Subunit conformation and dynamics in a heterodimeric protein: studies of the hybrid isozyme of creatine kinase

Several physical properties of creatine kinase (EC 2.7.3.2) isozymes MM (CK-MM, muscle-type) and BE (CK-BB, brain-type), both homodimers, and isozyme MB (CK-MB), a heterodimer, were compared to determine how formation of the hybrid modifies subunit conformation and dynamics. Circular dichroic spectra revealed additional a-helical content for the hybrid isozyme. Double-beam absorption difference spectra between CK-MB and a stoichiometric mixture of CK-MM and CE(-BB revealed decreased exposure of intrinsic chromophores in the hybrid. The relative intensity of the intrinsic fluorescence of CK-MB was between the two homodimers, but was 16% closer to the less fluorescent CK-MM. Steady state anisotropy spectra and decay of the anisotropy of CK derivatized on a single subunit with the fluorescent sulfhydryl reagent 5-[2-(iodoacetyl)amino-ethyl]aminonaphthalene-l-sulfonate indicated that the derivatized sites are more flexible in the heterodimer. The slow component in the anisotropy decay suggests that hybridization results in a small increase in the packing density or contraction of overall conformation of the B-subunit. The K-M for MgATP with singly derivatized CKMB was the same as the KM for the native enzyme. However, derivatization of a single subunit caused the V-max to decrease by greater than 50%, which indicates that subunit-subunit interactions may modulate the activity of CK. A model for assembly of CK-MB is proposed which includes subunit characteristics more similar to those found in the muscle-type homodimer than in the brain-type homodimer and increased flexibility of the active site domain of both subunits. (C) 1998 Elsevier Science B.V. All rights reserved.