Weak coupling of ATP hydrolysis to the chemical equilibrium of human nicotinamide phosphoribosyltransferase

Human nicotinamide phosphoribosyltransferase (NAMPT, EC 2.4.2.12) catalyzes the reversible synthesis of nicotinamide mononucleotide (NMN) and inorganic pyrophosphate (PP_i) 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 _m^NAM from 855 to 5 nM), and the K_eq 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⁺ (K_i = 0.14 μM) and NADH (K_i = 0.22 μM), an apparent regulatory feedback mechanism.