Flexible Exchange Rates

Abstract

Biochemistry![Figure][1] CREDIT: WATT ET AL., PROC. NATL. ACAD. SCI. U.S.A. 107 , 16823 (2010) The membrane-bound enzyme F-ATPase serves an important function in almost all walks of life. It makes ATP (from ADP and inorganic phosphate) by converting the passage of protons down an electrochemical gradient first into mechanical energy and thence into chemical energy. The parts of the enzyme (red and yellow) that bind ADP lie mostly outside the membrane, and three such binding sites are converted sequentially from open to loose to tight conformations. The energy contained in the movement of protons is extracted by forcing them through a membrane-embedded ring (brown) of identical subunits, each of which captures a single proton on a glutamate residue. Turning this ring rotates a central stalk (blue, purple, and green) at 6000 rpm, which drives the cycle of conformational changes. Watt et al. describe the crystal structure of the mitochondrial ring and show that it contains eight subunits, leading to a ratio of eight protons for three ATPs synthesized. What is puzzling is the range of stoichiometries documented thus far, with 10 to 15 subunits found in the F-ATPases of fungi, bacteria, and chloroplasts. All of these enzymes possess the canonical trio of ADP-binding sites, which means that the cost of each ATP varies from as little as 2.7 protons in animals to as many as 5in microbes. Proc. Natl. Acad. Sci. U.S.A. 107 , 16823 (2010). [1]: pending:yes