Sulfhydryl groups in photosynthetic energy conservation. VI. Subunit distribution of sulfhydryl groups and disulfide bonds in chloroplast coupling factor and ATPase activity.

The subunit distribution of sulfhydryl groups and disulfide bonds of spinach chloroplasts coupling factor I has been determined. Native coupling factor I with a latent ATPase activity has eight sulfhydryl groups distributed 4 : 2 : 0 : 0 : 2 in the alpha, beta, gamma, delta and epsilon subunits, respectively. Heat treatment of coupling factor I, in addition to the activation of its ATPase activity, induces a dithiol-disulfide interchange between the gamma and the alpha subunit, changing the sulfhydryl groups' distribution to 2 : 2 : 2 : 0 : 2. Reduction of disulfide bonds of coupling factor I by dithioerythritol during heat treatment gives a subunit distribution of 4 : 4 : 4 : 0 : 2, suggesting that native coupling factor I has three disulfide bonds, two in the gamma subunit and one in one of the beta subunits. The results suggest an asymmetric redox state of some of the subunits of coupling factor I and an asymmetric positioning of some of them in the molecular structure of coupling factor I.