Читать книгу Plant Nucleotide Metabolism - Hiroshi Ashihara - Страница 63

AMP and GMP are synthesized from IMP. AMP is formed by replacing the carboxyl group at C6 with an amino group from aspartate. GTP is the donor for the energy-rich phosphate bond to form adenylosuccinate (SAMP) (step 11, Figure 4.1, Reaction 11). The reaction is catalysed by SAMP synthase (ASS, EC 6.3.4.4).

The cDNA encoding ASS (pur11) has been isolated and characterized from A. thaliana. This enzyme is a known target for herbicides and antibiotics (Fonné-Pfister et al. 1996). The structure of ASS has been investigated using recombinant ASS proteins from A. thaliana and Tritium aestivum expressed in E. coli. Comparison with the known structures from E. coli revealed that the overall fold is very similar to that of the E. coli protein. The longer N terminus in the plant sequences is at the same place as the longer C terminus of the E. coli sequence in the 3D structure. The GDP-binding sites have one additional hydrogen-bonding partner, which is a plausible explanation for the lower Km value (Prade et al. 2000).

The removal of fumarate to form AMP is catalysed by ASL (step 12, Figure 4.1, Reaction 12). ASL also catalyses step 8 of the purine biosynthesis de novo.