Ca2+-induced structural changes in phosphorylase kinase detected by small-angle X-ray scattering

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Phosphorylase kinase (PhK) is a large, Ca(2+)-dependent enzyme regulating glycogenolysis.
• PhK exists as a hexadecameric complex with D(2) symmetry, composed of two octameric lobes and connecting bridges.
• Previous studies indicated Ca(2+) binding affects PhK subunits, but interrelationships were unclear.

Purpose of the Study:

• To investigate Ca(2+)-induced structural changes in phosphorylase kinase in solution.
• To elucidate the relationship between Ca(2+) binding and conformational alterations in PhK subunits and inter-lobe regions.

Main Methods:

• Small-angle X-ray scattering (SAXS) was employed on nonactivated and Ca(2+)-activated PhK.
• Computer-generated models based on electron microscopy data were used to interpret SAXS profiles.
• Theoretical scattering profiles from models were fitted to experimental data.

Main Results:

• Ca(2+) binding did not alter the overall dimensions of the PhK complex.
• SAXS data revealed a shift in scattering density distribution, indicating conformational changes.
• Models accurately represented experimental data, showing structural differences between activated and nonactivated states.

Conclusions:

• Calcium binding induces conformational changes in both the lobes and interlobal bridges of phosphorylase kinase.
• These findings provide insights into the mechanism of PhK activation by Ca(2+).
• The study highlights the utility of SAXS combined with modeling for analyzing enzyme dynamics.