Artificial Metalloprotease with Active Site Comprising Aldehyde Group and Cu(II)Cyclen Complex

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Abstract
To design artificial proteases that cleave peptide backbones of a wide range of proteins at selected sites, artificial active sites comprising the Cu(II) complex of cyclen (Cu(II)Cyc) and aldehyde group were synthesized on a cross-linked polystyrene. The aldehyde group was employed as the binding site in view of its ability of reversible formation of imine bonds with ε-amino groups of Lys residues exposed on the surface of proteins and Cu(II)Cyc as the catalytic group for peptide hydrolysis. The two polymeric artificial metalloproteases synthesized in the present study cleaved all of the protein substrates examined (myoglobin, γ-globulin, bovine serum albumin, human serum albumin, lysozyme, and ovalbumin), manifesting saturation kinetic behavior. At 50 °C and pH 9.0 or 9.5, Km was (1.3−22) × 10-4 M, comparable to those of natural proteases, and kcat was (6.0−25) × 10-4 s-1, corresponding to half-lives of 4.6−19 min. Intermediacy of the imine complexes formed between the aldehyde group of the catalyst and the ε-amino groups of Lys residues of the substrates was confirmed by the trapping experiment with NaB(OAc)3H. MALDI-TOF MS of the proteolytic reaction mixtures revealed formation of various cleavage products. Structures of some of the cleavage products were determined by using carboxypeptidase A and trypsin. Among various cleavage sites thus identified, Gln(91)−Ser(92) and Ala(94)−Thr(95) were the major initial cleavage sites in the degradation of myoglobin by the two catalysts. The selective cleavage of Gln(91)−Ser(92) and Ala(94)−Thr(95) was attributed to general acid assistance in peptide cleavage by Tyr(146) located in proximity to the two peptide bonds. Broad substrate selectivity, high cleavage-site selectivity, and high proteolytic rate are achieved, therefore, by positioning the aldehyde group in proximity to Cu(II)Cyc attached to a cross-linked polystyrene.
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