Cooperative changes in conformation

Enzyme active sites and receptors rarely interact with hgands without some attendant change in conformation, and the ability to detect and quantify a conformational change hes at the heart of contemporary biochemical kinetics. See Induced Fit Model Fluorescence Spectroscopy Linked Functions Flemoglobin Cooperativity... 

A model used to explain cooperativity on the basis of ligand-induced changes in conformation that may or may not alter the subunit-subunit interfaces of oligomeric enzymes and receptors. This model has also been referred to as the Adair-Koshland-Nemethy-Filmer model (AKNF model), the induced-fit model, and the sequential model. 

Near-UV CD depends critically on the atomic environment and closeness of packing of the aromatic residues, including their accessibility to solvent. Although the structural information is limited strictly to the immediate neighborhood of the relatively few aromatic residues, the generally cooperative nature of protein conformation means that changes in aromatic CD frequently reflect more widespread changes in conformation and dynamics. 

The modified hemoglobin should not show cooperativity. Although the imidazole in solution will bind to the heme iron (in place of histidine) and will facilitate oxygen binding, the imidazole lacks the crucial connection to the particular a helix that must move so as to transmit the change in conformation. 

It is the differences in hydrogen bond strengths that are relevant. These might be of the order of a few hundred calories per mole or less thus 80 kcal. is equivalent to upwards of 102 or 103 of such bonds. As shown quantitatively below, AH is related to the statistical number of neighboring polypeptide residues which cooperatively change their conformational state in the course of the transition. 

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