The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum reveals its substrate and reaction specificity.
Cystathionine gamma-synthase catalyses the committed step of de novo methionine biosynthesis in micro-organisms and plants, making the enzyme an attractive target for the design of new antibiotics and herbicides. The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum has been solved by Patterson search techniques using the structure of Escherichia coli cystathionine gamma-synthase. The model was refined at 2.9 A resolution to a crystallographic R -factor of 20.1 % (Rfree25.0 %). The physiological substrates of the enzyme, L-homoserine phosphate and L-cysteine, were modelled into the unliganded structure. These complexes support the proposed ping-pong mechanism for catalysis and illustrate the dissimilar substrate specificities of bacterial and plant cystathionine gamma-synthases on a molecular level. The main difference arises from the binding modes of the distal substrate groups (O -acetyl/succinyl versusO -phosphate). Central in fixing the distal phosphate of the plant CGS substrate is an exposed lysine residue that is strictly conserved in plant cystathionine gamma-synthases whereas bacterial enzymes carry a glycine residue at this position. General insight regarding the reaction specificity of transsulphuration enzymes is gained by the comparison to cystathionine beta-lyase from E. coli, indicating the mechanistic importance of a second substrate binding site for L-cysteine which leads to different chemical reaction types.
Steegborn
C
C
Abteilung Strukturforschung, Am Klopferspitz 18a, Planegg-Martinsried, D-82152, Germany. steegbo@biochem.mpg.de
Messerschmidt
A
A
Laber
B
B
Streber
W
W
Huber
R
R
Clausen
T
T
D000595:Amino Acid Sequence; D001665:Binding Sites; D019757:Carbon-Oxygen Lyases / Q000737:chemistry* / Q000378:metabolism*; D002384:Catalysis; D003460:Crystallization; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology; D006714:Homoserine / Q000031:analogs & derivatives / Q000378:metabolism; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D008190:Lyases / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010947:Plants, Toxic; D017433:Protein Structure, Secondary; D011732:Pyridoxal Phosphate / Q000378:metabolism; D013379:Substrate Specificity; D014026:Nicotiana / Q000201:enzymology*
C011066:O-phosphohomoserine; D011732:Pyridoxal Phosphate; D006714:Homoserine; C022696:O-succinylhomoserine (thiol)-lyase; D008190:Lyases; D019757:Carbon-Oxygen Lyases; C019965:cystathionine beta-lyase; D003545:Cysteine