PROBING LOCAL MOBILITY IN CARBONIC-ANHYDRASE - EPR OF SPIN-LABELED SH-GROUPS INTRODUCED BY SITE-DIRECTED MUTAGENESIS

Cloned human carbonic anhydrase, HCAII, and mutants thereof have been investigated by spin-probing methods during its unfolding caused by gu anidine.HCl. The spin-probe, N-(2,2,5,5-tetramethyl-1-ylooxypyrrolidin -3-yl) iodoacetamide, has been regiospecifically introduced into cyste ines by site-directed mutagenesis in various positions of the protein structure. Here we focus on EPR spectra of three different spin-labell ed enzymes at various guanidine.HCl concentrations (at equilibrium). T he following spin-labelled mutants are discussed: W16C/C206S, W97C/C20 6S and F176C/C206S. The EPR spectra of the three mutants differ marked ly at low guanidine.HCl concentration (0-1 mol dm-3) particularly with in the series W97C/C206S and F176C/C206S, showing the characteristic a nisotropic slow motional features. The W16C/C206S label position is mu ch more mobile in the folded structures. The rotational correlation ti mes reflect the local environment of the spin-probe in the folded enzy me: in W97C it is located in the core, near the active centre, in W16C and F176C at more peripheral positions. All samples gave EPR spectra characteristic of a 'free' unfolded protein chain at guanidine-HCl con centrations of ca. 3 mol dm-3 and above, and could be characterised by using one component in lineshape simulations. The spectra of the W97C /C206S and F176C/C206S samples in low concentrations of guanidine.HCl (between ca. 0.1 and 2.0 mol dm-3) could only be reproduced in simulat ions by introducing several components associated with rather differen t rotational correlation times. This seems to imply the co-existence o f at least two dynamic structures in equilibrium during the intermedia te stages of the unfolding process. It is compatible with earlier sugg estions of a folding intermediate based on optical characterisation.