As shown by kinetic experiments with competitive inhibitors like Thr(Bu<sup>t</sup>)-Phe-X peptide ligands are bound by hydrophobic interactions at two nonpolar subsites S<sub>1</sub> and S<sub>2</sub>. This contact region prefers with high selectivity L-configuration in P<sub>1</sub> and P<sub>2</sub> position. The additional binding P<sub>3</sub>-S<sub>3</sub> differs in polarity and stereospecifical requirement. The nitroxide-labelled inhibitor Thr(Bu<sup>t</sup> > Phe-SL is competitively bound at two types of binding sites. The main type, obtained from electron spin resonance spectroscopy, is in agreement with results from kinetic studies and represents the binding of two inhibitor molecules per subunit of the enzyme.