Sirtuins use NAD+ as a cosubstrate to remove acetyl moieties from lysines on histones and proteins, releasing NAM and O-acetyl-ADP-ribose (Houtkooper et al., 2010a). The consumption of NAD+ during deacetylation is what separates sirtuins, as type III lysine deactylases (KDACs), from type I, II, and IV KDACs. SIRT1, SIRT2, and SIRT3 have strong deacetylase activity (Imai et al., 2000, North et al., 2003, Schwer et al., 2002, Vaziri et al., 2001), while SIRT4, SIRT5, and SIRT6 are weak in comparison. However, deacetylation is not the singular function of all sirtuins, as SIRT4 can act as a lipoamidase (Mathias et al., 2014) and, along with SIRT6, as an NAD+-dependent mono-ADP-ribosyltransferase (Haigis et al., 2006, Liszt et al., 2005). SIRT6 can also efficiently remove long-chain fatty acyl groups from lysine residues (Jiang et al., 2013), while SIRT5 has strong desuccinylase, demalonylase, and deglutarylase enzymatic activities (Du et al., 2011, Tan et al., 2014). Finally, SIRT7 is an NAD+-dependent deacetylase with few known substrates, including p53 in vitro (Vakhrusheva et al., 2008), PAF53 in HeLa cells (Chen et al., 2013), and GABPβ1 in vivo (Ryu et al., 2014). Recent evidence indicates that long-chain deacylation is a general feature of all mammalian sirtuins (Feldman et al., 2013). For example, SIRT1, SIRT2, and SIRT3 can also act as effective decrotonylases (Bao et al., 2014, Feldman et al., 2013). The general deacylase activity of sirtuins, however, can differ in their preferential activity toward certain acyl chain lengths (Feldman et al., 2013).
Physiological Roles of Sirtuins
Generally, most sirtuins are activated during times of energy deficit and reduced carbohydrate energy sources, triggering cellular adaptations that improve metabolic efficiency (Figure 3A). For example, SIRT1 activity increases during exercise (Cantó et al., 2009), CR (Chen et al., 2008), fasting (Cantó et al., 2010, Rodgers et al., 2005), or low glucose availability (Fulco et al., 2008), all of which correlate with higher NAD+ levels. Due to space limitations, we will only briefly summarize the roles of sirtuins. For further information, we refer the reader to recent specialized reviews (Boutant and Canto, 2014, Chang and Guarente, 2013, Houtkooper et al., 2012).
Different nuclear sirtuin orthologs have been shown to influence lifespan in yeast, worms, flies, and mice (Bauer et al., 2009, Boily et al., 2008, Kaeberlein et al., 1999, Kanfi et al., 2012, Rogina and Helfand, 2004, Tissenbaum and Guarente, 2001). Accordingly, the deacetylation of transcription factors, cofactors, and histones by SIRT1 was shown to be important to enhance mitochondrial metabolism (Boily et al., 2008, Cantó et al., 2009, Cantó et al., 2010, Feige et al., 2008, Menzies et al., 2013, Price et al., 2012, Rodgers et al., 2005). Further evidence indicates that SIRT1 is key to linking nutrients to circadian rhythm (Asher et al., 2008, Chang and Guarente, 2013, Nakahata et al., 2008). The tight link between sirtuins and metabolism was reinforced by findings indicating that a moderate SIRT1 overexpression in mice could prevent metabolic and age-related complications, including insulin resistance, obesity, and hepatic steatosis (Banks et al., 2008, Herranz et al., 2010, Pfluger et al., 2008). In addition, pharmacological SIRT1 activation protects against lifespan reductions prompted by HFDs (Baur et al., 2006, Minor et al., 2011). Similarly, SIRT6 overexpression has been shown to increase mouse lifespan (Kanfi et al., 2012). Oppositely, loss-of-function models for SIRT1, SIRT3, and SIRT7 have been linked to a higher susceptibility to metabolic and age-related disease or reduced maximal lifespan (Boutant and Canto, 2014, Hirschey et al., 2011, Ryu et al., 2014, Vakhrusheva et al., 2008), while the absence of SIRT6 causes severe hypoglycemia, leading to mortality within the first month of life (Mostoslavsky et al., 2006, Zhong et al., 2010). Sirt2- and Sirt5-deficient mice, however, do not display an overt metabolic phenotype in the basal state (Beirowski et al., 2011, Bobrowska et al., 2012, Yu et al., 2013), while Sirt4 deficiency, in contrast to most sirtuins, enhances oxidative metabolism (Laurent et al., 2013).
Sirtuins as NAD+ Sensors
Their ability to use NAD+ as a substrate led to speculation that sirtuins could act as metabolic sensors. The activity of sirtuins for a given intracellular NAD+ level is defined by the Michaelis constant, Κm, for the reaction. This constant describes the NAD+ concentration when the reaction rate is half of the maximum during NAD+ excess. The estimated total intracellular content of NAD+ in mammals ranges from ∼200 to ∼500 μM (Bai et al., 2011b, Hong et al., 2014, Houtkooper et al., 2010a, Schmidt et al., 2004). The Κm of SIRT1 for NAD+ has been reported to be in the range of 94–96 μM in mammals (Table 1) (Gerhart-Hines et al., 2011, Pacholec et al., 2010). The Km for NAD+, however, can differ very significantly between sirtuins. For example, the Κm for NAD+ of SIRT2, SIRT3, SIRT4, SIRT5, and SIRT6 are reported as 83 μM (Borra et al., 2004), 880 μM (Hirschey et al., 2011), 35 μM (Laurent et al., 2013), 980 μM (Fischer et al., 2012), and 26 μM (Pan et al., 2011), respectively. The affinity of SIRT7 for NAD+ has not been reported to our knowledge. The above numbers help to classify sirtuins into two different categories. First, there are sirtuins, such as SIRT2, SIRT4, and SIRT6, whose activity is unlikely to be rate-limited by NAD+, as NAD+ availability is considerably higher than their Κm values. In contrast, there are other sirtuins, such as SIRT1, SIRT3, and SIRT5, whose Km for NAD+ falls within the range for physiological changes in NAD+. In this respect, it is important to note that SIRT1 is a nuclear enzyme, and NAD+ concentrations in the nucleus are below 100 μM, while NAD+ levels in the mitochondria can reach millimolar values, suggesting that NAD+ could limit SIRT3 and SIRT5 based on their Km values.