The structure of NMN AT closely resembles that of phosphopantetheine adenylyltransferase. Remarkably, in spite of the fact that the two enzymes share the same fold and hexameric assembly, a striking difference in their quaternary structure is observed. Moreover, on the basis of structural similarity including the HXGH motif, we identify NMN AT as a novel member of the newly proposed superfamily of nucleotidyltransferase alpha/beta phosphodiesterases. Our structural data suggest that the catalytic mechanism does not rely on the direct involvement of any protein residues and is likely to be carried out through optimal positioning of substrates and transition-state stabilisation, as is proposed for other members of the nucleotidyltransferase alpha/beta phosphodiesterase superfamily.

Synechocystis sp. slr0787 open reading frame encodes a 339 residue polypeptide with a predicted molecular mass of 38.5 kDa. Its deduced amino acid sequence shows extensive homology with known separate sequences of proteins from the thermophilic archaeon Methanococcus jannaschii. The N-terminal domain is highly homologous to the archaeal NMN adenylyltransferase, which catalyzes NAD synthesis from NMN and ATP. The C-terminal domain shares homology with the archaeal ADP-ribose pyrophosphatase, a member of thè Nudix' hydrolase family. The slr0787 gene has been cloned into a T7-based vector for expression in Escherichia coli cells. The recombinant protein has been purified to homogeneity and demonstrated to possess both NMN adenylyltransferase and ADP-ribose pyrophosphatase activities. Both activities have been characterized and compared to their archaeal counterparts.z 1999 Federation of European Biochemical Societies.

Sarcopenia and frailty are associated with several important health-related adverse events, including disability, loss of independence, institutionalization and mortality. Sarcopenia can be considered a biological substrate of frailty, and the prevalence of both these conditions progressively increases with age. Telomeres are nucleoprotein structures located at the end of linear chromosomes and implicated in cellular ageing, shorten with age, and are associated with various age-related diseases. In addition, telomere length (TL) is widely considered a molecular/cellular hallmark of the ageing process. This narrative review summarizes the knowledge about telomeres and analyzes for the first time a possible association of TL with sarcopenia and frailty. The overview provided by the present review suggests that leukocyte TL as single measurement, calculated by quantitative real-time polymerase chain reaction (qRT-PCR), cannot be considered a meaningful biological marker for complex, multidimensional age-related conditions, such as sarcopenia and frailty. Panels of biomarkers, including TL, may provide more accurate assessment and prediction of outcomes in these geriatric syndromes in elderly people.

The enzyme nicotinamide mononucleotide (NMN) adenylyltransferase (EC 2.7.7.1) catalyzes the synthesis of NAD ؉ and nicotinic acid adenine dinucleotide. It has been purified to homogeneity from cellular extracts of the thermophilic archaeon Sulfolobus solfataricus. Through a database search, a highly significant match was found between its N-terminal sequence and a hypothetical protein coded by the thermophilic archaeon Methanococcus jannaschii MJ0541 open reading frame (GenBank accession no. U67503). The MJ0541 gene was isolated, cloned into a T7-based vector, and expressed in Escherichia coli cells, yielding a high level of thermophilic NMN adenylyltransferase activity. The expressed protein was purified to homogeneity by a single-step chromatographic procedure. Both the subunit molecular mass and the N-terminal sequence of the pure recombinant protein were as expected from the deduced amino acid sequence of the MJ0541 open reading frame-encoded protein. Molecular and kinetic properties of the enzymes from both archaea are reported and compared with those already known for the mesophilic eukaryotic NMN adenylyltransferase. NADϩ synthesis can be accomplished either via de novo pathways or through preformed pyridine ring salvage routes (13). All such pathways converge to the reaction nicotinamide mononucleotide (NMN) (or nicotinic acid mononucleotide) ϩ ATP 7 NAD ϩ (or nicotinic acid adenine dinucleotide) ϩ PP i , which is catalyzed by the enzyme NMN adenylyltransferase (EC 2.7.7.1). Interestingly, this is the only enzyme in the biosynthetic pathway to be located in the cell nucleus (15). Numerous examples of a fluctuation of NMN adenylyltransferase activity during the DNA synthesis phase of the cell cycle have been reported (14,23,32). More recently, it has been proposed that the nuclear localization of the enzyme could be related to the consistent demand for NAD ϩ as a substrate for nuclear poly(ADP) ribosylation reactions (29, 34), thus suggesting a major role for the enzyme in cellular metabolism. In prokaryotes, the NMN adenylyltransferase gene, designated nadD, was mapped and demonstrated to be essential for viability (16). However it remains one of the few genes involved in the NAD ϩ biosynthetic pathway not to have been cloned and sequenced from any organism. The enzyme has been identified and characterized in its catalytic properties from several prokaryotic sources (6,10,26). It has been purified to homogeneity and extensively characterized by us from yeast, bull testis, and human placenta (4, 12, 24), but its instability and relatively low concentration in cell extracts precluded obtaining enzyme preparations suitable for gene isolation. We have previously reported the presence of NMN adenylyltransferase in the thermophilic archaeon Sulfolobus solfataricus (28). Purification to homogeneity of the S. solfataricus enzyme and determination of its N-terminal sequence allowed us to recognize the MJ0541 open reading frame (ORF) from the Methanococcus jannaschii genome sequence as the NMN adenylyltransferase ...

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