De Novo Maltotriose Biosynthesis from the Reducing End by Spinacia oleracea L. Chloroplasts

The distribution of (14)C in the various glucose residues of maltotriose was studied as a function of time of photosynthesis of isolated chloroplasts of spinach (Spinacia oleracea L.) using (14)CO(2). The distribution of label showed that the reducing-end glucose residue was labeled first and the label subsequently distributed to the second and third glucose residues at approximately equal rates.A mechanism for the distribution of label and the synthesis of maltotriose from the reducing end is presented. The mechanism has postulated to be the same as that for the maltose synthase recently described by Schilling. Maltose biosynthesis from alpha-d-glucose-1-phosphate was characterized as involving two glucosyl intermediates by a double displacement mechanism with inversion of configuration. The mode of enzymic action by which maltosyl intermediates were transferred to glucosyl intermediates was consistent with the fractional distribution of radioactivity found in each glucose unit of maltotriose during short term photosynthesis experiments.