Interactions substrat-flavine-hemoproteine et stabilite thermique du cytochrome b(2) (L-lactate deshydrogenase de la levure).
We studied the thermal inactivation of yeast cytochrome b(2), the native flavohaemoprotein (FHP(n)) and its derivatives, the haemoprotein (HP) obtained by dissociation of FMN from FHP(n) and the reconstituted flavohaemoprotein (FHP(r)) (haemoprotein + FMN). The initial velocity of inactivation is faster in the presence of FMN. The substrate (L-lactate) or the competitive inhibitor (oxolate) protects only the flavohaemoproteins (FHP(n) and FHP(r)) from thermal inactivation by decreasing this velocity. In each case, the "protection constant", K(p), is determined.Several interpretations can be proposed to explain the lack of protection of the haemoprotein by its substrate: 1) the structure of the substrate-binding site depends on the presence of the FMN; 2) the substrate binding site exists, but the affinity is too small to be detected; 3) a hypothetical scheme is proposed, suggesting that the protein can exist under different thermally sensitive forms following whether FMN or substrate is present.
Capeillere-Blandin
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Centre de Génétique Moléculaire du C.N.R.S., 91, Gif-sur-Yvette, France