Cooperative kinetics of human prostatic acid phosphatase.

The steady-state kinetics of hydrolysis reaction catalysed by human prostatic acid phosphatase (PAP) by using 1-naphthyl phosphate, phenyl phosphate and phosphotyrosine as substrates has been studied at pH 5.5. The substrate binding curves were sigmoidal and Hill cooperation coefficient h was higher than 1 for each of the examined compounds. Thus, human prostatic acid phosphatase kinetics exhibits positive cooperativity towards the studied substrates. The extent of cooperativity was found to depend on the substrate used and on enzyme concentration. The highest cooperativity of PAP was observed for 1-naphthyl phosphate and the lowest for phosphotyrosine. When prostatic phosphatase concentration increased, Hill cooperation coefficient (h) and half saturation constant (K(0.5)) both grew, but the catalytic constant (k(cat)) remained constant, for each of the substrates studied. Ligand-induced association-dissociation equilibrium of the active oligomeric species (monomer-dimer-tetramer-oligomers) is suggested.