Porphyrin-binding pocket

The monooxygenases [65-70] are characterized by the axial attachment of the heme cofactor to a thiolate functionality from a cysteine residue of the protein matrix. The binding pockets of monooxygenases are usually organized in such a way as to lead the substrate directly to the active FeO subunit of the porphyrin. This... 

Many other redox reactions are potentially amenable to antibody catalysis. For example, the chemistry of the P-450 cytochromes, including the hydroxylation of alkanes and the epoxidation of alkenes, can be mimicked with synthetic porphyrins. Incorporation of such molecules into antibody active sites could conceivably yield new catalysts that combine the intrinsic reactivity of the cofactor with the tailored selectivity of the binding pocket. Work is just beginning in this area, but preliminary studies with porphyrin haptens have yielded some interesting results.126-130 Novel redox chemistry can also be anticipated for antibodies containing metal ions, flavins, nicotinamide analogs, and other reactive moieties. 

To study this discrimination, generally expressed as an M value, the ratio of the O2 and CO equilibrium constant values, researchers have used sterically hindered porphyrins (capped, pocket and hybrid porphyrins) with a view to providing an environment which would permit normal binding of dioxygen whilst hindering the binding of carbon monoxide. Kinetically, the major effect is a decrease in CO association rates, consistent with steric blocking in productlike transition states. 

The X-ray crystal structures of the active sites of sperm whale oxyMb and an invertebrate oxyHb are shown in Figure 7. As anticipated from the picket-fence model, O2 binds to the active sites in a bent, end-on fashion trans to the proximal histidine. A comparison of the structures in Figures 5 and 7 shows that the torsional disorder of the Fe—O—O unit around the Fe—O bond axis in the picket-fence porphyrin O2 adduct is quenched in Mb/Hb, apparently by hydrogen bonding interactions with residues lining the 02-binding pocket—the so-called distal ... 

Shape selectivity of molecular recognition is one of the direct consequences of the lock-and-key mechanism. It may result from complementarity of interacting atoms as is the case in the system described in Fig. 4, or from steric restrictions in the binding pocket of the receptor. The latter principle is illustrated by shape-selective binding of amines to Zn(II) porphyrin complexes shown in Fig. 

Fig. 2 Dioxygen-binding myoglobin analogues as synthesized by Collman and coworkers.[ (A) Picket-fence porphyrin. (B) Pocket porphyrin. (C) Picnic-basket porphyrin.

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