Biochemistry/Glycogenolysis and glycogenesis

Structure of glycogen

Glycogen is ,one of the storage type of glucose, found in animals. It has branched shape which give glycogen two advantage:

1. Consuming less space
2. Allows to be brokendown and to be constructed fast. The reason for this that it leaves many ends for functioning of many enzymes at the same time.

The process which glycogen degradation happens called glycogenolysis whereas the construction of glycogen from glucose monomers named gycogenesis.

In glycogen, teo types of bond connects monomers. One of them is alpha-1,4 bond links linear ordered glucose residues. The other one has the name alpha-1,6 linkage provides connection between two parallel glucose chain. Via alpha-1,6 bond new branches can be made. Thus it is branching linkage.

Glycogen breakdown

Alpha-1,4 linkage is degraded by the glycogen phosphorylase, shortly phosphorylase. So phosphorylase can rupture interrraction between glucose residues that are in linear order. Phosphorylase breaks glucose residue as glucose 1- phosphate which needed to be further processed. Note that also parallel strand is cut with the target one however in figure to illustrate easier it is not mentioned. Phosphorylase continues until come near to the alpha-1,6. After that point, an enzyme having the opportunity of transferase and debranching activity takes the remaining job. Firstly, be means of its transferase activity, this enzyme transfer remained glucose residues, except the one located on branching point, to the end of the chain which degraded chain is linked. Now, the turn comes to disrupt the branching point, means that alpha-1,6 linkage. This process is carried out again by the enzyme also having debranching ability. By this ability, the enzyme yields glycogen that lost its one branch and glucose.

1st step

Glucose 1-phosphate gained by means of glycogen phosphorylase, is converted into glucose 6-phosphate that will be transported into endoplasmic reticulum where glucose 6-phosphate is divided into inorganic phosphate (Pi) and glucose molecule.Then, glucose is released form E.R. by the help of its transported, as it is true for Pi.

2nd step

Finally, glucose is mix into blood in order to reach organs.

Glycogen synthesis

In glycogen synthesis, NDP-glucoses(generally UDP-glucose) are used as reservoir for glucose.

Firstly, investigate what happens when a new branch is constructed on already existed glycogen polymer:

As a first step, a branching point must be provided. After branching point is constructed, now, new glucose residues are able to be inserted. However this addition is achieved by transferring a few existing residues to newly formed chain. After that new glucose molecules could join into glycogen with normal addition reactions. First two steps including construction of branching point and transfer process occurs via functioning of an enzyme that have both transferase and branching activity. Elongation of branched chain happens via the help of glycogen synthase.

Secondly, how glycogen synthesized, de novo? Glycogen synthesis begins with glycogenin, which behaves as primer for joining of new glucose residues. New glucose residues generated until chain reaches 8 unit. Then, new branches constructed. Glycogenin does not leave the structure after glycogen is formed, it preserves its position inside the glycogen.

Regulation

Controlling the activation of enzymes is a good way to regulate an intracellular events. Glycogen breakdown and built up are regulated via this way. Gycogenolysis utilizes the enzyme phosphorylase while glycogenesis makes use of glycogen synthase. Both of these enzyme have two form called a and b demonstrating that they are active or passive, a indicates enzyme is active as b indicates enzyme is in its deactivated form.

Glycogen phosphorylase: Phosphorylase a is the phosphorylated form of phosphorylase b. Glucagon provides phosphorylation of phosphorylase b and converts it phosphorylase a . As opposite to glucagon, insulin breaks phosphoryl group linked to phosphorylase a to change it to b form, in the presence of glucose. The responsibility of glucose is to bind phosphorylase A so as to force a conformational change. This conformational alteration lets a phosphatase(induced by means of insulin) to act on phosphorylase a. In addition to these, just allosterically linkage of glucose phosphorylase a also decrease the action phosphorylase a although it is not converted to its b form.

Glycogen synthase: Glycogen synthase b is the phosphorylated form of glycogen synthase a. Therefore, as opposite to phosphorylase ,whose activated form is phosphoylated, glycogen phosphorylase is inactive in its phosphorylated form. As insulin inhibits phosphorylation of glycogen syntase a, glucago provokes this phosphorylation.

References

• Nelson, D. L., Lehninger, A. L., & Cox, M. M. (2008). Lehninger principles of biochemistry.