Kinetic studies of the Serratia marcescens extracellular nuclease isoforms

Kinetic studies on the two major isoforms of Serratia marcescens nuclease, Sm2 and Sm1, have revealed them to be functionally equivalent. Both isoforms display marked substrate inhibition by DNA and RNA. They both require magnesium for optimal activity, but retain low catalytic activity in its absence. Both are moderately inhibited by mononucleotides including 5'-ATP, 5'-AMP, 5'-TTP and 3'5'-pTp. The two strongest mononucleotide inhibitors studied, 5'-ATP and 5'-AMP, display inhibition constants, KI, on the order of 10(-5) M. In assessing the strength of mononucleotide inhibition the type of nucleotide base appears to be more important than the number of phosphate moieties.