Journal
PROTEIN SCIENCE
Volume 14, Issue 8, Pages 2115-2124Publisher
WILEY
DOI: 10.1110/ps.051492805
Keywords
serine acetyltransferase; O-acetylserine sulfhydrylase; cysteine synthase; fluorescence; protein dynamics; enzymes; conformational changes
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Serine acetyltransferase is a key enzyme in the sulfur assimilation pathway of bacteria and plants, and is known to form a bienzyme complex with O-acetylserine sulfhydrylase, the last enzyme in the cysteine biosynthetic pathway. The biological function of the complex and the mechanism of reciprocal regulation of the constituent enzymes are still poorly understood. In this work the effect of complex formation on the O-acetylserine sulfhydrylase active site has been investigated exploiting the fluorescence properties of pyridoxal 5'-phosphate, which are sensitive to the cofactor micro-environment and to conformational changes within the protein matrix. The results indicate that both serine acetyltransferase and its C-terminal decapeptide bind to the (x-carboxyl subsite of O-acetylserine sulfhydrylase, triggering a transition from an open to a closed conformation. This finding suggests that serine acetyltransferase can inhibit O-acetylserine sulfhydrylase catalytic activity with a double mechanism, the competition with O-acetylserine for binding to the enzyme active site and the stabilization of a closed conformation that is less accessible to the natural substrate.
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