The Cori cycle describes the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles is transported to the liver and converted back to glucose, which then returns to the muscles to be used for energy. This is a key mechanism for maintaining blood glucose during prolonged exercise. The Cahill cycle involves the transport of alanine from muscle to the liver. The pentose phosphate pathway produces NADPH and ribose-5-phosphate. Glycogenolysis is the breakdown of glycogen, not the synthesis of glucose from lactate.

This patient has McArdle disease (glycogen storage disease type V), which is caused by a deficiency of myophosphorylase (muscle glycogen phosphorylase). This enzyme is required for the initial step of glycogenolysis in skeletal muscle. Its absence prevents muscles from accessing stored glycogen for energy during exercise, leading to cramps, myoglobinuria (dark urine), and a flat lactate curve during ischemic exercise. Glucose-6-phosphatase deficiency (von Gierke disease) affects the liver and causes fasting hypoglycemia. Liver glycogen phosphorylase deficiency (Hers disease) also causes hepatomegaly and mild hypoglycemia. Pyruvate kinase deficiency causes hemolytic anemia.

This presentation is characteristic of von Gierke disease (glycogen storage disease type I), caused by a deficiency of glucose-6-phosphatase. This enzyme is required for the final step of both gluconeogenesis and glycogenolysis. Its absence prevents the liver from releasing free glucose into the blood, leading to severe fasting hypoglycemia. The trapped glucose-6-phosphate is shunted into other pathways, causing lactic acidosis, hyperuricemia, and hyperlipidemia. The massive hepatomegaly is due to glycogen accumulation.

In the fed state, insulin signaling leads to the activation of protein phosphatase-1. This phosphatase dephosphorylates and activates glycogen synthase, the rate-limiting enzyme of glycogenesis, thus promoting the synthesis of glycogen for storage. Simultaneously, protein phosphatase-1 dephosphorylates and inactivates glycogen phosphorylase kinase and glycogen phosphorylase, inhibiting glycogen breakdown. Fructose-1,6-bisphosphatase is a gluconeogenic enzyme inhibited in the fed state. PFK-1 is allosterically activated, not directly by this phosphatase.

This presentation is characteristic of Cori disease (glycogen storage disease type III), caused by a deficiency of the debranching enzyme (α-1,6-glucosidase). This enzyme is needed to break down the α-1,6 linkages at branch points in glycogen. Its deficiency leads to the accumulation of abnormally structured glycogen (limit dextrin) with short outer chains. The clinical features include hepatomegaly and mild hypoglycemia, which are less severe than in von Gierke disease (glucose-6-phosphatase deficiency) because gluconeogenesis is intact.

Pyruvate kinase deficiency leads to decreased ATP production. In red blood cells, ATP is critical for maintaining the function of membrane ion pumps, such as the Na+/K+-ATPase. Failure of this pump leads to electrolyte imbalances, loss of the normal biconcave shape, and ultimately, extravascular hemolysis, resulting in chronic hemolytic anemia. While intermediates upstream of the enzymatic block (like 2,3-BPG) do increase and decrease oxygen affinity, the most direct cause of cell lysis is the membrane pump failure.

Glucokinase, found in the liver and pancreatic β-cells, has a high Km for glucose (around 10 mM). This means it is only significantly active when blood glucose levels are high, such as after a carbohydrate-rich meal, allowing the liver to effectively clear glucose from the portal circulation. In contrast, hexokinase has a low Km (~0.1 mM), meaning it is saturated at normal fasting glucose levels. Glucokinase is not inhibited by glucose-6-phosphate, unlike hexokinase.

Arsenate is structurally similar to inorganic phosphate. It can substitute for phosphate in the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. This forms an unstable 1-arseno-3-phosphoglycerate intermediate, which spontaneously hydrolyzes to 3-phosphoglycerate. This bypasses the substrate-level phosphorylation step normally catalyzed by phosphoglycerate kinase, where ATP is generated. As a result, glycolysis can proceed, but the 2 ATP molecules normally generated at this step are lost, resulting in a net yield of 0 ATP for the entire pathway.

This clinical picture is characteristic of Pompe disease (glycogen storage disease type II), a lysosomal storage disorder. It is caused by a deficiency of the lysosomal enzyme acid α-glucosidase (acid maltase). This enzyme is responsible for breaking down glycogen within the lysosome. Its absence leads to massive glycogen accumulation in lysosomes, particularly in cardiac and skeletal muscle cells, causing cardiomegaly and hypotonia. Unlike other glycogen storage diseases, it does not typically cause hypoglycemia because cytosolic glycogen metabolism is unaffected.

Glucagon signaling via cAMP and PKA aims to increase blood glucose. PKA phosphorylates glycogen phosphorylase kinase, which activates it. Activated glycogen phosphorylase kinase then phosphorylates and activates glycogen phosphorylase, the key enzyme for glycogenolysis. Simultaneously, PKA directly phosphorylates glycogen synthase, which inactivates it, thereby shutting down glycogen synthesis. This coordinated regulation ensures a net release of glucose from liver glycogen stores.