Enzyme activity is commonly controlled by allostery, where ligand binding at one site alters the activities of distant sites. Classical explanations for multisubunit proteins involve conformational transitions that are fundamentally deterministic. For example, in the Monod-Wyman-Changeaux (MWC) paradigm, conformational transitions occur simultaneously in all subunits. In the Koshland-Nemethy-Filmer (KNF) paradigm, conformational transitions only occur in ligand-bound subunits. In contrast, recent models predict conformational changes that are governed by probabilities rather than absolute rules. To better understand allostery at the molecular level, we applied a recently developed spectroscopic and calorimetric method to the interactions of a dimeric enzyme with two different ligands. We found that conformational transitions appear MWC-like for a ligand that binds at the dimer interface and KNF-like for a distal ligand. These results provide strong experimental support for probabilistic allosteric theory predictions that an enzyme can exhibit a mixture of MWC and KNF character, with the balance partly governed by subunit interface energies.
The rapid emergence and the prevalence of resistance mutations in HIV-1 reverse transcriptase (RT) underscore the need to identify RT inhibitors with novel binding modes and mechanisms of inhibition. Recently, two structurally distinct inhibitors, phosphonoformic acid (foscarnet) and INDOPY-1 were shown to disrupt the translocational equilibrium of RT during polymerization through trapping of the enzyme in the pre-and the post-translocation states, respectively. Here, we show that foscarnet and INDOPY-1 additionally display a shared novel inhibitory preference with respect to substrate primer identity. In RT-catalyzed reactions using RNA-primed substrates, translocation inhibitors were markedly less potent at blocking DNA polymerization than in equivalent DNA-primed assays; i.e. the inverse pattern observed with marketed non-nucleoside inhibitors that bind the allosteric pocket of RT. This potency profile was shown to correspond with reduced binding on RNA⅐DNA primer/template substrates versus DNA⅐DNA substrates. Furthermore, using site-specific footprinting with chimeric RNA⅐DNA primers, we demonstrate that the negative impact of the RNA primer on translocation inhibitor potency is overcome after 18 deoxyribonucleotide incorporations, where RT transitions primarily into polymerization-competent binding mode. In addition to providing a simple means to identify similarly acting translocation inhibitors, these findings suggest a broader role for the primer-influenced binding mode on RT translocation equilibrium and inhibitor sensitivity.HIV RT is a multifunctional enzyme that catalyzes the conversion of the single-stranded RNA HIV genome to doublestranded DNA that is then incorporated into the host genome by the HIV integrase. In the process, RT must support RNAdirected and DNA-directed DNA synthesis along with ribonuclease H (RNase H) 2 activities to create an integration-competent product. A key step during reverse transcription is the incomplete hydrolysis of the (ϩ)-stranded RNA that leaves behind a purine-rich RNA sequence referred to as the polypurine tract (PPT). The remnant RNA PPT sequence serves as a primer to initiate synthesis of the (ϩ)-strand DNA (second strand) (1-3). The unique structure of the RNA PPT sequence has been shown to be a key determinant of the RNase H cleavage specificity at the PPT/U3 junction (4). A recent report described the mechanism by which RT discriminates between polymerase and RNase H activities thereby enabling more efficient initiation of polymerization from the RNA PPT primer versus other remnant RNA primers (5). Using single-molecule spectroscopy experiments, it was shown that RT binds nucleic acid substrates in two distinct orientations in a manner that is governed by the sugar backbone composition of the four or five nucleotides at each end of the primer. Depending on the binding orientation, RT either initiates polymerization at the 3Ј-end of the primer (polymerase binding mode on a DNA primer), or alternatively, RNA hydrolysis through the RNase H domain (RNase H bindi...
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