Energy Generation Coupled with Decarboxylation Reactions in Lactic Acid Bacteria
Kei Nanatani and Keietsu Abe
from: Lactic Acid Bacteria and Bifidobacteria: Current Progress in Advanced Research (Edited by: Kenji Sonomoto and Atsushi Yokota). Caister Academic Press, U.K. (2011)
In bacteria, many biological reactions are sustained by metabolic energy present in the form of phosphoester bonds, in compounds such as ATP and phosphoenolpyruvate (PEP), or in the form of ion gradients, such as the proton motive force (PMF) and the sodium motive force. The two forms of metabolic energy can be interconverted by FoF1-ATPases, which catalyze the translocation of H+ (or Na+) concomitant with either the hydrolysis or synthesis of ATP. Nutrient transport in bacteria is usually thought to consume metabolic energy; however, over the last two decades, a new class of nutrient transport reactions has been identified, in which substrate transport generates rather than consumes energy. The reaction consists of two steps: (1) electrogenic exchange of a precursor (amino acid or other organic acid) with its intracellular metabolic product produced by decarboxylation, and (2) intracellular decarboxylation of the transported precursor. The precursor:product exchange causes a net charge movement, which generates a membrane potential of physiological polarity, and the intracellular decarboxylation consumes cytoplasmic protons to generate both a pH gradient of physiological polarity and an outward concentration gradient of the end-product, which drives precursor uptake. The combined activities constitute a metabolically-driven proton pump (proton-motive metabolic cycle), which provides sufficient energy to generate ATP in a process called "decarboxylative phosphorylation". Thus, the proton-motive metabolic cycle can be recognized as a new class of ATP generation system that is distinct from substrate-level phosphorylation, oxidative phosphorylation, and photo-phosphorylation. We consider that the proton-motive metabolic cycle could be made available as an artificial energy-supply system in various industrial fermentation organisms with the use of recombinant DNA technology in the future read more ...