Features which recur in the active site of enzymes, unrelated by evolution, are particularly worthy of chemical modelling. It is essential, however, that these small organic molecules maintain the spatial relationships found in the enzymic system. Models of the serine proteases](eg, lh, 2Zb, 32c) orient the anti lone pair of the carboxylate toward the imidazole, in contrast to the serine protease^,^ malate and lactate dehydr~ genase,~ thermolysin? and
