Start a live chat
1-800-870-4800
Sirtuins — the proteins
at the center of how cells
maintain themselves across time.
Every cell in the body carries a set of proteins whose job is cellular upkeep — regulating which genes are read, coordinating the response to damage, maintaining the metabolic systems that keep the cell functional. Sirtuins are among the most studied of these proteins. They are also NAD+-dependent — which is why the story of sirtuins and the story of NMN are, at their core, the same story.
I
The cellular maintenance system —
what sirtuins actually do.
A cell is not a passive object. It is a dynamic system that continuously monitors its own state, responds to damage, adjusts its gene expression to match its environment, and maintains the metabolic networks that keep it alive and functional. The proteins that coordinate this ongoing maintenance work are among the most consequential molecules in biology — and sirtuins, a family of seven proteins found in every mammalian cell, are among the most studied of them.
Sirtuins are classified as deacylases — enzymes that remove chemical tags from proteins, changing those proteins' behavior in the process. The proteins they target are not random. Sirtuins act on histones — the structural proteins around which DNA is wound — changing how tightly DNA is packaged and therefore which genes can be read. They act on metabolic enzymes, switching their activity on or off depending on the cell's energy state. They act on proteins involved in DNA repair, the stress response, and mitochondrial function. In each case, the sirtuin is not doing the downstream work itself — it is regulating the proteins that do, adjusting the cell's maintenance systems in response to what is happening around them.
What makes sirtuins particularly relevant to aging biology is a specific feature of how they work: they require NAD+ as a co-substrate to perform their deacylase activity. This is not a minor chemical detail. It means that sirtuin function is directly coupled to NAD+ availability — when NAD+ is abundant, sirtuins can operate at full capacity; when NAD+ declines, sirtuin activity is constrained by the availability of the substrate they consume. And as the previous articles in this series have documented, NAD+ declines substantially and systematically as the body ages.
Sirtuins do not work alone.
They work with NAD+.
Which means that when NAD+ falls,
the cell's maintenance system
operates on a reduced budget.
The Sirtuin Family
Seven proteins. Each one
NAD+-dependent. Each one distinct.
The seven mammalian sirtuins are not interchangeable. Each has a distinct cellular location, a distinct set of target proteins, and a distinct biological role. What they share is their dependency on NAD+ — and their collective involvement in the cellular processes most associated with how biology ages.
SIRT1
The master regulator
Nucleus & Cytoplasm
The most studied sirtuin. Deacetylates histones and a wide range of transcription factors — including p53, NF-κB, and PGC-1α — governing gene expression, the stress response, inflammation signaling, and mitochondrial biogenesis. SIRT1 is the primary link between NAD+ availability and the broad landscape of gene regulatory responses that change with age.
NAD+ dependency: consumes one molecule of NAD+ per deacetylation reaction — directly proportional to NAD+ availability
SIRT2
The cytoskeletal regulator
Cytoplasm (primarily)
Acts predominantly in the cytoplasm, deacetylating tubulin — a structural protein of the cell's internal scaffold — and several proteins involved in cell division and the cell cycle. SIRT2 has been studied in the context of neurodegeneration and cell cycle regulation, where its activity in aged cells has attracted interest from researchers working on cellular integrity over time.
NAD+ dependency: cytoplasmic NAD+ pool availability governs SIRT2 activity — a pool supplied by NMNAT2 in neurons
SIRT3
The mitochondrial overseer
Mitochondria (primary)
The dominant deacetylase of the mitochondrial matrix. SIRT3 regulates the activity of multiple mitochondrial enzymes involved in energy metabolism, fatty acid oxidation, and the management of reactive oxygen species. Its activity is closely tied to mitochondrial health — and the age-related decline of SIRT3 activity has been associated with the mitochondrial dysfunction that is one of the recognized hallmarks of cellular aging.
NAD+ dependency: draws on the mitochondrial NAD+ pool — supplied by NMNAT3, distinct from the nuclear and cytoplasmic pools
SIRT4
The metabolic gatekeeper
Mitochondria
SIRT4 operates primarily as a mitochondrial ADP-ribosyltransferase and deacylase, with roles in amino acid metabolism and the regulation of insulin secretion. It acts as a negative regulator of mitochondrial glutamate metabolism and has been studied in the context of metabolic aging and the cellular response to nutrient availability. Its biology is less fully characterized than SIRT1 or SIRT3, and understanding of its specific roles continues to develop.
NAD+ dependency: mitochondrial pool — activity shaped by the same mitochondrial NAD+ dynamics that govern SIRT3
SIRT5
The chemical tag specialist
Mitochondria / Cytoplasm
SIRT5 has weak deacetylase activity but strong desuccinylase and demalonylase activity — removing succinyl and malonyl groups from proteins. These modifications are distinct from the acetylation that most sirtuins target, and SIRT5's substrates include enzymes in the urea cycle, fatty acid oxidation, and energy metabolism. Its biology represents one of the more recently characterized dimensions of the sirtuin family's regulatory scope.
NAD+ dependency: requires NAD+ as a co-substrate for its desuccinylase and demalonylase reactions
SIRT6
The genome guardian
Nucleus
SIRT6 deacetylates specific histone residues at DNA damage sites and telomeres, playing a direct role in DNA double-strand break repair and telomere maintenance. It also regulates glucose and lipid metabolism through its effects on gene expression. SIRT6 has been described as a critical regulator of genomic stability — and its activity in maintaining the structural integrity of chromosomes over time places it at the intersection of DNA repair, metabolic regulation, and aging biology.
NAD+ dependency: nuclear NAD+ pool — same compartment as SIRT1, supplied by NMNAT1
SIRT7
The ribosomal regulator
Nucleus (nucleolus)
SIRT7 localizes primarily to the nucleolus — the nuclear subcompartment where ribosomal RNA is transcribed. It deacetylates histone H3K18, a modification associated with the transcriptional regulation of stress response genes. SIRT7 has been studied in the context of cellular stress responses, DNA repair coordination, and the regulation of protein synthesis capacity — an area of growing interest in aging biology as protein quality control declines with age.
NAD+ dependency: nucleolar NAD+ availability — subject to the same nuclear pool dynamics as SIRT1 and SIRT6
II
Why NAD+ dependency
is the defining feature of sirtuin biology in aging.
The NAD+ dependency of sirtuins is not simply a biochemical footnote. It is the mechanism that connects the molecular biology of cellular maintenance to the biology of aging — and it does so through a logical chain that, once understood, makes the relationship between NAD+ decline and the broad landscape of age-related cellular change considerably less mysterious.
Sirtuins consume NAD+ as they perform their deacylase reactions. This means their activity is not just enabled by NAD+ — it is limited by it. In a cell where NAD+ is abundant, sirtuins can respond to damage signals, adjust gene expression, support mitochondrial biogenesis, and coordinate repair processes at the rate those processes require. In a cell where NAD+ has declined — as it does in every tissue, in every person, across the middle and later decades of life — the same sirtuins are operating with less substrate available, and their collective output across all seven family members is correspondingly reduced.
The breadth of sirtuin biology — spanning gene regulation, DNA repair, mitochondrial function, metabolic control, telomere maintenance, and the stress response — means that this constraint is not narrow. It touches virtually every domain of cellular maintenance that aging biology has identified as declining with age. This is why the connection between NAD+ decline and the aging cell is not a single-pathway story. It is a systems-level story, with sirtuins as the molecular mechanism through which NAD+ availability translates into cellular maintenance capacity across the full scope of what that means.
The Scope of Sirtuin Biology
What the seven sirtuins collectively
govern inside the cell.
Domain 01
Gene expression and chromatin regulation
SIRT1, SIRT6, and SIRT7 all act on histones — the proteins that determine how tightly DNA is packaged and which genes are accessible for transcription. By deacetylating specific histone residues, these sirtuins regulate which genes can be read in response to cellular signals, nutrient status, and stress. The age-related dysregulation of gene expression patterns — one of the hallmarks of cellular aging — is connected to declining sirtuin activity in ways that continue to be characterized in the aging biology literature.
Domain 02
DNA repair and genomic integrity
SIRT1 and SIRT6 both play roles in DNA double-strand break repair — the response to the most severe form of DNA damage. SIRT6 specifically localizes to damage sites and telomeres, where it deacetylates histones to make the chromatin accessible for repair machinery. The accumulation of unrepaired DNA damage across decades of aging is one of the central biological processes of cellular deterioration, and sirtuin involvement in the repair response places NAD+ availability in direct relationship with the integrity of the genome over time.
Domain 03
Mitochondrial function and biogenesis
SIRT1, SIRT3, SIRT4, and SIRT5 each have roles in mitochondrial biology — whether through the regulation of PGC-1α (which governs mitochondrial biogenesis), the deacetylation of mitochondrial metabolic enzymes, or the management of reactive oxygen species. Mitochondrial dysfunction is one of the most consistently documented features of aging across tissues and species, and the sirtuin family's involvement in maintaining mitochondrial health is one of the primary mechanisms through which NAD+ biology intersects with this hallmark.
Domain 04
Metabolic regulation and stress response
Multiple sirtuins participate in the regulation of glucose and lipid metabolism — SIRT1 through PGC-1α and other transcription factors, SIRT3 through mitochondrial enzyme activity, SIRT4 through amino acid metabolism, SIRT6 through glucose transporter regulation. The sirtuin family functions, in aggregate, as a metabolic sensor — linking the cell's NAD+ status to its metabolic response. This connection between energy sensing and cellular maintenance is one of the reasons sirtuin biology is studied so intensively in the context of metabolic aging.
The Aging Sirtuin System
How the relationship between
NAD+ and sirtuins changes with age.
Adequate substrate. Full-capacity cellular maintenance.
NAD+ pool maintained by efficient NAMPT activity in the Salvage Pathway
All seven sirtuins have access to the NAD+ they need for their reactions
Gene expression regulation responds promptly to cellular signals
DNA damage is repaired at a rate that keeps pace with accumulation
Mitochondrial networks are maintained by active SIRT1 and SIRT3 signaling
Cellular stress response is rapid and complete
Declining substrate. Constrained maintenance across all seven sirtuins.
NAD+ pool reduced by declining NAMPT and rising CD38 — across all compartments
All seven sirtuins compete for a smaller NAD+ pool — collective output reduced
Gene expression regulation becomes less responsive and less precise
DNA repair is slower — damage accumulates faster than the response can address it
Mitochondrial biogenesis and quality control decline with reduced sirtuin signaling
Cellular stress response is attenuated — the cell's resilience narrows with time
The Sirtuin System in Numbers
What the NAD+–sirtuin relationship
looks like structurally.
7
Mammalian sirtuins — all NAD+-dependent, spanning nucleus, cytoplasm, and mitochondria
The seven sirtuins are distributed across three cellular compartments, each drawing on the NAD+ pool maintained by the NMNAT isoform specific to that compartment. This compartmentalization means that age-related NAD+ decline affects sirtuin activity not just globally but specifically within each cellular environment — nucleus, cytoplasm, and mitochondrial matrix each aging on their own NAD+ timeline.
1:1
Ratio of NAD+ molecules consumed per sirtuin deacylation reaction — the direct substrate relationship
Each deacylation reaction performed by a sirtuin consumes exactly one molecule of NAD+, releasing nicotinamide (which the Salvage Pathway can recycle) and the deacylated target protein. This stoichiometric relationship means sirtuin activity is directly and proportionally limited by NAD+ availability — not by a more complex allosteric mechanism, but by simple substrate availability. When NAD+ falls, sirtuin output falls in proportion.
3
Distinct cellular compartments with separate NAD+ pools governing different sirtuin subsets
The nuclear pool (NMNAT1 — governs SIRT1, SIRT6, SIRT7), the cytoplasmic pool (NMNAT2 — governs SIRT2), and the mitochondrial pool (NMNAT3 — governs SIRT3, SIRT4, SIRT5) are each maintained independently. This means the age-related decline of NAD+ does not affect all seven sirtuins equally or simultaneously — the dynamics of each compartment's production and consumption shape which sirtuins are most affected and when.
III
Sirtuins, NAD+, and NMN —
why the three belong in the same conversation.
The connection between sirtuins and NMN is not direct — NMN does not activate sirtuins, interact with them chemically, or influence their function other than through its role as an NAD+ precursor. The connection is architectural: NMN feeds the Salvage Pathway that maintains the NAD+ pools on which all seven sirtuins depend. Understanding sirtuin biology is understanding what is at stake when those pools decline.
This is why the sirtuin story is central to how the broader field thinks about NAD+ and aging. It is not simply that NAD+ declines. It is that NAD+ decline progressively limits the cellular maintenance system — across gene regulation, DNA repair, mitochondrial biology, and metabolic sensing — by constraining the enzymes whose activity coordinates that maintenance. The scope of that constraint, distributed across seven proteins operating in three cellular compartments, is why NAD+ biology has attracted the level of serious scientific attention it has over the past two decades.
The sirtuin biology described here reflects the current state of a field that continues to expand rapidly — new substrates, new compartment-specific dynamics, and new connections between sirtuin activity and specific aging phenotypes are being characterized on an ongoing basis. What is known is already substantial. What remains to be understood is likely more so. For the surrounding context of how NAD+ decline sets the stage for everything described here, the NAD+ aging article and the NMN–NAD+ relationship article provide the essential foundation. Both connect directly to Cellular Longevity — Pillar 03 of The Longevity Code.
NMN does not activate sirtuins.
It feeds the pools
on which all seven of them
depend to do their work.
Codeage · Pillar 03 · Cellular Longevity
Built for the
cellular long game.
Cellular Longevity is Pillar 03 of The Longevity Code — the dimension of the system built around NAD+ biology, mitochondrial health, and the science of cellular aging.
Explore Cellular Longevity →
