PHOTOREGULATION OF ENZYME-ACTIVITY - PHOTOCHROMIC, TRANSITION-STATE-ANALOG INHIBITORS OF CYSTEINE AND SERINE PROTEASES

Two azobenzene derivatives, the aldehyde 1 and the boronic acid 2, were synthesized and shown to be reversible, photoregulatable inhibitors of the cysteine protease, papain, and the serine proteases, chymotrypsin and subtilisin. trans-1 was found to be a potent, reversible inhibitor of papain (K(i) = 2.1 muM) while its cis isomer was about 40 times weaker. Irradiation of a solution containing papain and trans-1 with 330 > lambda > 370 nm light resulted in a photostationary state mixture of 83% cis-1 and a corresponding 500% increase in enzyme activity. The original activity level was subsequently restored by irradiation with lambda > 400 nm light which converted the inhibitor back to its trans configuration. The cycle was repeated to give an identical change in enzyme activity. Similar results were obtained with 2 as a photoswitchable inhibitor of chymotrypsin although the change in enzyme activity was much smaller (13%). In the case of subtilisin, the relative inhibitory abilities of trans-2 and cis-2 were found to be dependent upon the conditions of the experiment, particularly the concentration of KCl. In the absence of KCl, cis-2 was a better inhibitor of subtilisin than trans-2, but in the presence of 0.5 M KCl the reverse trend was observed.