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Archived Comments for: Evolution of vacuolar proton pyrophosphatase domains and volutin granules: clues into the early evolutionary origin of the acidocalcisome

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  1. Error in logic in article

    Adrian Goldman, University of Helsinki

    26 January 2015

    This article posits in the discussion that acidocalcisomes could have been used as a source of energy polyphosphate to drive the V-PPase and create a proton motive force in the LUCA. This is not possible. In modern organisms, the V-PPase has the intracellular region of the protein, which hydrolyses PPi, facing the cytoplasm. The extracellular region faces the vacuole/acidocalcisome/extracellular medium, into which H+/Na+ are pumped, thus acidifying it. That gradient is then available in principle for use by a similarly-oriented ATPase to synthesise ATP from ADP+Pi, which is the authors' hypothesis. This _requires_ that the PPi is on the intracellular side - which it isn't in the acidocalcisome. Alternatively, if the V-PPase were turned around to face the lumen side of the acidocalcisome, it would be pumping protons _out_ into the cytoplasm, thus acidifying and killing the cell! So, though I am completely comfortable with the idea that V-PPases are ancient enzymes, the posited role of V-PPase in energy metabolism in this paper can not be true.

    In addition, the structure of two V-PPases were solved after this paper was written, and it is _completely_ clear that polyphosphate is not a viable substrate. It would protrude out of the hydrolytic centre, preventing closure and thus preventing pumping.

    Competing interests