text: SBPase is a homodimeric protein, meaning that it is made up of two identical subunits.  The size of this protein varies between species, but is about 92,000 Da (two 46,000 Da subunits) in cucumber plant leaves.  The key functional domain controlling SBPase function involves a disulfide bond between two cysteine residues. These two cysteine residues, Cys52 and Cys57, appear to be located in a flexible loop between the two subunits of the homodimer, near the active site of the enzyme. Reduction of this regulatory disulfide bond by thioredoxin incites a conformational change in the active site, activating the enzyme. Additionally, SBPase requires the presence of magnesium (Mg) to be functionally active.  SBPase is bound to the stroma-facing side of the thylakoid membrane in the chloroplast in a plant. Some studies have suggested the SBPase may be part of a large (900 kDa) multi-enzyme complex along with a number of other photosynthetic enzymes.