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.

Figure 1. Schematic overview of NAD(P) biosynthetic pathways

The major known pathways of NAD(P) synthesis are presented. Main endogenous precursors of NAD(P) synthesis are Nam, NA and L-tryptophan. Nicotinamidase is only found in yeast. NamR, Nam riboside; NA/NamPRT, NA/Nam phosphoribosyltransferase; NADS, NAD synthetase.

Figure 2. Signalling derivatives of NAD(P)

The ADP-ribosyl moiety commonly shared by all derivatives is illustrated in structural detail and adumbrated in light blue, the red asterisk indicates the site of ADP-ribosyl attachment to the acceptor. The individual portion of each derivative is presented in red and the 2′-phosphate group of NADP⁺ in its derivatives is shown in dark blue. ADPr, ADP-ribose; mAPDr, mono-ADPr; pADPr, poly-ADPr; 2′P-cADPr, 2′-phosphate cADPr.

Figure 3. Generation and utilization of NADPH in eukaryotic cells

Cyt P450, cytochrome P450; Trx, thioredoxin.

Figure 4. Partial multiple sequence alignment of NADKs from several organisms

Amino acid sequences of human NADK (protein ID NP_075394); A. thaliana NADK-1, NADK-2 and NADK-3 (NP_974347, NP_564145 and NP_177980); S. cerevisiae NADK-1/Utr1p, NADK-2/Yef1p and NADK-3/Pos5p (P21373, NP_010873 and NP_015136); E. coli YfjB (NP_417105); and M. tuberculosis Ppnk (BAB21478) were compared using Clustal W [170]. Identical and similar residues are highlighted in black and grey respectively. The two conserved NADK motifs are boxed in red.

Figure 5. Subcellular distribution of NADP metabolism in eukaryotic cells

NADK isoforms described in mammals, yeast and Arabidopsis are highlighted in blue, red and green respectively. Transhydrogenase and ADP-ribosyl cyclase have not been detected in yeast.