| Abstract
| - Resonance Raman (RR) spectra are reported for mesoporphyrin IX bound to the Fab fragmentof the ferrochelatase antibody 7G12. Binding induces activation of a Raman band at 680 cm-1, which isassigned to an out-of-plane porphyrin vibration, γ15. This is exactly the predicted effect of distortingmesoporphyrin to the geometry of N-methylmesoporphyrin IX, the 7G12 hapten, based on DFT/CIS modelingof the RR spectrum. The modeling also shows that the pyrrole ring that is tilted out of the porphyrin planebears a nitrogen lone pair, which is therefore available for coordination by an incoming metal ion. The 680cm-1 band intensity is ∼3 times higher for the affinity-matured antibody than for the germline precursorantibody, while intermediate values are found for variants in which germline residues are mutated to matureresidues or vice versa. Thus, RR spectroscopy reveals an evolution from weak substrate distortion in thegermline antibody to strong substrate distortion in the affinity-matured antibody, and supports the view thatcatalysis involves a substrate strain mechanism.
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