Structure of Soybean Serine Acetyltransferase and Formation of the Cysteine Regulatory Complex as a Molecular Chaperone

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. 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 Asp¹⁵⁴ form a catalytic dyad for general base catalysis and that His¹⁸⁹ may stabilize the oxyanion reaction intermediate. Glu¹⁷⁷ helps to position Arg²⁰³ and His²⁰⁴ and the β1c-β2c loop for serine 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 SAT in 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.