in response to a dynamic nutrient environment , cells must assess their metabolic state to decide whether to grow , survive , or die .
it has become evident that metabolites themselves must feed back to regulate gene expression , signal transduction , and various protein activities in cellular decision - making processes .
these small molecule metabolites play critical roles in relaying metabolic information to their protein and nucleic acid counterparts . however , despite increased recognition of such reciprocal interplay , many aspects of the mechanisms through which metabolites exert their influence on cellular regulatory mechanisms are still being unraveled . amongst the thousands of metabolites present in the cellular milieu at any given time , which might represent the  sentinel
one well - known signature of metabolic state is amp , which indicates cellular energy charge and accumulates upon atp insufficiency .
amp regulates the activity of the amp - activated protein kinase ( ampk ) , which phosphorylates many proteins involved in cellular energy homeostasis .
another example is nad , which indicates the cellular redox status as a ratio of nad to nadh .
herein , we discuss the hypothesis that acetyl - coa represents an additional prominent gauge of the cell s metabolic state with substantial influence on numerous biological regulatory mechanisms .
acetyl - coa is a metabolite derived from glucose , fatty acid , and amino acid catabolism . during glycolysis , glucose is broken down into two three - carbon molecules of pyruvate .
the mitochondrial pyruvate dehydrogenase complex then catalyzes the oxidative decarboxylation of pyruvate to produce acetyl - coa , a two - carbon acetyl unit that is ligated to the acyl - group carrier , coa . in the mitochondria ,
citrate synthase then catalyzes the condensation of the acetyl moiety of acetyl - coa with oxaloacetate to yield a six - carbon citrate molecule .
citrate can proceed to be oxidized via the tca cycle , or alternatively it can be transported to the cytosol as a substrate for the enzyme atp citrate lyase , which cleaves citrate to regenerate acetyl - coa and oxaloacetate   ( fig .
the function of this pathway is to direct acetyl - coa away from the mitochondria and back to the cytosol for the synthesis of fatty acids and sterols . as such , cells can store excess carbohydrates as fat .
thus , the function of the atp citrate lyase enzyme offers a clue to the logic and direction of carbon flow 
acetyl - coa units are shipped out of the mitochondria in the form of citrate when carbon sources are abundant , indicating a favorable nutrient state .
nucleocytosolic pools of acetyl - coa are also utilized for histone acetylation and the activation of gene expression .
atp citrate lyase was shown to provide a source of acetyl - coa for histone acetylation in mammalian cells .
the budding yeast saccharomyces cerevisiae , which lacks atp citrate lyase , relies on acetyl - coa synthetase enzymes to supply acetyl - coa for histone acetylation .
moreover , a special cohort of yeast genes important for growth , such as those required for ribosome biogenesis and the g1 cyclin cln3 , are especially dependent on histone acetylation for their activation . as such , the expression of these growth genes
is closely coupled to acetyl - coa as an indicator of the cell s nutritional state .
thus , when carbon sources are abundant , nucleocytosolic amounts of acetyl - coa accumulate and facilitate the processes of lipid synthesis and histone acetylation ( fig .
during starvation , cells must typically shift from growth to survival mode and alter metabolism towards functions important for viability . instead of shipping acetyl units out to the cytosol
, there is now a greater requirement for acetyl - coa to be oxidized in the mitochondria for atp synthesis ( fig .
coa synthesis is induced to activate fatty acids as fatty acyl - coas , which can then be transported into mitochondria via the carnitine shuttle for -oxidation . as a result
, acetyl - coa is generated in the mitochondria for oxidation or other possible fates . in the liver , mitochondrial acetyl - coa
is used to synthesize ketone bodies ( acetoacetate and -hydroxybutyrate ) as alternative fuel sources for the brain and heart under conditions of carbohydrate scarcity . under such conditions ,
lower nucleocytosolic acetyl - coa will also limit fatty acid synthesis , histone acetylation , and other growth - related processes .
atp citrate lyase is inhibited under these situations at both the transcriptional and post - translational levels .
depletion of nucleocytosolic acetyl - coa also represents a cue to induce autophagy . in yeast , the expression of a core autophagy gene ( atg7 ) is repressed by acetyl - coa . during the yeast metabolic cycle ( ymc ) , many atg genes ( including atg1 and atg8 ) are repressed during growth phases when acetyl - coa levels rise , and are activated only during the stress / survival phases when acetyl - coa levels fall . more generally , many other genes with functions involved in stress and survival are induced concomitantly with core autophagy genes .
these genes tend to be less dependent on histone acetylation for their activation , perhaps due to reduced availability of acetyl - coa . in mammalian cells ,
autophagy regulation by acetyl - coa occurs in a manner dependent on the p300 acetyltransferase .
thus , the regulation of autophagy by acetyl - coa may occur primarily at the level of transcriptional control of core autophagy genes .
taken together , under fasted or carbon - poor states , nucleocytosolic amounts of acetyl - coa decrease in cells , while mechanisms to channel acetyl - coa into the mitochondria are engaged .
these considerations support a model in which the subcellular compartmentalization of acetyl - coa units undergoes a major shift during starvation , and the utilization of these acetyl units is re - purposed to support survival strategies ( fig .
it is perhaps no coincidence that acetyl - coa doubles as the acetyl donor for protein acetylation modifications ( including histone acetylation ) ( fig .
the abundance of protein acetylation modifications could therefore reflect the cell s metabolic state to regulate various protein activities .
studies performed under carbon - rich conditions where acetyl - coa synthesis is not limiting may mask the contributions of this metabolite in cellular regulation . however , most organisms , as well as particular tissue microenvironments in vivo experience challenges in the nutrient environment that might limit acetyl - coa biosynthesis or availability ( e.g. , carbon starvation or hypoxia ) .
recent studies have begun to provide compelling evidence that many protein acetylation modifications are indeed modulated by acetyl - coa availability . besides histones , the acetyl - coa synthetase family of enzymes was also identified to be regulated by reversible acetylation [ 2931 ] .
the acetylation of an active site lysine residue was observed to inhibit the activity of acetyl - coa synthetase as a mechanism of feedback inhibition in response to high acetyl - coa [ 3234 ] .
the deacetylation of these enzymes , catalyzed by sirtuins , restores their activity [ 3234 ] .
subsequent mass spectrometry surveys have now revealed that thousands of other proteins , including many other metabolic enzymes , can be acetylated [ 3538 ] . in some cases ,
although the majority of these modifications were found to be inhibitory , several were reported to be activating . in some instances ,
the acetylation of particular metabolic enzymes was responsive to glucose levels in the media , suggesting that they could be linked to intracellular acetyl - coa abundance .
whether specific acetyltransferase enzymes catalyze the majority of these acetylation modifications present on metabolic enzymes is not yet clear .
the yeast metabolic cycle ( ymc ) offers a system to investigate whether particular acetylation modifications might be coupled to acetyl - coa itself .
studies of yeast cells undergoing the ymc during continuous , glucose - limited growth in a chemostat have revealed periodic changes in intracellular acetyl - coa amounts as yeast cells alternate between growth and quiescent - like phases .
several proteins are dynamically acetylated precisely in phase with the observed acetyl - coa oscillations .
these include histones , several components of the transcriptional coactivator saga , a subunit of the swi / snf chromatin remodeling complex snf2p , and a transcriptional coactivator of ribosomal subunit gene expression ifh1p .
interestingly , the dynamic acetylation of all of these proteins is dependent on the acetyltransferase gcn5p , suggesting this enzyme has the capability of acetylating its substrates in tune with acetyl - coa fluctuations in vivo .
consistent with this hypothesis , mutations within gcn5p slow growth , disrupt the yeast metabolic cycle , or alter the cell s responsiveness to acetate .
moreover , acetylation of saga subunits appears to aid its recruitment to growth genes . a brief survey of other acetylated proteins that are not known to be gcn5p substrates showed they are not dynamically acetylated across the ymc .
an analysis of the genomic regions bound by these acetylated histones revealed that several marks , in particular h3k9ac , were present predominantly at growth genes , specifically during the growth phase of the ymc when acetyl - coa levels rise .
these considerations suggest that the acetylation of these nuclear - localized proteins collectively functions to promote the activation of growth genes in response to a burst of nucleocytosolic acetyl - coa .
given the thousands of newly identified acetylated proteins , a pertinent question is what proportion of each protein is acetylated ?
recent studies aiming to determine the stoichiometry of acetylated sites estimate that for many proteins , only a small fraction of the peptides are actually acetylated .
however , nuclear proteins , including histones and transcription factors , were estimated to be acetylated at much higher stoichiometry .
conventional shotgun detection of peptides by mass spectrometry is biased towards abundant proteins , so perhaps it is unsurprising that a small fraction of a very abundant protein that is acetylated could be scored as a positive .
moreover , lysine residues on proteins can react spontaneously with thioesters such as acetyl - coa or other acyl - coa metabolites , resulting in non - enzymatic acetylation or acylation [ 4448 ] .
non - enzymatic acetylation or acylation may be especially prominent within the mitochondria , which is thought to have higher acetyl - coa concentrations and higher ph , thereby increasing the nucleophilicity of lysyl side chains .
thus , while some non - enzymatic acetylation or acylation events could have evolved to be regulatory , the possibility also exists that many of these modifications could be spurious .
these considerations must be taken into account when determining the physiological significance of any detected acetylation site . moreover , there are limitations to mutation of a lysine residue to either arginine or glutamine .
these mutations are not always accurate acetylated or deacetylated lysine mimics , and could perturb protein function independent of site - specific acetylation . as such , it can be challenging to demonstrate whether a particular acetylation modification is functionally important in vivo . to help address these issues , methods for site - specific incorporation of acetyllysine , as well as
the use of these and other methods will help clarify the extent through which protein acetylation modifications are responsive to acetyl - coa fluctuations in a regulatory manner , either enzymatically or non - enzymatically .
the accumulation of acetyl - coa in subcellular compartments may also necessitate the activity of deacetylase enzymes to remove non - enzymatic acetylation modifications that could intentionally or unintentionally compromise protein function . such a  repair  or  detoxification 
consistent with this idea , hyperacetylation of mitochondrial enzymes occurs in the absence of mitochondrial sirt3 [ 5557 ] , and deacetylation of these enzymes typically increases their activity .
moreover , the expression of sirt3 is increased specifically under fasting states , in response to high - fat diets , or during exercise - conditions that all promote increased mitochondrial acetyl - coa .
likewise , the potential of proteins to be modified by other acyl - coa metabolites besides acetyl - coa is supported by the discovery of a wide variety of acylation modifications present on proteins , along with associated sirtuins that preferentially catalyze their removal [ 5861 ] .
evidence that sirtuins evolved specifically to remove non - enzymatic protein acylation as a form of protein quality control has been summarized in a recent review . in this model ,
failure of sirtuins to remove aberrant acylation modifications would hinder the function of effected proteins and consequently lead to dysfunctions in metabolism and susceptibility to disease .
however , deacylation of a lysine residue can also be executed using water as a nucleophile without a requirement for nad , a mechanism employed by many histone deacetylases ( hdacs ) .
the dependency of sirtuins on nad have led to the hypothesis that their activity could be regulated by fluctuations in nad concentrations . however
, such dependency on nad may serve an additional purpose and enable the removal of the acyl group via covalent attachment to adp - ribose , to produce o - acyl - adp - ribose metabolites , which themselves may have biological functions . moreover , if the acyl group were liberated as a free carboxylate , then the respective acyl - coa synthetase enzymes could potentially convert these free carboxylates back to acyl - coa metabolites , facilitating re - acylation and thus leading to a futile cycle ( fig .
in summary , there is now compelling evidence that acetyl - coa represents a fundamental gauge of cellular metabolic state that is monitored by the cell by way of distinctive protein acetylation modifications .
perhaps it is no coincidence that nature chose to carefully monitor the abundance of acetyl - coa as a proxy for its metabolic state due to its requirement in the biosynthesis of cellular building blocks , two carbons at a time .
some of these acetyl - coa - responsive modifications may be established by a delicate balance between the opposing activities of acetyltransferase and deacetylase enzymes , while others could be set in a non - enzymatic manner .
understanding the sources and fates , as well as the movement of acetyl groups between subcellular compartments reveals an underlying logic to metabolic strategies employed under growth versus survival , fed versus fasted , or normal versus tumorigenic metabolic states .
based on recent stoichiometry studies , it is tempting to speculate that acetylation originally evolved as a means to link nuclear activities with acetyl - coa equivalents produced in the mitochondria
. however , a consequence of using electrophilic acyl - coa metabolites in cellular metabolism and regulation is their tendency to react spontaneously with nucleophilic moieties on proteins such as lysine residues . as such
, the accumulation of particular acyl - coa metabolites in various cellular compartments may have necessitated a mechanism to remove unintended acylation modifications on proteins .
future studies will continue to reveal the mechanisms and consequences of employing acetyl - coa and other acyl - coas in cellular metabolism - their reciprocal influence on metabolism and cell regulation should no longer be overlooked .
 acetyl - coa represents a key node in metabolism due to its intersection with many metabolic pathways and transformations . 
 emerging evidence reveals that cells monitor the levels of acetyl - coa as a key indicator of their metabolic state , through distinctive protein acetylation modifications dependent on this metabolite . 
 we offer the following conceptual model for understanding the role of this sentinel metabolite in metabolic regulation . 
 high nucleocytosolic acetyl - coa amounts are a signature of a  growth  or  fed  state and promote its utilization for lipid synthesis and histone acetylation . 
 in contrast , under  survival  or 
  fasted  states , acetyl - coa is preferentially directed into the mitochondria to promote mitochondrial - dependent activities such as the synthesis of atp and ketone bodies . 
 fluctuations in acetyl - coa within these subcellular compartments enable the substrate - level regulation of acetylation modifications , but also necessitates the function of sirtuin deacetylases to catalyze removal of spontaneous modifications that might be unintended . 
 thus , understanding the sources , fates , and consequences of acetyl - coa as a carrier of two - carbon units has started to reveal its underappreciated but profound influence on the regulation of numerous life processes . 
Introduction
Growth or Fed State - High acetyl-CoA in cytosol/nucleus
Survival or Fasted State - High acetyl-CoA in mitochondria
Sensing of acetyl-CoA through protein acetylation modifications
Stoichiometry of acetylation modifications
Implications for sirtuin function
Summary and perspective