Riboflavin Binding in Egg‐White Flavoprotein: the Role of Tryptophan and Tyrosine

In egg‐white apoflavoprotein six of the nine tyrosines and three of the nine tryptophans have been shown to be accessible to ethylene glycol using solvent perturbation difference spectroscopy. For egg‐yolk flavoprotein the same ratio of exposed tyrosines to tryptophans was found, but the native protein structure proved to be more stable against denaturation by solvent. A single tryptophan and a single tyrosine have been demonstrated to be essential for binding of riboflavin by a combination of chemical modification and affinity chromatography techniques. Two of the exposed tyrosines and two of the exposed tryptophans could be modified without loss of binding capacity. Only the essential tyrosine was found to be protected by bound riboflavin against nitration with tetranitromethane, whereas the essential tryptophan was found not to be protected against alkylation by dimethyl‐(2‐hydroxy‐5‐nitrobenzyl)‐sulfonium bromide. Indocyanine green, a hydrophobic probe, binds to egg‐white flavoprotein, with a K d of 10 −6 M, most probably at the cofactor binding site. An intramolecular dihydroflavin · nicotinamide complex is bound to the apoprotein ( K d ∼ 10 −4 M) in a geometry still allowing charge transfer interaction between dihydro‐flavin and nicotinamide. These data lead us to propose that egg‐white flavoprotein binds riboflavin at a hydrophobic site near to or at the suface of the protein. Tyrosine is probably involved in a stacking interaction with riboflavin and tryptophan is also essential for binding. The dimethylbenzene part of the flavin nucleus appears to be buried in egg‐white flavoprotein in contrast to flavodoxins where it has been shown to be exposed.