Changes at the N-terminus of human brain creatine kinase during a transition between inactive folding intermediate and active enzyme

Hydrogen/deuterium exchange studies of native rabbit MM-CK dynamics

Creatine kinase (CK) isoenzymes catalyse the reversible transfer of a phosphoryl group from ATP onto creatine. This reaction plays a very important role in the regulation of intracellular ATP concentrations in excitable tissues. CK isoenzymes are highly resistant to proteases in native conditions. To appreciate localized backbone dynamics, kinetics of amide hydrogen exchange with deuterium was measured by pulse-labeling the dimeric cytosolic muscle CK isoenzyme. Upon exchange, the protein was digested with pepsin, and the deuterium content of the resulting peptides was determined by liquid chromatography coupled to mass spectrometry (MS). The deuteration kinetics of 47 peptides identified by MS/MS and covering 96% of the CK backbone were analyzed. Four deuteration patterns have been recognized: The less deuterated peptides are located in the saddle-shaped core of CK, whereas most of the highly deuterated peptides are close to the surface and located around the entrance to the active site. Their exchange kinetics are discussed by comparison with the known secondary and tertiary structures of CK with the goal to reveal the conformational dynamics of the protein. Some of the observed dynamic motions may be linked to the conformational changes associated with substrate binding and catalytic mechanism.

Generation of an active monomer of rabbit muscle creatine kinase by site-directed mutagenesis: the effect of quaternary structure on catalysis and stability

Cytosolic creatine kinase exists in native form as a dimer; however, the reasons for this quaternary structure are unclear, given that there is no evidence of active site communication and more primitive guanidino kinases are monomers. Three fully conserved residues found in one-half of the dimer interface of the rabbit muscle creatine kinase (rmCK) were selectively changed to alanine by site-directed mutagenesis. Four mutants were prepared, overexpressed, and purified: R147A, R151A, D209A, and R147A/R151A. Both the R147A and R147A/R151A were confirmed by size-exclusion chromatography and analytical ultracentrifugation to be monomers, whereas R151A was dimeric and D209A appeared to be an equilibrium mixture of dimers and monomers. Kinetic analysis showed that the monomeric mutants, R147A and R147A/R151A, showed substantial enzymatic activity. Substrate binding affinity by R147A/R151A was reduced approximately 10-fold, although k(cat) was 60% of the wild-type enzyme. Unlike the R147A/R151A, the kinetic data for the R147A mutant could not be fit to a random-order rapid-equilibrium mechanism characteristic of the wild-type, but could only be fit to an ordered mechanism with creatine binding first. Substrate binding affinities were also significantly lower for the R147A mutant, but k(cat) was 11% that of the native enzyme. Fluorescence measurements using 1-anilinonaphthalene-8-sufonate showed that increased amounts of hydrophobic surface area are exposed in all of the mutants, with the monomeric mutants having the greatest amounts of unfolding. Thermal inactivation profiles demonstrated that protein stability is significantly decreased in the monomeric mutants compared to wild-type. Denaturation experiments measuring lambda(max) of the intrinsic fluorescence as a function of guanidine hydrochloride concentration helped confirm the quaternary structures and indicated that the general unfolding pathway of all the mutants are similar to that of the wild-type. Collectively, the data show that dimerization is not a prerequisite for activity, but there is loss of structure and stability upon formation of a CK monomer.

Role of C-terminal sequences in the folding of muscle creatine kinase

Proteinase K selectively nicks the native homodimeric muscle creatine kinase (MM-CK) into two 37.1 kDa N-terminal (K1) and two 5.8 kDa C-terminal (K2) fragments that remain firmly associated in a native-like, although inactive, heterotetrameric structure. This truncated protein has been named (K1K2)(2). To analyze the role of the C-terminal peptide in the protein structure acquisition, we studied in vitro refolding of the guanidinium chloride-denatured (K1K2)(2). Although they never reassociate with K2, in selected conditions the K1 fragments refold slowly to a dimeric state as shown by size exclusion chromatography data. This K1 dimer exhibits a fluorescence emission lambda max of 335 nm, a high degree of tyrosine exposure, strongly binds ANS but not MgADP, a CK substrate, and according to these structural characteristics, could be a dimeric molten globule species. We propose a folding model that takes into account the existence of a new transient intermediate state in the MM-CK refolding process. Besides two monomeric premolten and molten globule kinetic intermediates and the active final dimeric form, an inactive dimer, with partly compacted monomers, must ephemerally exist. Our results strongly suggest that the C-terminal end of the protein accelerates folding and plays a critical role for monomer final packing into a native-like conformation. The data also indicate that MM-CK catalytic efficiency is only acquired after dimerization.

Evidence for kinetic intermediate states during the refolding of GdnHCl-denatured MM-creatine kinase. Characterization of a trapped monomeric species

The kinetics of refolding of guanidinium chloride-denatured rabbit MM-creatine kinase was investigated. Recovery of enzymatic activity is biphasic, depending on the temperature but not on the protein or DTT concentration. Only 45% of the original, active dimeric form is recovered even after several hours of refolding. The reactivation yield is limited by the accumulation of a highly stable but nonproductive monomeric species. The ratio of "correct" to "incorrect" forms depends on the duration of exposure to the denaturant, which may be consistent with the existence of a heterogeneous population of unfolded states with regard to proline isomerization. The first fast reaction observed during renaturation results in the appearance of collapsed monomeric states, displaying features of a pre-molten globule state. These burst species are rapidly transformed into more structured monomers resembling a molten globule state possessing a partially folded C-terminal domain. A proportion of these latter transient intermediates (45%) associates into an active dimer, while the remainder (55%) is trapped by reshuffling in a monomeric dead-end product. Our results strongly indicate that (i) the dimeric state is a prerequisite for the expression of catalytic activity, (ii) the kinetic intermediates of refolding are very similar to those observed during equilibrium unfolding, and (iii) refolding of creatine kinase in these conditions is limited by the accumulation of inactive misfolded nondimerizable monomer.

Protease digestion studies of an equilibrium intermediate in the unfolding of creatine kinase

Protease digestion experiments have been used to characterize the structure of an equilibrium intermediate in the unfolding of creatine kinase (CK) by low concentrations (0.625 M) of guanidine hydrochloride (GdnHCl). Eighteen of the major products of digestion by trypsin, chymotrypsin and endoproteinase Glu-C have been identified by microsequencing after separation by SDS/PAGE and electroblotting on poly(vinylidene difluoride) membranes. The C-terminal portion (Gly215 to Lys380) was much more resistant to digestion than the N-terminal portion (Pro1 to Gly133), although the area most sensitive to proteolysis was in the middle of the CK sequence (Arg134 to Arg214). These experiments are consistent with the two-domain model for the CK monomer. The structure of the intermediate is proposed to consist of a folded C-terminal domain and a partly folded N-terminal domain separated by an unfolded central linker. Protease susceptibility is clustered within two N-terminal regions and one central region. These regions are evidently exposed as a result of the partial unfolding and/or separation of the N-terminal domain. Further evidence for the structure of this intermediate comes from gel filtration studies. Treatment of CK with 0.625 M GdnHCl resulted in slow aggregation at 37 degrees C, but not at 12 degrees C, a phenomenon previously reported for phosphoglycerate kinase. The aggregation did not occur at higher GdnHCl concentrations and was unaffected by a reducing agent. It is proposed that aggregation is a consequence of non-specific interactions between hydrophobic regions, possibly domain/domain interfaces, which become exposed in the intermediate.

Tissue- and cell-specific distribution of creatine kinase B: a new and highly specific monoclonal antibody for use in immunohistochemistry

A synthetic 17-mer peptide corresponding to an unique sequence in the amino-terminal region of human creatine kinase B was used to raise a new and highly B-subunit-specific monoclonal antibody, CK-BYK/21E10. We show here that the monoclonal antibody is suitable for immunohistochemistry of unfixed frozen sections as well as formaldehyde- or Bouin-fixed, paraffin-embedded sections of human, rabbit, and mouse tissues. Moreover, in the study of cell- and tissue-specific distribution patterns, parallel Western blot analysis and immunoelectron microscopy is possible using this antibody. Our analyses demonstrate that creatine kinase B expression is restricted to a specific subset of cell types in various tissues. In brain, the B-subunit was found only in neurocytes, but not in glia cells. High expression was also observed in inner segments of photoreceptor cells and the outer plexiform layer of the retina, in the parietal cells of the stomach and in gut enterocytes, gallbladder and epithelial cells of the urogenital system. The possible roles of the creatine kinase/phosphocreatine-ATP system in these tissues are discussed.