Theoretical Insights into the Mechanism of Quinolinate Phosphoribosyl Transferase

Aleksandr Rozenberg, Rutgers University Jeehiun Katherine Lee, Rutgers University

Abstract We describe here a theoretical study into the catalytic mechanism of QPRTase, wherein we probe the energetics of the possible mechanistic pathways: i) decarboxylation of quinolinic acid (QA) before 5'-phosphoribosyl-1'-pyrophosphate (PRPP) addition and ii) addition of PRPP before decarboxylation. We first explored which form of quinolinic acid: the neutral (dicarboxylic acid), monocarboxylate, dicarboxylate or a decarboxylated species–is the best nucleophile. We next examined possible substrates for decarboxylation, and the associated energetics. Analysis of our computational results leads us to propose the following mechanism: QPRTase first binds the quinolinic acid (QA) monocarboxylate anion; PRPP then binds to the active site and reacts with the QA monoanion, leading to pyrophosphate departure, thus forming quinolinic acid mononucleotide (QAMN). Decarboxylation then occurs at the C2 position; final protonation at C2 yields NAMN. Our calculations are the first to show the energetic viability of the putative QAMN intermediate. Furthermore, we show that the monocarboxylate form of QAMN will decarboxylate much more favorably energetically than will the dicarboxylate form of QAMN. We also are able to discount decarboxylation as the first step, both energetically and also because the substrate in such a mechanism would prefer to decarboxylate at the C3 position, not the C2 position; decarboxylation at C3 is unrelated to the QPRTase mechanism and would not ultimately yield the desired biological NAMN product. We also discuss our results in the context of existing experimental data.

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