L-Aspartate Semialdehyde and a 6-Deoxy-5-ketohexose 1-Phosphate Are the Precursors to the Aromatic Amino Acids in Methanocaldococcus jannaschii

摘要

No orthologs are present in the genomes of the archaea encoding genes for the first two steps in the biosynthesis of the aromatic amino acids leading to 3-dehydroquinate (DHQ).The absence of these genes prompted me to examine the nature of the reactions involved in the archaeal pathway leading to DHQ in Methanocaldococcus jannaschii.Here I report that 6-deoxy-5-ketofructose 1-phosphate and L-aspartate semialdehyde are precursors to DHQ.The sugar,which is derived from glucose 6-P,supplies a "hydroxyacetone" fragment,which,via a transaldolase reaction,undergoes an aldol condensation with the L-aspartate semialdehyde to form 2-amino-3,7-dideoxy-D-threo-hept-6-ulosonic acid.Despite the fact that both hydroxyacetone and hydroxyacetone-P were measured in the cell extracts and confirmed to arise from glucose 6-P,neither compound was found to serve as a precursor to DHQ.This amino sugar then undergoes a NAD dependent oxidative deamination to produce 3,7-dideoxy-D-threo-hept-2,6-diulosonic acid which cyclizes to 3-dehydroquinate.The protein product of the M.jannaschii MJ0400 gene catalyzes the transaldolase reaction and the protein product of the MJ1249 gene catalyzes the oxidative deamination and the cyclization reactions.The DHQ is readily converted into dehydroshikimate and shikimate in M.jannaschii cell extracts,consistent with the remaining steps and genes in the pathway being the same as in the established shikimate pathway.