The evolution of an allosteric site in phosphorylase

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Glycogen phosphorylases feature a conserved catalytic core with variable regulatory sites.
• Understanding allosteric regulation requires comparing isozyme structures.
• Focus is on differences in glucose 6-phosphate (Glc-6-P) binding sites between isozymes.

Purpose of the Study:

• To elucidate the structural principles of allosteric regulation in glycogen phosphorylases.
• To compare the glucose 6-phosphate (Glc-6-P) binding sites of yeast and muscle phosphorylase isozymes.

Main Methods:

• Refinement of the crystal structure of Glc-6-P inhibited yeast phosphorylase b to 2.6 Å resolution.
• Comparative analysis with existing structures of muscle phosphorylase.

Main Results:

• Glc-6-P binds to yeast phosphorylase b in a novel manner, engaging unique secondary structural elements.
• While structural connections between Glc-6-P and catalytic sites are conserved, specific interactions differ.
• Muscle phosphorylase exhibits a bi-functional switch for Glc-6-P binding, adapted for adenine interaction.

Conclusions:

• The Glc-6-P binding site has evolved from yeast to vertebrate muscle phosphorylase.
• Evolutionary modifications created a bi-functional switch, incorporating an adenine-binding subsite for AMP activation.
• These changes maintain crucial links to the catalytic site, enabling complex allosteric control.