Interaction of Brain-Type Creatine Kinase with its Transition State Analog: Kinetics of Inhibition and Conformational Changes

Abstract

The effects of components of the transition state analog (creatine, MgADP, planar anion) on the kinetics and conformation of creatine kinase isozyme BB from monkey brain was studied. From analysis of the reaction time course using the pH stat assay, it was shown that during accumulation of the reaction products (ADP and creatine phosphate), among several anions added, nitrate proved the most effective in inhibiting catalytic activity. Maximum inhibition (77%) was achieved with 50 mM nitrate. The K_m for ATP was 0.48 mM and in the presence of 2.5 mM nitrate, 2.2 mM; for ATP in the presence of the dead-end complex, creatine and ADP, the apparent Km was 2.0 mM and theK wasO.16mM; in the presence of the transition state analog, MgADP + NO₃” + creatine, the K was estimated to be 0.04 mM.

Ultraviolet difference spectra of creatine kinase revealed significant differences only in the presence of the complete mixture of the components of the transition state analog. Comparison of gel nitration elution profiles for creatine kinase in the absence and presence of the complete mixture of components of the transition state analog did not reveal any differences in elution volume. Addition of components of the transition state analog to creatine kinase resulted in only a marginal change in intrinsic fluorescence. The presence of the components of the transition state analog increased the rate of reactivity of the enzyme with trinitrobenzenesulfonic acid from k = 6.06 ±0.05M⁻¹min to 6.96 ± 0.11 M⁻¹min⁻¹.

This study provides evidence that, like the muscle isozyme of creatine kinase, the brain form is effectively inhibited by the transition state analog. However, the inhibition is accompanied by small changes in the overall conformation of the protein. This adds to the evidence that the functional differences of the isozymic forms of creatine kinase cannot be attributed to differences in kinetic properties.

Key Words: :

• Creatine kinase
• transition state analog
• conformational changes
• brain enzyme
• muscle enzyme