Starch is synthesized in plant leaves during the day, in order to serve as an energy source at night. Starch is stored as granulates. These insoluble highly branched chains have to be phosphorylated in order to be accessible for degrading enzymes. The enzyme glucan, water dikinase (GWD) phosphorylates at the C-6 position of a glucose molecule, close to the chains 1,6-alpha branching bonds. A second enzyme, phosphoglucan, water dikinase (PWD) phosphorylates the glucose molecule at the C-3 position. A loss of these enzymes, for example a loss of the GWD, leads to a starch excess (sex) phenotype. Because starch cannot be phosphorylated, it accumulates in the plastid.
After the phosphorylation, the first degrading enzyme, beta-amylase (BAM) is able to attack the glucose chain at its non-reducing end. Maltose is released as the main product of starch degradation. If the glucose chain consists of three or less molecules, BAM cannot release maltose. A second enzyme, disproportionating enzyme-1 (DPE1), combines two maltotriose molecules. From this chain, a glucose molecule is released. Now, BAM can release another maltose molecule from the remaining chain. This cycle repeats until starch is degraded completely. If BAM comes close to the phosphorylated branching point of the glucose chain, it can no longer release maltose. In order for the phosphorylated chain to be degraded, the enzyme isoamylase (ISA) is required