Glycogen Metabolism Tournament
Two onboarding diagrams place glycogen metabolism inside carbohydrate metabolism. Then eight MCAT-DoK rounds: branching every 8-12 residues, the limit-dextrin / Cori-disease story, the GSD I-V mnemonic, the regulatory difference between liver and muscle phosphorylase, the surprising ATP cost of glucose release, and reciprocal phosphorylation logic.
Where the Glycogen metabolism fits in Metabolism of carbohydrates
Glycogen metabolism is the cell's branched-glucose buffer. Glycogenesis stores glucose during the fed state; glycogenolysis mobilizes it during fasting (in liver) or contraction (in muscle). To enter the tournament, click the highlighted Glycogen metabolism box.
Click the highlighted Glycogen metabolism box to continue.
What this tournament tests
Each task maps to a distinct MCAT cognitive demand. The first two orient you in the broader topology; the next eight test the high-yield mechanism, regulation, sequence and quantitative reasoning that consistently appear on test day.
The Bigger Picture
Locate glycogen metabolism inside Reactome's Metabolism of Carbohydrates overview.
Whole-Pathway Overview
Pan and zoom the WikiPathways glycogen-metabolism figure before you play.
Fill in the Blank
Recall that branching every 8-12 residues maximizes phosphorylase access.
Disruptor
Diagnose Cori (Type III) disease from the limit-dextrin morphology.
Sequence Ordering
Walk glycogenesis from G6P through glycogenin to a branched glycogen polymer.
Match the Pairs
Pair each glycogen-storage disease (I-V) with its missing enzyme.
Numeric Input
Show that liver releases free glucose with zero net ATP cost via phosphorolysis.
Select All That Apply
Discriminate true facts about MUSCLE glycogen from common MCAT confusions.
Odd One Out
Spot the glycolysis enzyme hiding among the glycogen-only enzymes.
Reciprocal Regulation
Reason about how one phosphorylation cascade flips synthesis OFF + breakdown ON.
Public leaderboard
Your score posts to a global, persistent leaderboard scored by points first, time as tiebreaker.
Glycogen metabolism in 60 seconds
Glycogen is built on a tyrosine residue of glycogenin (the autocatalytic primer). The chain extends through α-1,4 bonds via glycogen synthase (UDP-glucose donor); branches go in every 8-12 residues via the α-1,6 transferase activity of branching enzyme. Breakdown is the mirror image: phosphorylase takes glucose-1-P off α-1,4 chains; debranching enzyme handles the limit dextrin.
Reciprocal regulation is the textbook MCAT principle. Glucagon (or epinephrine) -> PKA -> phosphorylase kinase phosphorylates BOTH phosphorylase (ON) and synthase (OFF). PP1, activated downstream of insulin, dephosphorylates BOTH (synthase ON, phosphorylase OFF). Same switch, two opposite outcomes - no futile cycling.
Liver vs muscle is the biggest MCAT trap. Liver = blood-glucose homeostasis; has G6Pase; responds to glucagon; phosphorylase a inhibited by free glucose. Muscle = local energy; no G6Pase (so no free glucose released); no glucagon receptor; phosphorylase b allosterically activated by AMP + Ca2+ (via the calmodulin subunit of phosphorylase kinase).
GSDs I-V: Very Poor Carbohydrate Metabolism = Von Gierke (G6Pase), Pompe (lysosomal acid α-glucosidase, classic cardiomegaly + early death), Cori (debrancher), Andersen (brancher), McArdle (muscle phosphorylase, exercise intolerance + 'second wind').
FAQ
Why is glycogen branched?
Branches multiply the number of non-reducing ends, which is where phosphorylase works. More branches -> faster mobilization. Branching also keeps glycogen highly soluble; a fully linear polysaccharide would crystallize like cellulose.
Why does Pompe disease specifically affect the heart?
Pompe (Type II GSD) is a lysosomal acid α-glucosidase deficiency. Glycogen accumulates in lysosomes of every tissue, but the heart and skeletal muscle handle the heaviest glycogen load and lysosomal rupture leads to cardiomegaly and respiratory failure in classic infantile-onset Pompe.
Do I need an account to play?
No. The tournament is fully public. You get a randomized handle and your score posts to the public leaderboard at the bottom of this page.
Keep going
Where the G1P -> G6P from glycogenolysis goes once it enters glycolysis (via phosphoglucomutase).
Liver runs gluconeogenesis and glycogenolysis simultaneously during prolonged fasting.
Overview diagram: Reactome Pathway R-HSA-71387, licensed CC BY 4.0.