Enkephalinase structure

Fig. 3.13. The 3D structure of neprilysin (enkephalinase, EC 3.4.24.11) obtained from ldmt.pdb [79]. This structure was determined for the enkephal-inase-phosphoramidon complex, but the inhibitor has been removed to unmask the catalytic center, a) Structure of the enzyme, with the Glu and the two His residues of the HEXXH zinc-binding motif shown in blue, b) Zoom on the catalytic center, revealing the spatial arrangement of the zinc-binding residues.

Independently, simple peptide hydroxamic acids (Z-Gly-L-Leu-NHOH and others) were first observed to inhibit the metalloprotease thermolysin in 1977 9,101 The structure was then further improved to the hydroxamidoalkylmalonyl-peptide moiety by considering the substrate specificity of thermolysin and other metalloproteases 10-121 A summary of hydroxamic acids reported to be inhibitors of various metalloenzymes up to 1983 has been published 131 In 1985 hydroxamido-benzylsuccinyl-L-alanine (kelatorphan) was synthesized and found to be one of the best enkephalinase inhibitors 141... 

Enkephalinase inhibitors have been developed from early studies which utilized simple dipeptides as probes for identifying the structural requirements of the active site of the enzyme [78] together with information evolved from the development of ACE inhibitors. The different enkephalinase inhibitors have been classified according to the zinc binding ligand in the inhibitor molecule namely sulphydryl, carboxyalkyl and phosphoryl [33,71,91] (see Figure 6.5). 

A range of penicillins have been examined as competitive reversible inhibitors of enkephalinase [119,120]. Carfecillin K = 0.18 M) was the most potent inhibitor in the series, whereas cloxacillin (Aj = 27.5 //M), ampicillin (A = 41 //M), nafcillin (A = 59 //M) and carbenicillin (Aj =158 juM) had moderate potency and benzylpenicillin (A = 885 jUM), mezlocillin (Aj = 437 juM) and azlocillin (Kj = 556 //M) were weak inhibitors. Structure-activity relationships within the series have been rationalized... 

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