Glycogenesis

Glycogen synthesis is the construction of the memory form of glucose ( dextrose), the glycogen. This is composed of long, branched chains composed of individual glucose molecules. The process takes place in most living things.

This makes single glucose molecules can be connected, they must be implemented by phosphorylation in UDP -glucose and thus activated first. This link creates a branched molecule. This structure has the advantage that glycogen can be reduced about in the muscle during contraction faster energy delivery to many ends simultaneously.

The reverse process is glycogenolysis.

Construction steps

The glycogen synthesis consists of several steps:

Hexokinase reaction

In the first step, glucose is converted to glucose-6 -phosphate. This is the glucokinase ( hexokinase IV) also phosphorylated in the glycolysis of glucose; the reaction consumes per glucose unit is a molecule of ATP as an energy supplier, and phosphate. An ADP - molecule is produced as a by-product:

ATP ADP

Phosphoglucomutase reaction

Glucose -6-phosphate by phosphoglucomutase is isomerized to glucose -1-phosphate, in which the equilibrium reaction is shifted due to the continuous removal of the glucose-1- phosphate by the next reaction in the right side. As an intermediate product of the reaction generated glucose -1 ,6- bisphosphate:

Glucose-1- phosphate transferase reaction UTP

The actual activation of the glucose -1-phosphate occurs by the reaction with uridine triphosphate (UTP). In this case, a portion of the UTP is linked, while the two outer phosphate residues (beta- and gamma- phosphate) of the UTP are cleaved to the phosphate group of the glucose. The result is activated UDP -glucose.

The reaction is catalysed by the glucose -1 -phosphate UDP - transferase.

Glycogen synthase reaction

UDP-glucose is used for the actual synthesis of glycogen, the UDP-glucose transferred through glycogen synthase to an existing glycogen and uridine diphosphate (UDP) is cleaved. For the initial establishment of glycogen is a starter molecule, a so-called core protein, is needed which is provided by the glycogenin. This is the center of each glycogen molecule. It has even some molecules α -1 ,4- glycosidically linked glucose required of glycogen synthase as a primer - this enzyme slides as the slider of a zipper to the existing chain of glucose molecules along and can not determine a start point itself. This reaction triggers a chain unbranched glycogen.

Branching enzyme

1,4- α -glucan branching enzyme or branching enzyme serves to produce branching in the linear chain: it cuts the strand every 7 to 12 glucose units and inserts the cut-off piece of alpha-1 ,6 -glycosidic branching as lateral to a minimum of 11 molecules long chain again.