Glycogen Structure & Synthesis
ENDOGENOUS GLUCOSE POLYMERS
Glycogen
• Synthesized in liver & muscle (insulin: glucagon is high)
• Branched structure
• Linear segments: glucose monomers linked with alpha (1,4) glycosidic bonds
• Branch points: alpha (1,6) glycosidic bonds
• Branch point functions: i. solubilize glycogen ii. create terminal sugars for release
EXOGENOUS GLUCOSE POLYMERS
• Dietary
Amylopectin (starch)
• Fewer branches than glycogen
• Obtained from: potatoes, rice, etc.
Cellulose
• No branches
• Obtained from plants
• Glucose monomers linked with beta (1,4) glycosidic bonds
• Humans lack enzymes to break beta (1,4) glycosidic bonds
GLYCOGEN SYNTHESIS
1. Glucose + ATP --> Glucose 6P + ADP
• Hexokinase (M) and Glucokinse (L)
2. Glucose 6P --> Glucose 1P (reversible)
• Phosphoglucomutase
3. Glucose 1P + UTP --> UDP-glucose + PPi
• PPi + H2O --> 2Pi (drives reaction forward)
• UDP-glucose = substrate for glycogen synthesis
4. UDP-glucose + glycogen polymer --> glycogen polymer (+1 glucose residue) + UDP
• Glycogen synthase: alpha (1,4) glycosidic bonds (adds 1 glucose-residue/rxn)
5. Branching enzyme adds branches
• Breaks off (at least) 6 terminal residues from linear portion to make branch
• Catalyzes alpha (1,6) linkage
Glycogenin: primer for glycogen chain
• Catalyzes first 4-8 glucose residues
• First glucose binds tyrosine residue in glycogenin
• Glycogen synthase adds glucose residues to preexisting glucose polymer
CLINICAL CORRELATION
Type IV Glycogen Storage Disease: Anderson's Disease
• Branching-enzyme deficiency
• Presents as long, linear polymers of glucose
• Visible at very young age, produces cell damage
• Aka amylopectosis (amylopectin ~ glycogen w/ less branching)