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purF K326Q

It is the coding sequence of the K326Q mutant of glutamine phosphoribosylpyrophosphate amidotransferase that catalyzes the transformation from PRPP to PRA in de novo purine biosynthesis. We introduced a mutation K326Q to the enzyme based on "Identification of Sites for Feedback Regulation of Glutamine 5-Phosphoribosylpyrophosphate Amidotransferase by Nucleotides and Relationship to Residues Important for Catalysis"(Gaochao Zhou et al. ,1993) to remit the feedback inhibition by GMP.

Usage and Biology

De novo purine nucleotide synthesis is a necessary procedure in almost every organism to satisfy the basic living and reproducing needs. In the prime step of the pathway, PRPP is transformed from R5P with the catalysis of Prs. An ATP provides a pyrophosphate and turns into AMP in the same procedure.(fig.1)

Figure 1: phosphoribosylpyrophosphate synthase catalyzes the transformation from R5P to PRPP

The formation of PRA is the first committed step of the pathway. PurF catalyzes the attachment of an amino group to C-1 of PRPP.(fig.2)

Figure 2: glutamine phosphoribosylpyrophosphate amidotransferase catalyzes the transformation from R5P to PRPP in de novo purine biosynthesis

The resulting PRA is highly volatile. Purine rings are then built up on this structure. The second step is the addition of three atoms from glycine to turn PRA into GRA, helped along by enzyme PurD and energy provided by an ATP.(fig.3).

Figure 3: glycinamide ribonucleotide synthetase catalyzes the transformation from PRA to GAR in de novo purine biosynthesis

Then N10-formyltetrahydrofolate formylates the glycine amino group which turns GAR into FGAR, and a nitrogen is contributed by glutamine transforming FGAR to FGAM, before dehydration and ring closure yield the AIR by PurM.(fig.4)

Figure 4: aminoimidazole ribonucleotide synthetase catalyzes the transformation from FGAM to AIR in de novo purine biosynthesis

To complete the second ring in the purine structure, a carboxyl group is first added and transforms AIR into CAIR. Then, CAIR is transformed into SACAIR with enzyme PurC and energy provided by an ATP.(fig.5)

Figure 5: succinylaminoimidazolecarboxamide ribonucleotide synthetase catalyzes the transformation from CAIR to SAICAR in de novo purine biosynthesis

After another 3 steps, we have the first intermediate with a complete purine ring is inosinate (IMP). The total pathway of de novo purine biosynthesis is showed below.(fig.6)

Figure 6: The total pathway of de novo purine biosynthesis

Relieve the feedback inhibition

Gaochao Z.(1993) pointed out that, at concentrations of GMP which completely inhibited the wild type, the K326Q enzyme retained full activity. The experiment demonstrated that inactivation of glutamine PRPP amidotransferase was caused by covalent labeling by FSBA. It appears that Lys326 may have been labeled by FSBA in the secondary phase of the reaction which did not contribute to inactivation, so we introduced a mutation

We overexpressed PurF K326Q to achieve the mass synthesis of purine. Consequently, more purines than before participant in the circuits we designed and were used to synthesize uric acid when co-expressed with rhXOR.

References

1.Lehninger, A. L., Nelson, D. L. 1., & Cox, M. M. (2008). Lehninger principles of biochemistry (5th ed.). New York ; New Delhi: W.H. Freeman.
2. Sequence and Features