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Title: Structure of soybean serine acetyltransferase and formation of the cysteine regulatory complex as a molecular chaperone

Author
item YI, HANKUIL - Washington University
item DEY, SANGHAMITRA - Washington University
item KUMARAN, SANGARALINGAM - Council Of Scientific And Industrial Research (CSIR)
item LEE, SOON GOO - Washington University
item Krishnan, Hari
item JEZ, JOSEPH - Washington University

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2013
Publication Date: 12/20/2013
Publication URL: http://handle.nal.usda.gov/10113/58800
Citation: Yi, H., Dey, S., Kumaran, S., Lee, S., Krishnan, H.B., Jez, J.M. 2013. Structure of soybean serine acetyltransferase and formation of the cysteine regulatory complex as a molecular chaperone. Journal of Biological Chemistry. 288(51):36463-36472.

Interpretive Summary: Soybean is a rich source of protein. Unfortunately, soybean proteins contain low amounts of two important amino acids, methionine and cysteine, that are vital for optimal growth of humans and animals. Therefore, attempts are being made to increase the amount of these two amino acids in soybeans by manipulating enzymes involved in sulfur assimilation. Serine acetyltransferase (SAT) catalyzes the limiting reaction in the biosynthesis of cysteine. In addition, this enzyme forms a complex with O-acetylserine sulfhydrylase (OASS), another key enzyme in sulfur assimilation. The formation of the enzyme complex is a critical biochemical control feature in plant sulfur metabolism. In this study, we present the x-ray crystal structure of soybean SAT providing new details on how this enzyme functions in plant sulfur assimilation. The information obtained from this study will help biotechnologists to genetically manipulate the sulfur-assimilatory pathway so that we can improve the overall quality of soybean seed proteins. Superior quality soy proteins can be utilized to meet the nutritional requirements of the multitude of malnourished people around the world.

Technical Abstract: Serine acetyltransferase (SAT) catalyzes the limiting reaction in plant and microbial biosynthesis of cysteine. In addition to its enzymatic function, SAT forms a macromolecular complex with O-acetylserine sulfhydrylase (OASS). Formation of the cysteine regulatory complex (CRC) is a critical biochemical control feature in plant sulfur metabolism. Here we present the 1.75 - 3.0 Å resolution x-ray crystal structures of soybean (Glycine max) SAT (GmSAT) in apoenzyme, serine bound, and CoA bound forms. The GmSAT•serine and GmSAT•CoA structures provide new details on substrate interactions in the active site. The crystal structures and analysis of site-directed mutants suggest that His169 and Asp168 form a catalytic dyad for general base catalysis and that His189 stabilizes the oxyanion reaction intermediate. For binding of serine, Glu177 and His204 stabilize movement of the beta1c-beta 2c loop to position Arg203 for substrate binding. A similar role for ionic interactions formed by Lys230 is required for CoA binding. The GmSAT structures also identify Arg253 as important for the enhanced catalytic efficiency of SATin the CRC and suggest that movement of the residue may stabilize CoA binding in the macromolecular complex. Differences in the effect of cold on GmSAT activity in the isolated enzyme versus the enzyme in the CRC were also observed. A role for CRC formation as a molecular chaperone to maintain SAT activity in response to an environmental stress is proposed for this multienzyme complex in plants.