Starch biosynthesis: experiments on how starch granules grow in vivo

Four varieties of starch granules from potato, wheat, maize, and rice were fractionated into homogeneous 10-microm-sized ranges. The size with the largest amount of granules was reacted with ADP-[(14)C]Glc, washed, and peeled into 7-9 layers, using a controlled peeling process, involving 90:10 volume proportions of Me(2)SO-H(2)O at 10 degrees C. All of the starches showed biosynthesis of starch throughout the granules. Starch synthase activities were determined for each of the layers. Three of the starches had a relatively large amount of synthase activity in the second layer, with only a small amount in the first layer. Potato starch had the largest amount of activity in the first layer. Starch synthase activity was found to alternate between higher and lower activities throughout all of the varieties of granules, showing that the synthesis was not uniform and also was not exclusively occurring at the surface of the starch granules, which had previously been hypothesized. From these results and our previous studies on the mechanism of starch chain elongation by the addition of d-glucose to the reducing end of a growing chain that is covalently attached to the active site of starch synthase, a hypothesis is proposed for how starch granules grow in vivo.