Sirtuin Proteins (SIRT1-7)

• Basic Medical Biochemistry - A Clinical Approach, 6e, p. 1106

Cellular Localization Overview

SIRT1 - Nuclear/Cytosolic

• Location: Predominantly nucleus; can shuttle to cytosol in certain neurons and cell types
• Enzymatic activity: Strong NAD+-dependent deacetylase
• Key functions:
  • Regulates gene expression by deacetylating histones (H1, H3, H4)
  • Deacetylates and activates eNOS (endothelial nitric oxide synthase) - cardiovascular protective
  • Regulates FOXO transcription factors - promotes longevity genes
  • Involved in caloric restriction response and metabolic adaptation
  • Protects against oxidative stress; loss of SIRT1 is linked to aging and COPD/COPD exacerbation
  • Reduces inflammation by inhibiting NF-kB
• Disease relevance: Cardiovascular protection, cancer, neurodegeneration, metabolic syndrome

SIRT2 - Cytoplasmic

• Location: Cytoplasm (moves to nucleus during G2/M cell cycle transition, where it deacetylates histone H4K16)
• Enzymatic activity: Deacetylase
• Key functions:
  • Deacetylates and activates PEPCK (phosphoenolpyruvate carboxykinase) - promotes gluconeogenesis under low-nutrient conditions
  • Inhibits ACLY (ATP-citrate lyase) - suppresses lipid synthesis
  • Regulates mitosis and cell cycle progression
  • Involved in tubulin deacetylation (cytoskeletal regulation)
• Disease relevance: Metabolic disease, neurodegeneration (Parkinson's)

SIRT3 - Mitochondrial

• Location: Mitochondrial matrix
• Enzymatic activity: Strong deacetylase
• Key functions:
  • Master regulator of mitochondrial protein acetylation
  • Deacetylates and activates LCAD/VLCAD (long-chain acyl CoA dehydrogenase) - stimulates fatty acid oxidation during fasting
  • Activates IDH2 and enhances the TCA cycle (cellular respiration)
  • Regulates mitochondrial ROS production
  • Upregulated during fasting in the liver
• Key note: Mice lacking SIRT3 accumulate long-chain fatty acid intermediates and triglycerides in the liver during fasting - Basic Medical Biochemistry, p. 1107
• Disease relevance: Metabolic syndrome, cancer, aging

SIRT4 - Mitochondrial

• Location: Mitochondria
• Enzymatic activity: ADP-ribosyltransferase (not primarily a deacetylase)
• Key functions:
  • ADP-ribosylates and inhibits glutamate dehydrogenase (GDH) in pancreatic beta cells - reduces insulin secretion in response to amino acids
  • Inhibits PDH (pyruvate dehydrogenase) activity - reduces conversion of pyruvate to acetyl-CoA
  • Opposes some of SIRT3's effects on GDH
• Disease relevance: Diabetes/insulin regulation, cancer metabolism

SIRT5 - Mitochondrial

• Location: Mitochondria
• Enzymatic activity: Weak deacetylase; primary activities are desuccinylase, demalonylase, and deglutarylase
• Key functions:
  • Regulates protein lysine succinylation, malonylation, and glutarylation - post-translational modifications beyond acetylation
  • Inhibits PDH activity
  • May disrupt glutamine metabolism through glutaminase (GLS)
  • Represses IDH2 activity
• Disease relevance: Cancer, metabolic disorders

SIRT6 - Nuclear (Chromatin)

• Location: Nucleus, principally in chromatin
• Enzymatic activity: NAD+-dependent mono-ADP-ribosyltransferase + long-chain fatty acid deacetylase + deacetylase
• Key functions:
  • Promotes genomic integrity - activates DNA repair enzymes via deacylation
  • Regulates metabolic adaptations to caloric restriction
  • Anti-aging: serves dual functions in metabolism and DNA repair
  • Regulates NADPH oxidase activity - cardiovascular protective
  • Antiaging molecule; loss is linked to accelerated aging phenotypes
• Textbook note: "An increase in sirtuin-6 serves dual functions: contributes to metabolic adaptations of caloric restriction AND promotes genomic integrity by activating DNA repair enzymes" - Robbins & Cotran Pathologic Basis of Disease
• Disease relevance: Aging, cancer, cardiovascular disease

SIRT7 - Nuclear (Nucleolus)

• Location: Nucleolus
• Enzymatic activity: Deacetylase (H3K18 deacetylation)
• Key functions:
  • Regulates RNA polymerase I - controls ribosomal RNA (rRNA) transcription
  • Plays a role in ribosome biogenesis
  • Involved in DNA damage response and chromatin remodeling
  • Regulates cardiac function and stress response
• Disease relevance: Cancer (often overexpressed), cardiac hypertrophy

Summary Table

Why Sirtuins Matter Clinically

1. Aging & Longevity: Caloric restriction activates sirtuins (especially SIRT1 and SIRT6), which extend lifespan in multiple organisms. Whether this fully applies in humans is still under investigation - Medical Physiology
2. Cardiovascular disease: Age-related NAD+ depletion impairs sirtuin activity, reducing cardiovascular protection (eNOS regulation, mitochondrial function, NADPH oxidase control) - Fuster and Hurst's The Heart, 15e
3. Cancer: Sirtuins act as both tumor suppressors and promoters depending on context
4. Metabolic disease: SIRT3 and SIRT4 directly regulate fatty acid oxidation and insulin secretion
5. Therapeutic targets: Both sirtuin activators (e.g., resveratrol targets SIRT1) and inhibitors are under active clinical investigation - Nature Signal Transduction review

Cofactor of Sirtuin Proteins: NAD+

Why NAD+ is the Cofactor (Not Just a Substrate)

The Sirtuin Reaction in Detail

Acetyl-protein + NAD+  →  Deacetylated protein + Nicotinamide + 2'-O-Acetyl-ADP-ribose

1. NAD+ binds to the sirtuin catalytic domain
2. The glycosidic bond between nicotinamide and ADP-ribose in NAD+ is cleaved
3. The acetyl group from the protein substrate (on a lysine residue) is transferred to the 2'-hydroxyl of the ribose in ADP-ribose
4. Products released: deacetylated protein + nicotinamide + 2'-O-acetyl-ADP-ribose

• Basic Medical Biochemistry - A Clinical Approach, 6e, p. 1106

Key Implications of NAD+ Dependency

1. Metabolic/Energy Sensing

• NAD+ levels rise when the cell is in a low-energy/fasting state (high NAD+/NADH ratio)
• This directly activates sirtuins - linking caloric restriction to sirtuin-mediated longevity
• "The sirtuins are a class of seven proteins that respond to dietary energy via the cofactor NAD" - Harrison's Principles of Internal Medicine, 22e

2. Feedback Inhibition by Nicotinamide

• Nicotinamide (a product of the reaction) is an endogenous inhibitor of sirtuin activity
• It can re-bind the active site and block further reactions (product inhibition)
• This creates a built-in regulatory brake

3. NAD+ Depletion in Aging

• NAD+ levels decline with age
• This impairs sirtuin activity, leading to loss of cardiovascular protection, metabolic regulation, and DNA repair
• NAD+ supplementation (e.g., NMN, NR - nicotinamide riboside) is being actively studied to restore sirtuin activity

4. Resveratrol Connection

• Resveratrol (found in red wine) is a proposed SIRT1 activator
• Increased sirtuin activity via "genetic manipulation, resveratrol, or NAD supplementation has been associated with increased lifespan" - Harrison's, 22e

NAD+ Biosynthesis and Sirtuins

• De novo pathway - from tryptophan
• Salvage pathway - from nicotinamide (the very byproduct of sirtuin reactions), recycled back to NAD+ by NAMPT (nicotinamide phosphoribosyltransferase) - the rate-limiting enzyme

Summary