A cofactor approach to copper-dependent catalytic antibodies

A strategy for the preparation of semisynthetic copper(II)-based catalytic metalloproteins is described in which a metal-binding bis-imidazole cofactor is incorporated into the combining site of the aldolase antibody 38C2. Antibody 38C2 features a large hydrophobic-combining site pocket with a highly nucleophilic lysine residue, Lys(H93), that can be covalently modified. A comparison of several lactone and anhydride reagents shows that the latter are the most effective and general derivatizing agents for the 38C2 Lys residue. A bis-imidazole anhydride (5) was efficiently prepared from N-methyl imidazole. The 38C2–5-Cu conjugate was prepared by either (i) initial derivatization of 38C2 with 5 followed by metallation with CuCl(2), or (ii) precoordination of 5 with CuCl(2) followed by conjugation with 38C2. The resulting 38C2–5-Cu conjugate was an active catalyst for the hydrolysis of the coordinating picolinate ester 11, following Michaelis–Menten kinetics [k(cat)(11) = 2.3 min(−1) and K(m)(11) 2.2 mM] with a rate enhancement [k(cat)(11)k(uncat)(11)] of 2.1 × 10(5). Comparison of the second-order rate constants of the modified 38C2 and the Cu(II)-bis-imidazolyl complex k(6-CuCl(2)) gives a rate enhancement of 3.5 × 10(4) in favor of the antibody complex with an effective molarity of 76.7 M, revealing a significant catalytic benefit to the binding of the bis-imidazolyl ligand into 38C2.