V-shaped structure of glutamyl-tRNA reductase, the first enzyme of tRNA-dependent tetrapyrrole biosynthesis

Abstract

contributed equally to this work Processes vital to life such as respiration and photosynthesis critically depend on the availability of tetrapyrroles including hemes and chlorophylls. tRNA-dependent catalysis generally is associated with protein biosynthesis. An exception is the reduction of glutamyl-tRNA to glutamate-1-semialdehyde by the enzyme glutamyl-tRNA reductase. This reaction is the indispensable initiating step of tetrapyrrole biosynthesis in plants and most prokaryotes. The crystal structure of glutamyl-tRNA reductase from the archaeon Methanopyrus kandleri in complex with the substrate-like inhibitor glutamycin at 1.9 A Ê resolution reveals an extended yet planar V-shaped dimer. The well de®ned interactions of the inhibitor with the active site support a thioester-mediated reduction process. Modeling the glutamyl-tRNA onto each monomer reveals an extensive protein±tRNA interface. We furthermore propose a model whereby the large void of glutamyl-tRNA reductase is occupied by glutamate-1-semialdehyde-1,2-mutase, the subsequent enzyme of this pathway, allowing for the ef®cient synthesis of 5-aminolevulinic acid, the common precursor of all tetrapyrroles.

Complex Formation between Glutamyl-tRNA Reductase and Glutamate-1-semialdehyde 2,1-Aminomutase in Escherichia coli during the Initial Reactions of Porphyrin Biosynthesis

“…However they do not rule out the possibility that a very rapid GSA-AM reaction in the coupled assay might also result in the protection of GSA. Nevertheless, these findings are in clear agreement with the postulated substrate channeling pathway as indicated by x-ray crystallography and by modeling experiments (5) in which the intermediate aldehyde is prevented from exposure to the aqueous environment.…”

“…The modeled E. coli structures were generated using the program BRAGI (19). The model complex was created by placing GSA-AM in the open space delimited by GluTR and docking them along their 2-fold symmetry axes as described previously (5). The image was generated by using PyMOL (20).…”

“…Based on the three-dimensional structures of GluTR from M. kandleri and GSA-AM from Synechococcus sp. (10) a hypothetical model ensuring efficient ALA synthesis was proposed (5). We realized that the open space delimited by the GluTR monomers is remarkably similar to the volume occupied by GSA-AM.…”

“…Recently the catalytic mechanism of GluTR and its structural basis have been elucidated in a combined biochemical and structural investigation using the recombinant enzyme from the extreme thermophilic archaean Methanopyrus kandleri (4,5). The crystal structure of GluTR reveals an unusual extended V-shaped dimer with each monomer consisting of three distinct domains arranged along a curved "spinal" ␣-helix.…”

Domain structures of chlorophyllide �a oxygenase of green plants and Prochlorothrix hollandica in relation to catalytic functions

“…It has been reported that some enzymes for chlorophyll biosynthesis are associated with other proteins and that this fe ature is important for the enzymatic activities. Glutamyl-tRNA reductase, which is involved in 5-amonolevulinic acid syntheses, has been suggested to form complexes with glutamate-1-semialdehyde-1, 2-mutase, an enzyme required for the subsequent step (Moser et al 2001). It is thought that this structure promotes the efficient synthesis of 5-amonolebulinic acids.…”