By systematic syntheses and measurements of their equilibrium substrate binding of two sets of peptides which encompass the binding sites (a) for the magnesium chelates of the nucleotide substrates (MgATP2- and MgADP1-) of rabbit muscle myokinase and (b) of the uncomplexed nucleotide substrates (ADP3- and AMP2-) of the myokinase, it is hoped to chemically outline and deduce the requirements of the binding of the substrates to ATP-AMP transphosphorylase. To provide information on the basic process of ATP-transphosphorylation, a comprehensive physico-chemical approach, which will include analyses of their primary structures, has been initiated or will be continued on several enzymes which have in common the ability to catalyze the transfer of a phosphoryl group (or pyrophosphoryl group) of ATP to a suitable acceptor, but which differ markedly in their substrate specificity requirements or range of requirements for the acceptor. It is presumed that certain physical and chemical similarities as well as significant differences will manifest themselves in detailed studies on these enzyme proteins and their catalyzed reactions, so that eventually a broad picture will emerge and which will permit one to define more clearly the chemical and physical requirements for transphosphorylation as well as to permit a delineation of individual requirements which may be demanded for the specificity functions. To this aim, the studies will encompass the following series of enzymes: ATP-AMP transphosphorylase (adenylate kinase) isoenzymes from the muscle and liver of rabbit and calf; nucleoside triphosphate-nucleoside diphosphate transphosphorylase (nucleoside diphosphokinase) from yeast; ATP-creatine transphophorylase isoenzymes from rabbit and calf; ATP-thiamine pyrophosphokinase from porcine brain. In the case of the last enzyme, attempts will be made first to secure a homogeneous preparation and to provide a preliminary characterization, physically, chemically and kinetically.