Biochemistry of Ageing

Introduction

• Ageing is a complex, progressive, and irreversible biological process.

• It is characterized by a gradual decline in physiological and cellular functions.

• There is a reduction in homeostatic capacity, leading to decreased stress adaptation.

• Ageing increases susceptibility to diseases and ultimately results in death.

• From a biochemical perspective, ageing involves cumulative molecular and cellular damage.

• Major biomolecules affected include DNA, proteins, lipids, and carbohydrates.

• There is progressive dysregulation of metabolic pathways and cellular signaling mechanisms.

• These biochemical changes impair normal cellular functions.

• Structural and functional integrity of tissues and organs gradually deteriorates.

• Ageing is influenced by multiple factors such as genetic makeup and epigenetic modifications.

• Environmental exposures play a significant role in the ageing process.

• Nutritional status and dietary habits affect the rate of ageing.

• Lifestyle factors such as physical activity, stress, and habits modulate ageing.

• Modern biochemistry considers ageing as a multifactorial process.

• Ageing is not governed by a single pathway but by the interaction of multiple interconnected molecular mechanisms.

 

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Theories of Ageing 

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1. Free Radical Theory of Ageing

• Proposed by Denham Harman.
• Ageing results from cumulative damage caused by reactive oxygen species (ROS) such as superoxide (O₂•⁻), hydroxyl radical (•OH), and hydrogen peroxide (H₂O₂).
• ROS are generated mainly in mitochondria during oxidative phosphorylation.
• Oxidative damage affects:
  • Lipids → lipid peroxidation of membranes
  • Proteins → oxidation, misfolding, loss of enzymatic activity
  • DNA → mutations, strand breaks, base modifications (e.g., 8-oxo-dG)

2. Mitochondrial Theory of Ageing

• Mitochondria are both the source and target of ROS.
• Accumulation of mutations in mitochondrial DNA (mtDNA) leads to defective electron transport chain.
• Reduced ATP production and increased ROS generation create a vicious cycle.
• Decline in mitochondrial biogenesis and mitophagy contributes to ageing.

3. Telomere Shortening Theory

• Telomeres are repetitive DNA sequences (TTAGGG) at chromosome ends.
• With each cell division, telomeres shorten due to the end-replication problem.
• Critically short telomeres trigger cellular senescence or apoptosis.
• Reduced telomerase activity in somatic cells accelerates ageing.

4. Glycation and Cross-Linking Theory

• Non-enzymatic glycation of proteins by glucose leads to formation of advanced glycation end products (AGEs).
• AGEs cause:
  • Protein cross-linking
  • Loss of elasticity of tissues (skin, blood vessels)
  • Altered enzyme activity
• AGEs interact with RAGE receptors, inducing inflammation and oxidative stress.

5. Genetic and Epigenetic Theory

• Ageing is regulated by longevity-associated genes (e.g., sirtuins, FOXO, mTOR).
• Epigenetic changes include:
  • DNA methylation drift
  • Histone modification changes
  • Altered chromatin structure
• These changes affect gene expression patterns during ageing.

 

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Biochemical Changes During Ageing

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1. Oxidative Stress and Free Radical Accumulation

• Increased production of reactive oxygen species (ROS)

• Decline in antioxidant defenses:

  • ↓ Superoxide dismutase (SOD)

  • ↓ Catalase

  • ↓ Glutathione peroxidase

  • ↓ Reduced glutathione (GSH)

Biochemical consequences:

• Lipid peroxidation (↑ MDA, 4-HNE)

• Protein oxidation

• DNA base modifications

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2. Mitochondrial Dysfunction

• Accumulation of mutations in mitochondrial DNA

• Decreased oxidative phosphorylation

• Reduced ATP synthesis

• Increased electron leakage → enhanced ROS generation

Impact:

• Reduced cellular energy

• Muscle weakness

• Neurodegeneration

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3. Alterations in Protein Metabolism

• Decreased protein synthesis

• Increased protein degradation

• Accumulation of misfolded and cross-linked proteins

• Decline in proteasomal and autophagic activity

Examples:

• Amyloid-β in Alzheimer’s disease

• Advanced glycation end-products (AGEs)

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4. Changes in Lipid Metabolism

• Increased total cholesterol and LDL

• Decreased HDL

• Increased triglycerides

• Accumulation of oxidized lipids

Leads to:

• Atherosclerosis

• Cardiovascular disease

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5. Carbohydrate Metabolism Alterations

• Reduced insulin sensitivity

• Impaired glucose tolerance

• Increased hepatic gluconeogenesis

• Post-prandial hyperglycemia

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6. Hormonal Changes

• ↓ Growth hormone and IGF-1

• ↓ Sex hormones (estrogen, testosterone)

• ↓ DHEA

• Altered cortisol rhythm

• Reduced melatonin secretion

Consequences:

• Sarcopenia

• Osteoporosis

• Sleep disturbances

• Immune dysfunction

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7. Nucleic Acid and Genomic Changes

• Accumulation of DNA damage

• Reduced DNA repair efficiency

• Telomere shortening

• Genomic instability

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8. Cellular Senescence

• Permanent cell cycle arrest

• Increased expression of p16INK4a and p21

• Senescence-associated secretory phenotype (SASP)

Effects:

• Chronic inflammation

• Tissue dysfunction

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9. Chronic Low-Grade Inflammation (Inflammaging)

• Increased circulating inflammatory markers:

  • IL-6

  • TNF-α

  • CRP

• Due to immune dysregulation and senescent cell accumulation

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10. Calcium and Bone Metabolism

• Reduced vitamin D synthesis

• Decreased calcium absorption

• Secondary hyperparathyroidism

Results:

• Osteopenia

• Osteoporosis

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11. Altered Water and Electrolyte Balance

• Decreased total body water

• Reduced thirst sensation

• Impaired renal concentrating ability

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12. Changes in Enzyme Activity

• Reduced activity of metabolic enzymes

• Altered isoenzyme patterns

• Reduced adaptability to metabolic stress

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13. Impaired Autophagy

• Reduced clearance of damaged organelles

• Accumulation of dysfunctional mitochondria

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Summary Table: Biochemical Changes in Ageing

 

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Role of Oxidative Stress in Ageing

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Mechanisms Linking Oxidative Stress to Ageing

1. Mitochondrial Dysfunction

• Mitochondria are both sources and targets of ROS

• ROS-induced damage to mitochondrial DNA (mtDNA) leads to:

  • Reduced ATP production

  • Increased electron leakage → more ROS

• This creates a vicious cycle accelerating cellular ageing

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2. DNA Damage and Genomic Instability

• ROS cause:

  • Base modifications (e.g., 8-oxo-deoxyguanosine)

  • Single and double-strand DNA breaks

• Accumulation of unrepaired DNA damage leads to:

  • Cellular senescence

  • Mutations

  • Apoptosis

• Telomeres are particularly sensitive to oxidative damage, resulting in telomere shortening

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3. Lipid Peroxidation

• ROS attack polyunsaturated fatty acids in membranes

• Formation of toxic aldehydes:

  • Malondialdehyde (MDA)

  • 4-hydroxynonenal (4-HNE)

• Consequences:

  • Loss of membrane fluidity

  • Altered membrane-bound enzyme and receptor functions

  • Cellular dysfunction and death

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4. Protein Oxidation

• Oxidative modification of amino acid side chains

• Protein cross-linking and fragmentation

• Accumulation of damaged and misfolded proteins

• Decline in:

  • Proteasomal activity

  • Autophagy

• Leads to functional impairment of enzymes and structural proteins

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5. Cellular Senescence

• Oxidative stress activates:

  • p53

  • p21

  • p16INK4a pathways

• Results in irreversible cell cycle arrest

• Senescent cells secrete pro-inflammatory cytokines (SASP – Senescence Associated Secretory Phenotype), contributing to chronic low-grade inflammation (“inflammaging”)

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Oxidative Stress and Age-Related Diseases

Oxidative stress plays a central role in the pathogenesis of many ageing-associated disorders:

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Antioxidant Defense Systems and Ageing

Enzymatic Antioxidants

• Superoxide dismutase (SOD)

• Catalase

• Glutathione peroxidase (GPx)

Non-Enzymatic Antioxidants

• Glutathione (GSH)

• Vitamin C

• Vitamin E

• Carotenoids

• Polyphenols

With ageing, antioxidant capacity declines, while ROS production increases, tipping the balance toward oxidative damage.

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Hormesis and Redox Signaling

• Low levels of ROS act as signaling molecules regulating:

  • Cell proliferation

  • Stress response pathways

• Mild oxidative stress can induce protective mechanisms (hormesis)

• Excessive ROS → pathological ageing

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Role of Lifestyle and Nutrition

• Caloric restriction reduces ROS production and enhances longevity
• Regular physical exercise improves antioxidant enzyme activity
• Diet rich in fruits, vegetables, and polyphenols supports redox balance

 

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Hormonal and Metabolic Changes in Ageing

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• Ageing is associated with progressive endocrine and metabolic alterations that affect growth, energy homeostasis, body composition, stress response, glucose and lipid metabolism.
• These changes contribute to reduced physiological reserve, increased susceptibility to chronic diseases, and altered response to stress. The ageing process involves both hormonal decline and tissue resistance to hormones.

I. Hormonal Changes in Ageing

1. Growth Hormone (GH) and Insulin-like Growth Factor-1 (IGF-1)

• Progressive decline with age (somatopause)

• Reduced GH secretion amplitude and frequency

• Decreased IGF-1 levels

Consequences:

• Loss of lean body mass (sarcopenia)

• Increased fat mass

• Reduced bone density

• Decreased protein synthesis

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2. Sex Hormones

a) Estrogen (Females)

• Sharp decline after menopause

• Reduced ovarian estrogen production

Effects:

• Osteoporosis

• Increased cardiovascular risk

• Skin thinning

• Urogenital atrophy

b) Testosterone (Males)

• Gradual decline (andropause / late-onset hypogonadism)

Effects:

• Decreased muscle mass and strength

• Reduced libido

• Increased visceral fat

• Insulin resistance

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3. Thyroid Hormones

• ↓ Triiodothyronine (T3)

• Normal or slightly ↑ Thyroxine (T4)

• TSH usually normal or slightly elevated

Clinical Impact:

• Reduced basal metabolic rate (BMR)

• Cold intolerance

• Fatigue

• Slower metabolic processes

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4. Insulin

• Decreased insulin sensitivity (insulin resistance)

• Impaired glucose uptake in muscle and adipose tissue

Results:

• Impaired glucose tolerance

• Increased risk of type 2 diabetes mellitus

• Post-prandial hyperglycemia

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5. Cortisol

• Basal cortisol levels may increase

• Flattened circadian rhythm

• Reduced feedback inhibition

Effects:

• Muscle wasting

• Immunosuppression

• Central obesity

• Cognitive decline

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6. Aldosterone and Renin

• Reduced renin-angiotensin-aldosterone activity

• ↓ Aldosterone secretion

Consequences:

• Impaired sodium conservation

• Dehydration

• Orthostatic hypotension

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7. Dehydroepiandrosterone (DHEA)

• Progressive decline with age (adrenopause)

Effects:

• Reduced immune function

• Decreased muscle strength

• Reduced well-being

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8. Melatonin

• Reduced nocturnal secretion

Effects:

• Sleep disturbances

• Altered circadian rhythm

• Increased oxidative stress

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II. Metabolic Changes in Ageing

1. Basal Metabolic Rate (BMR)

• Declines by ~1–2% per decade after 30 years

• Due to reduced lean body mass and thyroid hormone activity

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2. Body Composition

• ↓ Lean muscle mass (sarcopenia)

• ↑ Fat mass, especially visceral fat

• ↓ Bone mineral density (osteopenia/osteoporosis)

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3. Carbohydrate Metabolism

• Reduced glucose tolerance

• Increased hepatic gluconeogenesis

• Reduced muscle glucose uptake

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4. Lipid Metabolism

• ↑ Total cholesterol and LDL

• ↓ HDL cholesterol

• Increased triglycerides

Leads to:

• Atherosclerosis

• Cardiovascular disease

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5. Protein Metabolism

• Decreased protein synthesis

• Increased protein breakdown

• Reduced anabolic response to amino acids

Results:

• Sarcopenia

• Poor wound healing

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6. Calcium and Vitamin D Metabolism

• ↓ Vitamin D synthesis in skin

• ↓ Intestinal calcium absorption

• Secondary hyperparathyroidism

Consequences:

• Bone loss

• Increased fracture risk

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7. Water and Electrolyte Balance

• ↓ Total body water

• Reduced thirst sensation

• Impaired renal concentrating ability

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III. Interrelationship Between Hormonal and Metabolic Changes

• Hormonal decline leads to altered metabolism

• Metabolic dysfunction worsens hormonal responsiveness

• Combined effects result in:

  • Frailty

  • Reduced stress tolerance

  • Increased disease susceptibility

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IV. Clinical Significance

• Explains increased prevalence of:

  • Diabetes mellitus

  • Osteoporosis

  • Cardiovascular diseases

  • Sarcopenia

• Alters pharmacokinetics and drug responses

• Important for geriatric care and preventive strategies

 

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Cellular Senescence

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Cellular senescence is a stable form of cell cycle arrest characterized by resistance to apoptosis, altered gene expression, and the secretion of bioactive molecules, occurring in response to cellular damage or stress.

Types of Cellular Senescence

1. Replicative Senescence

• Occurs due to repeated cell divisions

• Caused by telomere shortening

• Associated with the Hayflick limit

• Common in somatic cells

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2. Stress-Induced Premature Senescence (SIPS)

• Occurs independent of telomere length

• Triggered by:

  • Oxidative stress

  • DNA damage

  • Oncogene activation

  • Radiation, toxins

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3. Oncogene-Induced Senescence (OIS)

• Caused by aberrant oncogene activation (e.g., Ras, BRAF)

• Acts as a tumor-suppressive mechanism

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Molecular Mechanisms of Cellular Senescence

1. DNA Damage Response (DDR)

• Activation of:

  • ATM/ATR kinases

  • p53 pathway

• Leads to increased expression of p21

• Results in G1/S cell cycle arrest

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2. p16INK4a–Rb Pathway

• p16 inhibits cyclin-dependent kinases (CDK4/6)

• Maintains retinoblastoma protein (Rb) in hypophosphorylated state

• Blocks E2F-mediated transcription

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3. Telomere Dysfunction

• Critically shortened telomeres activate DDR

• Telomeres lose protective shelterin complex

• Triggers replicative senescence

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4. Mitochondrial Dysfunction

• Increased ROS production

• Mitochondrial DNA damage

• Altered bioenergetics (↓ ATP)

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5. Epigenetic Alterations

• Senescence-associated heterochromatin foci (SAHF)

• DNA methylation changes

• Histone modifications

 

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Biochemical Basis of Age-Related Diseases

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1. Oxidative Stress and Free Radical Damage

• Ageing is associated with increased generation of reactive oxygen species (ROS) and reduced antioxidant defenses.

• ROS damage:

  • DNA → mutations, strand breaks

  • Proteins → loss of enzymatic activity

  • Lipids → lipid peroxidation

Disease association:

• Alzheimer’s disease – oxidative neuronal damage

• Atherosclerosis – LDL oxidation

• Cancer – genomic instability

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2. Mitochondrial Dysfunction

• Accumulation of mtDNA mutations

• Reduced oxidative phosphorylation

• Decreased ATP production

• Increased electron leakage → more ROS

Leads to:

• Neurodegenerative diseases

• Sarcopenia

• Cardiomyopathy

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3. Genomic Instability and DNA Damage

• Impaired DNA repair mechanisms

• Telomere shortening

• Accumulation of somatic mutations

Associated diseases:

• Cancer

• Age-related immune dysfunction

• Neurodegeneration

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4. Cellular Senescence

• Permanent cell cycle arrest due to DNA damage and telomere attrition

• Secretion of inflammatory mediators (SASP)

Consequences:

• Chronic inflammation (inflammaging)

• Tissue dysfunction

• Osteoarthritis

• Atherosclerosis

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5. Altered Protein Homeostasis (Proteostasis)

• Reduced protein synthesis

• Impaired folding (ER stress)

• Decreased proteasomal and autophagic degradation

Results in:

• Accumulation of misfolded proteins

• Amyloid-β plaques (Alzheimer’s)

• α-synuclein aggregates (Parkinson’s)

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6. Chronic Low-Grade Inflammation (Inflammaging)

• Persistent elevation of pro-inflammatory cytokines:

  • IL-6

  • TNF-α

  • CRP

• Due to immune dysregulation and senescent cells

Leads to:

• Cardiovascular diseases

• Type 2 diabetes

• Frailty syndrome

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7. Hormonal Dysregulation

• ↓ Growth hormone, IGF-1

• ↓ Sex hormones (estrogen, testosterone)

• Altered cortisol rhythm

• Insulin resistance

Metabolic consequences:

• Sarcopenia

• Osteoporosis

• Obesity

• Diabetes mellitus

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8. Impaired Autophagy

• Decline in cellular “self-cleaning” mechanisms

• Accumulation of damaged organelles

Disease links:

• Neurodegenerative disorders

• Cardiomyopathy

• Metabolic syndrome

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9. Epigenetic Alterations

• DNA methylation changes

• Histone modifications

• Altered chromatin remodeling

Effects:

• Dysregulated gene expression

• Increased disease susceptibility

• Cancer development

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10. Metabolic Dysregulation

• Reduced insulin sensitivity

• Altered lipid metabolism

• Increased visceral adiposity

Leads to:

• Type 2 diabetes mellitus

• Dyslipidemia

• Cardiovascular diseases

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11. Stem Cell Exhaustion

• Reduced regenerative capacity

• Decline in tissue repair

Consequences:

• Delayed wound healing

• Bone marrow failure

• Immune senescence

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Biochemical Basis of Major Age-Related Diseases

 

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Anti-Ageing Interventions

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I. Lifestyle-Based Anti-Ageing Interventions

1. Caloric Restriction (CR)

• Reduction of calorie intake without malnutrition

• Most consistently proven lifespan-extending intervention

Biochemical effects:

• ↓ ROS production

• ↑ Insulin sensitivity

• ↓ IGF-1 signaling

• Activation of AMPK and SIRT1

• Inhibition of mTOR pathway

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2. Physical Exercise

• Aerobic + resistance training

Anti-ageing mechanisms:

• ↑ Mitochondrial biogenesis (via PGC-1α)

• ↑ Antioxidant enzymes (SOD, catalase)

• ↓ Chronic inflammation

• Preservation of muscle mass and bone density

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3. Nutrition and Diet Quality

• Diet rich in:

  • Antioxidants (vitamins C, E, carotenoids)

  • Polyphenols (resveratrol, flavonoids)

  • Omega-3 fatty acids

Benefits:

• Reduced oxidative stress

• Improved lipid and glucose metabolism

• Reduced inflammaging

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4. Sleep and Circadian Rhythm Regulation

• Adequate sleep restores hormonal balance

• Maintains melatonin secretion

• Reduces oxidative and inflammatory stress

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II. Pharmacological Anti-Ageing Interventions

1. Antioxidants

• Vitamins C, E

• Glutathione

• Coenzyme Q10

• Alpha-lipoic acid

Role:

• Neutralize free radicals

• Protect macromolecules from oxidative damage
  (Note: Excess supplementation may blunt adaptive stress responses)

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2. Caloric Restriction Mimetics

• Metformin

• Resveratrol

• Rapamycin

Mechanisms:

• AMPK activation

• mTOR inhibition

• Improved insulin sensitivity

• Enhanced autophagy

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3. Senotherapeutics

a) Senolytics

• Selectively eliminate senescent cells

• Examples:

  • Dasatinib + Quercetin

  • Fisetin

b) Senomorphics

• Suppress SASP without killing cells

• Examples:

  • Metformin

  • Rapamycin

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4. Hormone Replacement Therapy (HRT)

• Estrogen (post-menopausal women)

• Testosterone (selected elderly males)

• Growth hormone (limited use)

Caution:

• Risk of cancer, cardiovascular disease

• Should be individualized and monitored

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III. Molecular and Cellular Interventions

1. Enhancement of Autophagy

• Fasting

• Exercise

• mTOR inhibitors

Benefits:

• Removal of damaged proteins and organelles

• Neuroprotection

• Metabolic homeostasis

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2. Mitochondrial Protection

• Coenzyme Q10

• NAD⁺ precursors (NMN, NR)

• Exercise-induced mitochondrial biogenesis

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3. Telomere Maintenance

• Telomerase activation (experimental)

• Lifestyle factors reduce telomere attrition

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4. Epigenetic Modulation

• DNA methylation modifiers

• Histone modification via diet and lifestyle

• Reversal of age-associated epigenetic drift

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IV. Anti-Inflammatory Strategies

• Omega-3 fatty acids

• NSAIDs (limited role)

• Control of obesity and metabolic syndrome

• Gut microbiome modulation

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V. Stem Cell and Regenerative Therapies

• Stem cell transplantation (experimental)

• Tissue regeneration

• Repair of damaged organs

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VI. Psychosocial and Cognitive Interventions

• Stress reduction

• Mental stimulation

• Social engagement

Biochemical impact:

• ↓ Cortisol

• Improved neuroplasticity

• Reduced neurodegeneration

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VII. Summary Table: Anti-Ageing Interventions and Targets