Operation of Cofactors in Two-Stage Photophosphorylation

Chloroplasts illuminated at p H 6 accumulate a limited amount of “XE” — an unidentified component which can phosphorylate ADP in a dark reaction at p H 8. The maximum formation of XE requires 15 to 30 seconds’ light if a cyclic cofactor is present, and otherwise as much as 4 minutes’ light. Ferricyanide and cyclic cofactors provide the highest XE yields, with other cofactors giving a lower value. The cofactorless system appears to have two components of which one is ascorbate dependent. The results suggest that XE is located at or near the phosphorylation site as studied in one stage photophosphorylation at p H 8.