GLYCOGEN - STRUCTURE & SYNTHESIS - GLYCOGENESIS

• Glycogen is the storage form of carbohydrate
• Liver Glycogen stores are increased in well fed state and depleted in fasting state
• Muscle Glycogen supplies ATP during muscle contraction

STRUCTURE OF GLYCOGEN:

• It is a homopolysaccharide composed of alpha-D glucose units
• Glucose units are held together linearly by alpha 1,4 linkages
• After 8-10 units of linear glucose units , branches are formed by alpha 1,6 linkages
• Synthesis and degradation of glycogen occurs in the cytoplasm 
• It is stored in the cytoplasm as granules

SYNTHESIS OF GLYCOGEN - GLYCOGENESIS

• Synthesis of glycogen occurs from glucose in the cytoplasm

• Synthesis requires ATP, UTP, and  Glucose

Synthesis occurs in 4 steps

1.SYNTHESIS OF UDP GLUCOSE

1.A) Glucose is converted to Glucose-6-Phosphate in the presence of the enzymes glucokinase in liver and Hexokinase in muscle.

--Glucose-6-phosphate is converted to glucose - 1 - phosphate in the presence of phosphoglucomutase

1.B) Glucose- 1-Phosphate combines with UTP in the presence of UDP glucose pyrophosphorylase to form UDP glucose & high energy phosphate

2.INITIATION OF GLYCOGEN SYNTHESIS:

--For the initiation of glycogen synthesis,a part of pre-existing glycogen or a specific protein called as glycogenin acts as a primer.

--Primer is the first point where the actual synthesis atarts.here in glycogen synthesis - glycogenin or small part of glycogen itself acts as a primer where the furthur attachment of glucose units take place

.

--Glycogenin is a dimeric protein i.e. made up of two subunits

--The -OH group of amino acid tyrosine of glycogenin is the site where glucose unit is first attached.

--The process of addition of glucose units is called as glycosylation and it continues till 7 units of glucose are added.

--Glycogen Synthase helps in the transfer of glucose units from UDP glucose to glycogenin / glycogen primer and later glycogenin acts as an acceptor for the rest of glucose units.

3.FORMATION OF LINEAR UNITS IN GLYCOGEN:

--the glycogen synthase transfers glucose from UDP glucose to the non-reducing end of glycogenin/glycogen primer to form alpha ,4 linkages.

--Glycogen synthase can only form the straight alpha ,4 linkages.

--UDP released convert back to UTP by nucleoside diphosphate kinase. 

4.FORMATION OF BRANCHES IN GLYCOGEN:

--For the formation of branches in glycogen structure branching enzyme called as 

amylo [1,4] --> [1,6]  transglucosidase is required.

--It forms the alpha 1,6 glycosidic bonds which serves as branch in the glycogen structure.

--After the addition of 2-4 glucose units in a linear manner,the branching enzyme  transfers about 7-8 units of glucose from the non-reducing end(i.e end away from the prime) and binds at any other site

--The new 7-8 glucose units are held to the linear structure by alpha 1,6 linkages

--After this process of branching furthur new glucose units are added linearly to the newly formed branch by the enzyme glycogen synthase

--These steps result in the formation of a highly branched tree like structure of glycogen

In this whole process, 2 ATP are utilized

--   1 ATP for phosphorylation of glucose

 --  1ATP for the conversion of UDP to UTP