The power to reduce: pyridine nucleotides – small molecules with a multitude of functions

Abstract

The pyridine nucleotides NAD and NADP play vital roles in metabolic conversions as signal transducers and in cellular defence systems. Both coenzymes participate as electron carriers in energy transduction and biosynthetic processes. Their oxidized forms, NAD + and NADP + , have been identified as important elements of regulatory pathways. In particular, NAD + serves as a substrate for ADP-ribosylation reactions and for the Sir2 family of NAD + -dependent protein deacetylases as well as a precursor of the calcium mobilizing molecule cADPr (cyclic ADP-ribose). The conversions of NADP + into the 2 -phosphorylated form of cADPr or to its nicotinic acid derivative, NAADP, also result in the formation of potent intracellular calcium-signalling agents. Perhaps, the most critical function of NADP is in the maintenance of a pool of reducing equivalents which is essential to counteract oxidative damage and for other detoxifying reactions. It is well known that the NADPH/NADP + ratio is usually kept high, in favour of the reduced form. Research within the past few years has revealed important insights into how the NADPH pool is generated and maintained in different subcellular compartments. Moreover, tremendous progress in the molecular characterization of NAD kinases has established these enzymes as vital factors for cell survival. In the present review, we summarize recent advances in the understanding of the biosynthesis and signalling functions of NAD(P) and highlight the new insights into the molecular mechanisms of NADPH generation and their roles in cell physiology.

Regulatory crosstalk of the metabolic network

“…In addition of being targeted by metabolic activity, NAD(H) concentration and redox state is controlled by the activity of the NAD salvage pathway, the activity of NAD kinases and the synthesis of precursor molecules [37,38]. Furthermore, the NAD + :NADH ratio responds directly to stress situations [37].…”

“…The NAD + :NADH ratio influences chromatin-independent signalling events and modulates calcium homeostasis, gene expression (for instance, by targeting RNA polymerase II through PARP) and the immune response (via nuclear factor kB signalling) [37,38].…”

The transcription repressor NmrA is subject to proteolysis by three Aspergillus nidulans proteases

“…10 If the function of a redox sensing ability is to form a link between changes to cellular metabolism and the control of transcription, one might expect that NmrA would be more likely to have its biological effects modulated by binding reduced dinucleotides. 27 The rationale for this argument is that it follows from the high (mM range) NAD(H) concentration and the high NAD þ /NADH ratio (500-700) 28,29 that conversion of NAD þ to NADH will have the greatest relative effect on the NADH level. 27 In this respect we note that the Human protein HSCARG, which belongs to the NmrA-like structural family, binds NADPH with a 360-fold lower K D than that for NADP þ .…”

“…It appears therefore that although there have been substantial evolutionary changes to the amino acid sequence, there has been selective pressure for NmrA to retain the ability to bind oxidized dinucleotides. Most of the dinucleotide pool is believed to be protein bound 29 in vivo, and this latter observation has led to the caveat that 'estimates of the actual substrate availability, for example, for NAD(P) þ -mediated signaling processes are to be viewed with caution, at least, when based upon pyridine nucleotide concentrations in cellular extracts'. 29 As NmrA appears to be produced in vivo under most physiological conditions, there is no obvious reason why it should not form part of the cellular pool of protein that binds the majority of the dinucleotide pool in vivo.…”

“…Most of the dinucleotide pool is believed to be protein bound 29 in vivo, and this latter observation has led to the caveat that 'estimates of the actual substrate availability, for example, for NAD(P) þ -mediated signaling processes are to be viewed with caution, at least, when based upon pyridine nucleotide concentrations in cellular extracts'. 29 As NmrA appears to be produced in vivo under most physiological conditions, there is no obvious reason why it should not form part of the cellular pool of protein that binds the majority of the dinucleotide pool in vivo. Therefore, it is not unreasonable to infer that a significant proportion of the in vivo NmrA pool is likely to have bound NAD þ under most physiological conditions.…”