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

Hankuil Yi, Sanghamitra Dey, Sangaralingam Kumaran, Soon Goo Lee, Hari B. Krishnan, Joseph M. Jez
Journal of Biological Chemistry 2013 v.288 no.51 pp. 36463-36472
Glycine max, X-ray diffraction, biosynthesis, catalytic activity, coenzyme A, cold, cold stress, crystal structure, cysteine, enzyme activity, molecular chaperones, mutants, plant response, plant stress, protein binding, protein synthesis, serine, serine O-acetyltransferase, site-directed mutagenesis, soybeans, sulfur
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.