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Interaction of serine acetyltransferase with O-acetylserine sulfhydrylase active site: Evidence from fluorescence spectroscopy

¹ Department of Biochemistry and Molecular Biology and ² Department of Public Health, University of Parma, 43100 Parma, Italy
³ Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA
⁴ Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461, USA

(RECEIVED April 1, 2005; FINAL REVISION May 9, 2005; ACCEPTED May 9, 2005)

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 microenvironment and to conformational changes within the protein matrix. The results indicate that both serine acetyltransferase and its C-terminal decapeptide bind to the -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.

Keywords: serine acetyltransferase; O-acetylserine sulfhydrylase; cysteine synthase; fluorescence; protein dynamics; enzymes; conformational changes

Abbreviations: SAT, serine acetyltransferase • OASS, O-acetylserine sulfhydrylase • HiOASS/HiSAT, OASS and SAT from H. influenzae • StOASS/StSAT, OASS and SAT from S. typhimurium • PLP, pyridoxal 5'-phosphate • OAS, O-acetylserine • P₁₀, decapeptide corresponding to the last 10 residues of HiSAT • S₁₀, scrambled decapeptide

Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051492805.

Reprint requests to: Stefano Bettati, Department of Public Health, University of Parma, 43100 Parma, Italy; e-mail: stefano.bettati{at}unipr.it; fax: 39-0521-903712.

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