Abstract
In bacteria, metabolic energy present in the form of a phosphoesterbond (e. g. ATP or phosphoenolpyruvate [PEP] ) and ion gradients (e. g. H+-proton motive force [pmf] or Na+ sodium motive force) are used to sustain various biological reactions. The two forms of metabolic energy can be interconverted by FoF1 -ATPases that catalyze the translocation of H+(or Na+) concomitant with the hydrolysis/or synthesis of ATP. Nutrient transport by bacteria is usually thought of as consuming such metabolic energy. Over the last ten years, however, a new class of nutrient transport reactions has been identified, one in which substrate transport is virtually used to generate rather than consume energy. The reaction consists of two steps, (i)electrogenic exchange of precursor (amino acids or organic acids) with its intracellular metabolic product (decarboxylation), and (ii) intracellular decarboxylation of the transported precursor. The net charge movement through the precursor: product exchange generates a membrane potential of physiological polarity, and intracellular decarboxylation consumes cytoplasmic protons to generate both a pH gradient of physiological polarity and an outward concentration gradient of the end-product to drive precursor uptake. The combined activities constitute a metabolically-driven proton pump (proton-motive metabolic cycle), allowing cells to display decarboxylative phosphorylation. Thus, it can be recognized as a new class of ATP generation systems besides substrate-level phosphorylation, oxidative phosphorylation, and photo-phosphorylation. The proton motive metabolic cycles would be available for artificial energy-supply systems in various fermentation organisms with recombinant technology.
