Oliver Kensch, Tobias Restle, Birgitta M. Wöhrl, Roger S. Goody
and Heinz-Jürgen Steinhoff
X-ray crystallographic studies of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) complexed with or without substrates or inhibitors show that the heterodimeric enzyme adopts distinct conformations which differ in the orientation of the so called thumb subdomain in the large subunit. Site-directed spin labelling of mutated residue positions W24C and K287C is applied here to determine the distances between the fingers and thumb subdomains of liganded and unliganded RT in solution. The inter-spin distances of a DNA/DNA and a pseudoknot RNA complexed RT in solution was found to agree with the respective crystal data of the open and closed conformations. For the unliganded RT a temperature dependent equilibrium between these two states was observed. The fraction of the closed conformation decreased from 95% at 313 K to 65% at 273 K. The spectral separation between the two structures was facilitated by the use of a perdeuterated 15N-nitroxide methane-thiosulfonate spin label.