Abstract
Abstract The incorporation of water oxygen into ATP during photophosphorylation by chloroplasts varies from l0.5 to g1.0 atom of oxygen per mole of ATP formed. The ATP ⇄ HOH exchange, particularly in the presence of ascorbate and phenazine methosulfate or ferricyanide, is accompanied by small but significant Pi ⇄ HOH and Pi ⇄ ATP exchanges. The results are compatible with all exchanges occurring as a result of dynamic reversal of ATP formation at the catalytic site. Such interpretation is supported by ATP acceleration of a light-dependent Pi ⇄ HOH exchange, and can be rationalized with Pi and ADP inhibitory effects on the ATP ⇄ HOH exchange. Mg++ in excess of the ATP concentration is required for maximal ATP ⇄ HOH exchange. A small Pi ⇄ HOH exchange as well as a more prominent ATP ⇄ HOH exchange accompanies ATP synthesis in the dark resulting from an acid-base transition of chloroplasts.
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