GLYCOGEN METABOLISM

DEFINITION:

·          Glycogen is readily mobilized storage form of glucose.

·          It is very the large branched polymer of glucose residues that can   be broken down to yield glucose molecule when energy is needed.

·          Most of this glucose is linked by α-1,4-glycoside bonds and α -1,6-glycosidic bonds.

·         It is branched at about every tenth residues.

 

 

GLYCOGEN CHEMISTRY:

GLYCOGEN CHEMISTRY

·          Glycogen’s molecular weight is about 10⁷ Da and consist of polysaccharides chain each containing about 13 glucose residues.

·          Chain may be branches (inner layer) or unbranched (outer layer) and are arranged in 12 concentric circles.

 

Glycogen Chemistry

 

GLYCOGEN STORAGE SITES

 

·          Glycogen is stored mainly in the liver and muscles.

·          Liver contact of glycogen is greater than muscles.

·          Since the muscles mass of the body is considerably greater than that of the liver, about three-quarters of total  body glycogen is in muscles.

 

Function Of Glycogen Metabolism

Function Of Glycogen Metabolism 

PROCESSES OF GLYCOGEN METABOLISM

 

There are two main processes in glycogen metabolism.

·           Glycogenesis

·           Glycogenolysis 

 

Glycogenesis and Glycogenolysis

Glycogenesis and Glycogenolysis 

GLYCOGENESIS

Glycogenesis is the synthesis of glycogen from glucose.

There are four different steps of glycogen synthesis.

·          Activation of Glucose

·          Initiation

·          Elongation

·          Glycogen Branching 

 

STEP-1:   ACTIVATION OF GLUCOSE:

1.  The enzyme “Hexokinase” (in muscles) and “glucokinase” (in liver) convert glucose to glucose-6-phosphate by taking one phosphate group from ATP.

2.  Phosphoglucomutase catalysis the conversion of glucose-6-phosphate to glucose-1-phosphate.

3.  Uradine diphosphate glucose (UDP) is synthesized from glucose-1-phosphate and UTP by UDP-glucose pyrophosphate.

 

Step-1:                Activation of Glucose

 Activation of Glucose 

STEP-2:   INITIATION:

·          A small fragment of pre-existing glycogen must act as a primer to initiate glycogen synthesis (glycogenesis) called as “Glycogen”.

·          Glycogen receives glucose from UDP. The hydroxyl group of OH of amino acid tyrosine of Glycogenin is the site at which the The initial glucose unit is attached.

 

Step-2:                Initiation

Initiation

STEP-3:   ELONGATION:

·          Glycogen synthase is responsible for the formation of α-1,4-glycosidic linkages.

·          The enzyme transfer the glucose from UDP-glucose to non-reducing end of glycogen to form an α-1,4-glycosidic linkage

·         es.

 

STEP-4:   BRANCHING:

·          Glycogen synthase can catalyze the synthesis of linear unbranched molecules with α-1,4-glycosidic linkages.

·          Glycogen is a large branched tree-like structure.

·         The formation of branches is brought about by the action of a branching enzyme, namely “glycosyl-4,6-transferase”.

·         It only makes a linear structure, not a branched structure.

 

·          If any chain continues from C-1 to C-16. so enzyme “glycosyl-4,6 linkage transferase” will cut the chain from  C-10 to C-16 and join it with the carbon of another chain of C-7 to make branched structure. In other words “glycosyl α-1,4-linkage transferase” will be converted into “glycosyl α-1,6-linkage transferase”.

·          This leads to the formation of a new non-reducing ends, besides the existing one.

·          Glycogen is further elongated and branched, by the enzymes “Glycogen synthase” and “glycosyl-4,6 linkage transferase”