The "Super Mutant" of Yeast FMN Adenylyltransferase Enhances the Enzyme Turnover Rate by Attenuating Product Inhibition
FMN adenylyltransferase (FMNAT) is an essential enzyme catalyzing the last step of a two-step pathway converting riboflavin (vitamin B2) to FAD, the ubiquitous flavocoenzyme. A structure-based mutagenesis and steady-state kinetic analysis of yeast FMNAT unexpectedly revealed that mutant D181A had a much faster turnover rate than the wild type enzyme. Product inhibition analysis showed that wild type FMNAT is strongly inhibited by FAD, whereas D181A mutant enzyme has an attenuated product inhibition. These results provide a structural basis for the product inhibition of the enzyme and suggest that product release may be the rate-limiting step of the reaction.
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