Near- and far-ultraviolet circular dichroism of the catalytic subunit of adenosine cyclic 5'-monophosphate dependent protein kinase.

The circular dichroism spectrum of the catalytic subunit of cAMP-dependent protein kinase was measured in the far-UV (190-240 nm) and near-UV (250-300 nm) region. Data from the far-UV spectra were processed with the CONTIN program for estimation of globular protein secondary structure [ Provencher , S. W. (1982) CONTIN (Version 2) User's Manual, European Molecular Biology Laboratory, Heidelberg, West Germany]. The composition of the protein determined by this method was 49 +/- 2% alpha-helix, 20 +/- 4% beta-sheet, and 31 +/- 3% remainder. This composition changes when the protein is allowed to bind Kemptide , a synthetic peptide substrate, with more than half of the disordered portion of the protein taking the form of beta-sheet. A certain portion of the alpha-helical structure also appears to move into a beta-sheet form. The near-UV CD spectrum of catalytic subunit shows changes in aromatic amino acid dichroism associated with substrate binding. These changes can be ascribed with a fair degree of certainty to alterations in the orientation of a tyrosine residue at the surface of the protein. These findings are discussed in terms of previous work on induced dichroism in this enzyme with regard to control mechanisms operating at the active site.