Antibody macrocycle

Kukis. D.L. Diril, H. Greiner, D.P. De Nardo, G.L. Salako. Q.A. Meares. C.F. A comparative study of copper-67 radiolabeling and kinetic stabilities of antibody-macrocycle chelate conjugates. Cancer 1994, 73. 779. 

Antibodies also well catalyze the reactions where a molecule should he forced to adopt a particular and reactive conformation, thanks to privileged interactions with the amino acids of the binding site. For example, ahzymes with a ferrochelatase activity, 7G12, force the mesoporphyrin IX ring to adopt a distorted conformation favorable to the insertion of a Cu ion in the center of the macrocycle,thanks to an interaction with the HlOOc methionine which constrains one of the pyrrole rings to be left outside the plane of porphyrin (Figure 24). 

Antibodies were generated against a bent A-alky 1 mesoporpbyrin, which is a known inhibitor of the enzyme ferrochelatase. Ferrochelatase catalyzes the insertion of Fe(II) into protoporphyrin, as part of the heme biosynthetic pathway (Lavallee, 1988). Antibody 7G12 was found to catalyze the insertion of divalent metal ions into porphyrins, with a rate similar to thatfound for ferrochelatase (Cochran and Schultz, 1990). The structural data presented below supports the hypothesis that the transition state for porphyrin metallation involves a distortion of the macrocyclic ring system to facilitate metal insertion, and that this bent porphyrin acts as a good transition-state mimic in the development of catalytic antibody 7G12. 

Fig. 2 Structures of selected acyclic and macrocyclic BFCs useful for the radiometal labeling of biomolecules, including antibodies, antibody fragments, small peptides, and nonpeptide receptor ligands...

Tumor diagnostic and therapy using radiolabeled macrocycle-antibody conjugates 90CSR271. 

An example of this approach is demonstrated in an antibody mimic of the enzyme ferrochetalase (39). Ferrochelatase catalyzes the insertion of Fe + into protoporphyrin IX (3) as the last step in the heme biosynthetic pathway (40). Interestingly, N-alkylporphyrins are known to be potent inhibitors of this enzyme, because alkylation at one pyrrole lutrogen distorts the planarity of the porphyrin macrocycle (41). This finding was used in the design of hapten 4 to catalyze the incorporation of metal ions into mesoporphyrin IX (5) by eliciting an antibody that binds the substrate in a ring-strained conformation. 

Comparable to the infiuence of such structural well-defined macrocycles, cell-free extracts as well as antibodies also show strong catalytic effects. Hence, the use of organic compounds, which are able to form micelles, being active in water and easy to handle could lead to new insights and unexpected results for catalytic Diels-Alder... 

The construction of "a calixarene with four peptide loops" serves two functions in this article. It serves as a simplified substitute for an antibody, though we doubt that the intent of the authors is the design of potential therapeutic agents. More important, the calixarene serves to test the theory of antibody function sketched in the preceding discussion Is this really the way that antibodies work The authors note that earlier attempts to mimic antibodies have been unsuccessful, and they propose the alternative strategy, which is the heart of the article The search for antibody mimics has not yet yielded compact and robust frameworks that reproduce the essential features of the CDRs. Our strategy is to use a macrocyclic scaffold to which multiple peptide loops in stable hairpin-turn conformations can be attached (p. 2681). 

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