Accumulation of Neutral Amino Acids by Streptococcus jkecnlis

  • ESEKGY COUPLING
  • Published 2002

Abstract

We have studied the accumulation of amino acids by a strain of Sfrepfococcus faecalis (faecium) which does not carry out oxidative phosphorylation but relies upon glycolysis for metabolic energy. In the absence of exogenous substrate, cells take up limited amounts of glycine, L-alanine, L-serine, and L-threonine by exchange for components of the free amino acid pool. Net flux is much slower. When an energy source is provided, extensive net uptake takes place. Glycine and threonine accumulate largely as such, attaining concentration gradients of about 400 between cytoplasm and medium. Glycine, alanine, serine, and threonine undergo reciprocal exchange in starving cells, suggesting that they share a common transport system. A mutant, isolated by virtue of its resistance to cycloserine, was severely deficient in the transport of all four amino acids, both in the presence and in absence of an energy source. It thus appears that there is a single transport system for these amino acids, which can operate in two modes: tightly coupled exchange which is independent of metabolism, and net uptake linked to energy generation. The energy donor for metabolic accumulation is most probably ATP. Inhibition of accumulation by dicyclohexylcarbodiimide further implicates the membrane-bound ATPase in coupling glycolysis to the transport system. No evidence for an obligatory involvement of either K+ or Na+ was obtained, but inhibition by uncouplers strongly suggests that protons play a key role in energy coupling of amino acid transport. The effects of ionophores on amino acid accumulation are best understood in terms of Mitchell’s chemiosmotic hypothesis: extrusion of protons by the ATPase would generate a gradient of pH and of electrical potential (proton-motive force) across the membrane and establish a circulation of protons which is the immediate driving force for active transport. Metabolic accumulation (unlike exchange in starving cells) was totally blocked by proton-conducting uncouplers. These

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Cite this paper

@inproceedings{COUPLING2002AccumulationON, title={Accumulation of Neutral Amino Acids by Streptococcus jkecnlis}, author={ESEKGY COUPLING}, year={2002} }