Subtilisin boronate inhibitor

Subtilisin [11] and cholesterol esterase [12, 13] are also inhibited by a range of aliphatic and aromatic boronic acids. Work on inhibiting a-CHT and cell replication with a variety of boronic acids concluded that the a-CHT activity was associated with chromatin in normal and tumorous tissue of mice [14]. Certain boronic acids were demonstrated to inhibit protease activity in rat liver chromatin [15]. It was concluded that good boronate inhibitors of CHT, like PEBA, inhibit cell replication and that this effect is expected to be higher in rapidly proliferating cancer cells than in normal tissue [14]. 

Figure 11. Stereoview of a proposed boronate inhibitor to subtilisin showing favorable stacking interactions and hydrogen bridges...

The closest organic specie to the inorganic boric acid are the boronic acids generally described as R-B(OH)2. Boronic acids have been shown to act as inhibitors of the subtilisins. X-ray crystallographic studies of phenylboronic acid and phenyl-ethyl-boronic acid adducts with Subtilisin Novo have shown that they contain a covalent bond between the oxygen atom of the catalytic serine of the enzyme and the inhibitor boron atom (Matthews et al, 1975 and Lindquist Terry, 1974). The boron atom is co-ordinated tetrahedrally in the enzyme inhibitor complex. It is likely that boric acid itself interacts with the active site of the subtilisins in the same manner. 

The very first examples were some simple boronic acid compoimds [5, 6]. For example, phenylboronic acid and substituted phenylboronic acids were found to be strong competitive inhibitors of subtilisin and chymotrypsin [6]. More recendy, boronic acids have been used for the synthesis of inhibitors against thrombin [50,51], lactamases [52], dipeptidyl peptidases [53], and others [54, 55]. In this section, thrombin inhibitors will be used as examples. The rest will be summarized in a table at the end of the section. 

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