transport of branched-chain amino acids was characterized in intact cells of Corynebacterium glutamicum ATCC 13032. Uptake and accumulation of these amino acids occur via a common specific carrier with slightly different affinities for each substrate (Km[Ile] = 5.4 microM, Km[Leu] = 9.0 microM, Km[Val] = 9.5 microM). The maximal uptake rates for all three substrates were very similar (0.94 - 1.30 nmol/mg dw.min). The optimum of amino acid uptake was at pH 8.5 and the activation energy was determined to be 80 kJ/mol. The transport activity showed a marked dependence on the presence of Na+ ions and on the membrane potential, but was independent of an existing proton gradient. It is concluded, that uptake of branched-chain amino acid transport proceeds via a secondary active Na+-coupled symport mechanism.