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 His(169) and Asp(154) form a catalytic dyad for general base catalysis and that His(189) may stabilize the oxyanion reaction intermediate. Glu(177) helps to position Arg(203) and His(204) and the β1c-β2c loop for serine binding. A similar role for ionic interactions formed by Lys(230) is required for CoA binding. The GmSAT structures also identify Arg(253) 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.
From the Department of Biological Sciences, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, Korea.
Hankuil
H
Yi
Sanghamitra
S
Dey
Sangaralingam
S
Kumaran
Soon Goo
SG
Lee
Hari B
HB
Krishnan
Joseph M
JM
Jez
D000595:Amino Acid Sequence; D001665:Binding Sites; D003065:Coenzyme A / Q000737:chemistry / Q000378:metabolism; D018832:Molecular Chaperones / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D062105:Molecular Docking Simulation; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D012694:Serine / Q000737:chemistry / Q000378:metabolism; D051050:Serine O-Acetyltransferase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013025:Glycine max / Q000201:enzymology*
D018832:Molecular Chaperones; D010940:Plant Proteins; D012694:Serine; D051050:Serine O-Acetyltransferase; D003065:Coenzyme A
Biosynthesis; Crystal Structure; Enzyme Structure; Plant Biochemistry; Plant Molecular Biology