Extending the limits to enzymatic catalysis: Diffusion of ribonuclease A in one dimension

Bovine pancreatic ribonuclease A (RNase A) is a distributive endoribonuclea se that catalyzes the cleavage of the P-O-5' bond of RNA on the 3' side of pyrimidine residues. Here, RNase A is shown to cleave the P-O-5' bond of a pyrimidine ribonucleotide faster when the substrate is embedded within a lo nger tract of poly(adenylic acid) [poly(A)] or poly(deoxyadenylic acid) [po ly(dA)]. These data indicate that a ribonuclease can diffuse in one dimensi on along a single-stranded nucleic acid. This facilitated diffusion is medi ated by Coulombic interactions, as the extent is diminished by the addition of NaCl. RNase A is more effective at cleaving a pyrimidine ribonucleotide embedded within a poly(dA) tract than within a poly(deoxycytidylic acid) [ poly(dC)] tract. T45G RNase A, which catalyzes the processive cleavage of p oly(A) but the distributive cleavage of poly(cytidylic acid) [poly(C)], has the same preference. Apparently, processive catalysis by the T45G enzyme a rises from the expanded substrate specificity of the variant superimposed u pon an intrinsic ability to diffuse along poly(A), Homologous ribonucleases with cytotoxic activity may rely on facilitated diffusion along poly(A) ta ils for efficient degradation of the essential information encoded by cellu lar mRNA.