Nicotinic acid mononucleotide (BioDeep_00000003532)

描述信息

Nicotinic acid mononucleotide, also known as nicotinate ribonucleotide, belongs to the class of organic compounds known as nicotinic acid nucleotides. These are pyridine nucleotides in which the pyridine base is nicotinic acid or a derivative thereof. Nicotinic acid mononucleotide is an extremely weak basic (essentially neutral) compound (based on its pKa). Nicotinic acid mononucleotide an intermediate in the cofactor biosynthesis and the nicotinate and nicotinamide metabolism pathways. It is a substrate for nicotinamide riboside kinase, ectonucleotide pyrophosphatase/phosphodiesterase, nicotinamide mononucleotide adenylyltransferase, 5-nucleotidase, nicotinate-nucleotide pyrophosphorylase, and 5(3)-deoxyribonucleotidase.

Nicotinic acid mononucleotide is an intermediate in the metabolism of Nicotinate and nicotinamide. It is a substrate for Ectonucleotide pyrophosphatase/phosphodiesterase 2, Ectonucleotide pyrophosphatase/phosphodiesterase 1, Nicotinamide mononucleotide adenylyltransferase 3, Cytosolic 5-nucleotidase IA, Cytosolic 5-nucleotidase IB, Nicotinate-nucleotide pyrophosphorylase, 5(3)-deoxyribonucleotidase (cytosolic type), Cytosolic purine 5-nucleotidase, Nicotinamide mononucleotide adenylyltransferase 2, Ectonucleotide pyrophosphatase/phosphodiesterase 3, 5-nucleotidase, 5(3)-deoxyribonucleotidase (mitochondrial) and Nicotinamide mononucleotide adenylyltransferase 1. [HMDB]

NaMN is the most common mononucleotide intermediate (a hub) in NAD biogenesis. For example, in E. coli all three pyridine precursors are converted into NaMN (Table 1 and Figure 3(a)). Qa produced by the de novo Asp–DHAP pathway (genes nadB and nadA) is converted into NaMN by QAPRT (gene nadC). Salvage of both forms of niacin proceeds via NAPRT (gene pncB) either directly upon or after deamidation by NMDSE (gene pncA). Overall, more than 90\% of approximately 680 analyzed bacterial genomes contain at least one of the pathways leading to the formation of NaMN. Most of them (∼480 genomes) have the entire set of nadBAC genes for NaMN de novo synthesis from Asp that are often clustered on the chromosome and/or are co-regulated by the same transcription factors (see Section 7.08.3.1.2). Among the examples provided in Table 1, F. tularensis (Figure 4(c)) has all three genes of this de novo pathway forming a single operon-like cluster and supporting the growth of this organism in the absence of any pyridine precursors in the medium. More than half the genomes with the Asp–DHAP pathway also contain a deamidating niacin salvage pathway (genes pncAB) as do many representatives of the α-, β-, and γ-Proteobacteria, Actinobacteria, and Bacillus/Clostridium group. As already emphasized, the genomic reconstruction approach provides an assessment of the metabolic potential of an organism, which may or may not be realized under given conditions. For example, E. coli and B. subtilis can utilize both de novo and PncAB Nm salvage pathways under the same growth conditions, whereas in M. tuberculosis (having the same gene pattern) the latter pathway was considered nonfunctional, so that the entire NAD pool is generated by the de novo NadABC route. However, a recent study demonstrated the functional activity of the Nm salvage pathway in vivo, under hypoxic conditions in infected macrophages.221 This study also implicated the two downstream enzymes of NAD synthesis (NAMNAT and NADSYN) as attractive chemotherapeutic targets to treat acute and latent forms of tuberculosis.

In approximately 100 species, including many Cyanobacteria (e.g., Synechococcus spp.), Bacteroidetes (e.g., Chlorobium spp.) and Proteobacteria (e.g., Caulobacter crescentus, Zymomonas mobilis, Desulfovibrio spp., and Shewanella spp. representing α-, β-, δ-, and γ-groups, respectively) the Asp–DHAP pathway is the only route to NAD biogenesis. Among them, nearly all Helicobacter spp. (except H. hepaticus), contain only the two genes nadA and nadC but lack the first gene of the pathway (nadB), which is a likely subject of nonorthologous gene replacement. One case of NadB (ASPOX) replacement by the ASPDH enzyme in T. maritima (and methanogenic archaea) was discussed in Section 7.08.2.1. However, no orthologues of the established ASPDH could be identified in Helicobacter spp. as well as in approximately 15 other diverse bacterial species that have the nadAC but lack the nadB gene (e.g., all analyzed Corynebacterium spp. except for C. diphtheriae). Therefore, the identity of the ASPOX or ASPDH enzyme in these species is still unknown, representing one of the few remaining cases of ‘locally missing genes’220 in the NAD subsystem. All other bacterial species contain either both the nadA and nadB genes (plus nadC) or none.

In a limited number of bacteria (∼20 species), mostly in the two distant groups of Xanthomonadales (within γ-Proteobacteria) and Flavobacteriales (within Bacteroidetes), the Asp–DHAP pathway of Qa synthesis is replaced by the Kyn pathway. As described in Section 7.08.2.1.2, four out of five enzymes (TRDOX, KYNOX, KYNSE, and HADOX) in the bacterial version of this pathway are close homologues of the respective eukaryotic enzymes, whereas the KYNFA gene is a subject of multiple nonorthologous replacements. Although the identity of one alternative form of KYNFA (gene kynB) was established in a group of bacteria that have a partial Kyn pathway for Trp degradation to anthranilate (e.g., in P. aeruginosa or B. cereus57), none of the known KYNFA homologues are present in Xanthomonadales or Flavobacteriales. In a few species (e.g., Salinispora spp.) a complete gene set of the Kyn pathway genes co-occurs with a complete Asp–DHAP pathway. Further experiments would be required to establish to what extent and under what conditions these two pathways contribute to Qa formation. As discussed, the QAPRT enzyme is shared by both de novo pathways, and a respective gene, nadC is always found in the genomes containing one or the other pathway. Similarly, gene nadC always co-occurs with Qa de novo biosynthetic genes with one notable exception of two groups of Streptococci, S. pneumonaie and S. pyogenes. Although all other members of the Lactobacillales group also lack the Qa de novo biosynthetic machinery and rely entirely on niacin salvage, only these two human pathogens contain a nadC gene. The functional significance of this ‘out of context’ gene is unknown, but it is tempting to speculate that it may be involved in a yet-unknown pathway of Qa salvage from the human host.

Among approximately 150 bacterial species that lack de novo biosynthesis genes and rely on deamidating salvage of niacin (via NAPRT), the majority (∼100) are from the group of Firmicutes. Such a functional variant (illustrated for Staphylococcus aureus in Figure 4(b)) is characteristic of many bacterial pathogens, both Gram-positive and Gram-negative (e.g., Brucella, Bordetella, and Campylobacter spp. from α-, β-, and δ-Proteobacteria, Borrelia, and Treponema spp. from Spirochaetes). Most of the genomes in this group contain both pncA and pncB genes that are often clustered on the chromosome and/or are co-regulated (see Section 7.08.3.1.2). In some cases (e.g., within Mollicutes and Spirochaetales), only the pncB, but not the pncA gene, can be reliably identified, suggesting that either of these species can utilize only the deamidated form of niacin (Na) or that some of them contain an alternative (yet-unknown) NMASE.

Although the nondeamidating conversion of Nm into NMN (via NMPRT) appears to be present in approximately 50 bacterial species (mostly in β- and γ-Proteobacteria), it is hardly ever the only route of NAD biogenesis in these organisms. The only possible exception is observed in Mycoplasma genitalium and M. pneumoniae that contain the nadV gene as the only component of pyridine mononucleotide biosynthetic machinery. In some species (e.g., in Synechocystes spp.), the NMPRT–NMNAT route is committed primarily to the recycling of endogenous Nm. On the other hand, in F. tularensis (Figure 4(c)), NMPRT (gene nadV) together with NMNAT (of the nadM family) constitute the functional nondeamidating Nm salvage pathway as it supports the growth of the nadE′-mutant on Nm but not on Na (L. Sorci et al., unpublished). A similar nondeamidating Nm salvage pathway implemented by NMPRT and NMNAT (of the nadR family) is present in some (but not all) species of Pasteurellaceae in addition to (but never instead of) the RNm salvage pathway (see below), as initially demonstrated for H. ducreyi.128

A two-step conversion of NaMN into NAD via a NaAD intermediate (Route I in Figure 2) is present in the overwhelming majority of bacteria. The signature enzyme of Route I, NAMNAT of the NadD family is present in nearly all approximately 650 bacterial species that are expected to generate NaMN via de novo or salvage pathways (as illustrated by Figures 3(a) and 3(b)). All these species, without a single exception, also contain NADSYN (encoded by either a short or a long form of the nadE gene), which is required for this route. The species that lack the NadD/NadE signature represent several relatively rare functional variants, including:

1.
Route I of NAD synthesis (NaMN → NaAD → NAD) variant via a bifunctional NAMNAT/NMNAT enzyme of the NadM family is common for archaea (see Section 7.08.3.2), but it appears to be present in only a handful of bacteria, such as Acinetobacter, Deinococcus, and Thermus groups. Another unusual feature of the latter two groups is the absence of the classical NADKIN, a likely subject of a nonorthologous replacement that remains to be elucidated.

2.
Route II of NAD synthesis (NaMN → NMN → NAD). This route is implemented by a combination of the NMNAT of either the NadM family (as in F. tularensis) or the NadR family (as in M. succinoproducens and A. succinogenes) with NMNSYN of the NadE′ family. The case of F. tularensis described in Section 7.08.2.4 is illustrated in Figure 3(b). The rest of the NAD biosynthetic machinery in both species from the Pasteurellaceae group, beyond the shared Route II, is remarkably different from that in F. tularensis. Instead of de novo biosynthesis, they harbor a Na salvage pathway via NAPRT encoded by a pncB gene that is present in a chromosomal cluster with nadE′. Neither of these two genes are present in other Pasteurellaceae that lack the pyridine carboxylate amidation machinery (see below).

3.
Salvage of RNm (RNm → NMN → NAD). A genomic signature of this pathway, a combination of the PnuC-like transporter and a bifunctional NMNAT/RNMKIN of the NadR family, is present in many Enterobacteriaceae and in several other diverse species (e.g., in M. tuberculosis). However, in H. influenzae (Figure 3(d)) and related members of Pasteurellaceae, it is the only route of NAD biogenesis. As shown in Table 1, H. influenzae as well as many other members of this group have lost nearly all components of the rich NAD biosynthetic machinery that are present in their close phylogenetic neighbors (such as E. coli and many other Enterobacteriaceae). This pathway is an ultimate route for utilization of the so called V-factors (NADP, NAD, NMN, or RNm) that are required to support growth of H. influenzae. It was established that all other V-factors are degraded to RNm by a combination of periplasmic- and membrane-associated hydrolytic enzymes.222 Although PnuC was initially considered an NMN transporter,223 its recent detailed analysis in both H. influenzae and Salmonella confirmed that its actual physiological function is in the uptake of RNm coupled with the phosphorylation of RNM to NMN by RNMKIN.17,148,224 As already mentioned, H. ducreyi and several other V-factor-independent members of the Pasteurellaceae group (H. somnus, Actinobacillus pleuropneumoniae, and Actinomycetemcomitans) harbor the NMNAT enzyme (NadV) that allows them to grow in the presence of Nm (but not Na) in the medium (Section 7.08.2.2).

4.
Uptake of the intact NAD. Several groups of phylogenetically distant intracellular endosymbionts with extremely truncated genomes contain only a single enzyme, NADKIN, from the entire subsystem. Among them are all analyzed species of the Wolbachia, Rickettsia, and Blochmannia groups. These species are expected to uptake and utilize the intact NAD from their host while retaining the ability to convert it into NADP. Among all analyzed bacteria, only the group of Chlamydia does not have NADKIN and depends on the salvage of both NAD and NADP via a unique uptake system.157

A comprehensive genomic reconstruction of the metabolic potential (gene annotations and asserted pathways) across approximately 680 diverse bacterial genomes sets the stage for the accurate cross-genome projection and prediction of regulatory mechanisms that control the realization of this potential in a variety of species and growth conditions. In the next section, we summarize the recent accomplishments in the genomic reconstruction of NAD-related regulons in bacteria.

Nicotinic acid mononucleotide. CAS Common Chemistry. CAS, a division of the American Chemical Society, n.d. https://commonchemistry.cas.org/detail?cas_rn=321-02-8 (retrieved 2024-06-29) (CAS RN: 321-02-8). Licensed under the Attribution-Noncommercial 4.0 International License (CC BY-NC 4.0).

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