Essential sulfhydryl groups and the light-dependent transhydrogenase system of Rhodopseudomonas spheroides: localization of substrate binding sites and evidence for masked or buried sulfhydryl groups in the peripheral protein factor.

The light-dependent transhydrogenase system of Rhodopseudomonas spheroides which consists of a peripheral protein factor and a membrane-bound component contains essential sulfhydryl groups that are sensitive to p-hydroxymercuribenzoic acid (PMB). There are two types of sulfhydryl groups required for light-dependent (LD) transhydrogenation. One type is associated with the membrane-bound component and participates in or influences the binding of one of the substrates, NADH. A second type is associated with the peripheral protein factor and is not involved with the binding of the substrates. These sulfhydryl groups are masked or buried in the protein and are only exposed upon treatment of the peripheral protein factor with urea or trypsin. The peripheral protein factor may be an integral part of the transhydrogenase complex or may be required for the energization process. This factor appears to play an important role in the activation and control of LD-transhydrogenation.