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Subunit interactions and the allosteric response in phosphorylase.

S Sprang, R J Fletterick

    Biophysical Journal
    |October 1, 1980
    PubMed
    Summary

    Interactions between protein subunits in phosphorylase are key to its allosteric conformational changes. Phosphorylation of phosphorylase a (Pa) stabilizes its active form, while phosphorylase b (Pb) remains inactive due to disordered N-terminal residues and fewer salt bridges.

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    Structural relationships among regulated and unregulated phosphorylases.

    Annual review of biophysics and biomolecular structure·2001

    Area of Science:

    • Biochemistry
    • Structural Biology
    • Enzymology

    Background:

    • Phosphorylase exists in active (R) and inactive (T) conformations, influenced by phosphorylation.
    • Phosphorylase a (Pa) is predominantly in the R conformation, while phosphorylase b (Pb) is in the T conformation.
    • Understanding intersubunit interactions is crucial for elucidating allosteric regulation.

    Purpose of the Study:

    • To investigate the contribution of intersubunit interactions to allosteric conformational changes in phosphorylase.
    • To determine the atomic structure of glucose-inhibited (T) Pa and analyze its subunit interface.
    • To compare the structural features and interactions between Pa and Pb.

    Main Methods:

    • X-ray crystallography at 2.5-A resolution to determine the structure of glucose-inhibited (T) Pa.

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  • Calculation of solvent accessible surface area at the subunit interface.
  • Analysis of difference electron density between isomorphous Pa and Pb crystal structures.

    • Main Results:

    • The subunit contact in Pa involves 6% of the monomer surface, withdrawing 4,600 A2 from solvent, primarily in the N-terminal regulatory domain.
    • Four intersubunit salt bridges were identified, with two at the N-terminus being more solvent accessible.
    • Pb exhibits disordered N-terminal residues and lacks specific salt linkages present in Pa, correlating with a higher activation energy for the T to R transition.

    Conclusions:

    • Phosphorylation of phosphorylase activates the N-terminus, which acts as an intramolecular effector for the R state.
    • The N-terminal region and its intersubunit interactions are critical for homotropic cooperativity between phosphorylase subunits.
    • Structural differences in the N-terminus explain the distinct conformational preferences of Pa and Pb.