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)

Glycogen Structure & Synthesis

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