Human nicotinamide phosphoribosyltransferase (NAMPT, EC 22.214.171.124) catalyzes the reversible synthesis of nicotinamide mononucleotide (NMN) and inorganic pyrophosphate (PPi) from nicotinamide (NAM) and α-D-5- phosphoribosyl-l-pyrophosphate (PRPP). NAMPT, by capturing the energy provided by its facultative ATPase activity, allows the production of NMN at product:substrate ratios thermodynamically forbidden in the absence of ATP. With ATP hydrolysis coupled to NMN synthesis, the catalytic efficiency of the system is improved 1100-fold, substrate affinity dramatically increases (K mNAM from 855 to 5 nM), and the Keq shifts -2.1 kcal/mol toward NMN formation. ADP-ATP isotopic exchange experiments support the formation of a high-energy phosphorylated intermediate (phospho-H247) as the mechanism for altered catalytic efficiency during ATP hydrolysis. NAMPT captures only a small portion of the energy generated by ATP hydrolysis to shift the dynamic chemical equilibrium. Although the weak energetic coupling of ATP hydrolysis appears to be a nonoptimized enzymatic function, closer analysis of this remarkable protein reveals an enzyme designed to capture NAM with high efficiency at the expense of ATP hydrolysis. NMN is a rate-limiting precursor for recycling to the essential regulatory cofactor, nicotinamide adenine dinucleotide (NAD+). NMN synthesis by NAMPT is powerfully inhibited by both NAD+ (Ki = 0.14 μM) and NADH (Ki = 0.22 μM), an apparent regulatory feedback mechanism.
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