FUNCTIONAL DIFFERENCES BETWEEN DIMERIC AND OCTAMERIC MITOCHONDRIAL CREATINE-KINASE

Mitochondrial creatine kinase (Mi-CK) consists of octameric and dimeric molecules that are interconvertible. In the present study, the kinetic properties of purified chicken heart Mi-CK (Mi(b)-CK) dimers and octamers were investigated separately under highly controlled conditions. Gel-permeation chromatography was performed before and after kinetic measurements in order to clearly define the proportions of octamers and dimers. 'Dimeric' Mi-CK solutions consisted of greater than or equal to 90 % dimers throughout the experiment whereas 'octameric' Mi-CK solutions consisted in the beginning of 90 % octamers, but upon measuring with the highest concentrations of creatine (Cr) and ATP approximately one-third of the octamers dissociated into dimers. These proper controls enabled us to pinpoint the observed kinetic differences between dimers and octamers solely to the oligomeric state of Mi(b)-CK. Both dimeric and octameric Mi-CK displayed synergism in substrate binding (K-d values are higher than K-m values), meaning that binding of the first substrate facilitates subsequent binding of the second substrate. Most interestingly, K-m(Cr) and K-d(Cr) values are both 2-3 times higher for octameric than for dimeric Mi-CK. Thus, at low Cr concentrations, the dimer is kinetically favoured for the forward direction of the reaction (phosphorylcreatine synthesis) compared with the octamer. The possible physiological significance of the lower K-d(Cr) value of dimeric versus octameric Mi(b)-CK, as well as the apparent negative cooperativity of ATP binding at higher [Cr], are discussed within the context of a possible functional role for dimeric Mi(b)-CK in vivo.