Inactive PKA holoenzyme consists of
Inactive protein kinase A is a tetramer of two regulatory and two catalytic subunits. Binding of four molecules of cAMP to the regulatory subunits induces a conformational change that releases the catalytic subunits. Free catalytic subunits then phosphorylate downstream substrates. Reassociation of the holoenzyme occurs when cAMP levels fall, restoring the inactive state.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)