Control of glycogen deposition

Area of Science:

• Biochemistry
• Metabolic Regulation
• Cellular Biology

Background:

• Glycogen synthase (GS) traditionally viewed as the primary control point for glycogen synthesis.
• Emerging evidence indicates other factors and tissue-specific mechanisms are crucial for glycogen deposition.
• Glycogen synthesis regulation varies significantly between muscle and liver tissues.

Purpose of the Study:

• To elucidate the distinct regulatory mechanisms of glycogen synthesis in liver and muscle.
• To investigate the role of glucose-6-phosphate (Glc-6-P) and its sources in activating glycogen synthase (GS).
• To understand the interplay between GS and specific kinases (glucokinase/GK and hexokinase I/HK I) in different tissues.

Main Methods:

• Enzyme kinetics analysis of glycogen synthase (GS) activation by glucose-6-phosphate (Glc-6-P).
• Comparative study of Glc-6-P potency derived from glucokinase (GK) versus hexokinase I (HK I).
• Investigation of intracellular localization changes of GS and GK in response to glucose.

Main Results:

• Glc-6-P allosterically activates GS, likely via conformational changes enhancing phosphatase action.
• Glc-6-P generated by liver GK is more potent in activating liver GS than Glc-6-P from muscle HK I.
• In skeletal muscle, glycogen synthesis control is shared between glucose transport and GS.
• Glucose triggers translocation of liver GS and GK, correlating with stimulated glycogen synthesis.

Conclusions:

• Hepatic glycogen deposition is controlled by GS, which is itself regulated by GK, highlighting a hierarchical control system.
• Tissue-specific isoenzyme pairs (liver GS/GK and muscle GS/HK I) are tailored for distinct metabolic roles.
• Intracellular translocation of GS and GK represents an additional regulatory mechanism for orderly hepatic glycogen deposition.