Structural mechanism for glycogen phosphorylase control by phosphorylation and AMP

Area of Science:

• Structural biology
• Biochemistry
• Enzymology

Background:

• Glycogen phosphorylase (GP) exists in T (tense) and R (relaxed) states, differing in catalytic activity.
• Activation of GP involves structural transitions and allosteric regulation by AMP.
• Understanding these conformational changes is key to enzyme function.

Purpose of the Study:

• To elucidate the structural basis of glycogen phosphorylase a (GPa) and glycogen phosphorylase b (GPb) activation.
• To characterize the structural differences between R and T states of GPb and R state of GPa.
• To investigate the role of N-terminal residues and AMP binding in GP activation.

Main Methods:

• X-ray crystallography was used to determine the structures of R-state GPa and R/T-state GPb complexed with AMP.
• High-resolution structural analysis (2.2–2.9 Å) allowed detailed examination of atomic interactions.
• Comparison of crystal structures revealed conformational changes upon T to R state transition.

Main Results:

• R-state GPa structure is similar to R-state GPb, with key differences at Ser14 (phosphate vs. sulfate).
• N-terminal tail residues undergo significant conformational changes (extended to coiled) during the T to R transition, forming intersubunit contacts.
• AMP binds with significantly higher affinity to R-state GPb, involving additional hydrogen bonds and increased buried surface area.

Conclusions:

• N-terminal interactions in the R state are critical for stabilizing the active conformation and creating a high-affinity AMP binding site.
• The T to R transition involves coordinated tertiary and quaternary structural changes that are relayed to the catalytic site.
• Structural insights explain the allosteric activation mechanism of glycogen phosphorylase by AMP.