Nicotinamide phosphoribosyltransferase (NMPRTase) catalyzes the synthesis of nicotinamide mononucleotide (NMN) from nicotinamide (NM) and 5′-phosphoribosyl-1′-pyrophosphate (PRPP), thus playing an important role in the cyclic biosynthetic pathway of nicotinamide adenine dinucleotide (NAD) in the human body (Magni et al.
). NAD performs crucial roles in living cells by participating widely in redox reactions and also in signal transduction. In the latter, NAD is degraded and consumed by NAD cyclase, ADP ribosyltransferase etc.
(Guse et al.
; Ziegler, 2000
). Thus, in order to maintain the concentration of NAD, organisms have a system that recruits NAD in addition to the pathway for its de novo
biosynthesis. Nicotinamide (NM), a degradation product of NAD, is reused in cells via
the following two pathways. In the first path, NM is transformed to nicotinic acid (NA) by nicotinamide deaminase and then to nicotinic acid mononucleotide (NAMN) by phosphoribosyltransferase (NAPRTase), ultimately leading to NAD. However, in the human body, nicotinamide deaminase is only found in the case of infection by malarial parasites (Zerez et al.
); it is not found in healthy humans, so this path does not generally contribute. In the second path, the main recruitment path, NM reacts with 5′-phosphoribosyl-1′-pyrophosphate (PRPP) to form nicotinamide mononucleotide (NMN), which then leads to NAD.
It would be expected that the inhibition of NMPRTase would cause exhaustion of NAD and disorder of the NAD-requiring systems of mitochondrial respiratory and energy metabolism. This is supported by a report on a new anticancer drug, FK866 (Hasmann & Schemainda, 2003
), which causes cell apoptosis by inhibiting this enzyme.
NMPRTase has also been identified as pre-B-cell colony-enhancing factor (PBEF; Samal et al.
), a growth factor for early stage B cells, and has recently further been identified as visfatin, a cytokine that is secreted from visceral fat tissues. It has attracted attention as the ‘second insulin’ as visfatin binds to the insulin receptor and acts to lower the blood-sugar level (Fukuhara et al.
Recently, crystal structures of NMPRTase/PBEF/visfatin have been reported (Khan et al.
; Kim et al.
; Wang et al.
). Eight structures have been described: three murine structures (free form, PDB codes 2h3b
; complex with NMN, 2h3d
), three rat structures (free form, 2g95
; complex with NMN, 2g96
; complex with FK866, 2g97
) and two human structures (complex with NMN, 2gvg
; complex with FK866, 2gvj
). However, the free form of human NMPRTase has not yet been reported.
In this paper, we report the first crystallization of the free form of human NMPRTase using the microseeding technique (Thaller et al.
; Stura, 1999a
; Bergfors, 2003
), with the aim of structure determination of the enzyme itself and further analysis of its reaction mechanism by conducting inhibitor-soaking experiments on the enzyme.