INVOLVEMENT OF A TYROSINE RESIDUE IN THE ADP BINDING-SITE OF CREATINE-KINASE - A 2ND-DERIVATIVE UV-SPECTROSCOPY STUDY

Second-derivative spectroscopy was used to determine the percentage of tyrosine residues that are exposed to solvent in rabbit MM-creatine k inase. Six residues, among the ten present per monomer, are solvent-ex posed. The presence of creatine in the incubation medium does not modi fy this value. However, this number is decreased by one when the enzym e is incubated with saturating concentrations of MgADP. A dissociation constant for MgADP can be estimated and the obtained value (0.085 mM) is comparable to the Km for this substrate. Thus, a tyrosine residue is located near the MgADP binding site or is masked during protein con formational change induced by adenyl nucleotide binding.