Detection of interaction between cofactors at the active centers of homodimeric and homotetrameric enzymes is usually elusive by steady-state kinetic approaches and requires protein variants where such interactions are diminished or exaggerated. In this Account, evidence for active-center interactions will be presented for the following thiamin diphosphate-dependent enzymes: yeast pyruvate decarboxylase, benzoylformate decarboxylase, and examples from the 2-oxoacid dehydrogenase multienzyme complex class. The dissymmetry of active sites is especially evident in the X-ray structures of these enzymes with substrate/substrate analogues bound. Perturbations that reveal active center communication include use of chromophoric substrates and substitutions of active center residues on putative pathways.
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