BIOCHEMISTRY OF AGING

Dr. Gangadhar ChatterjeePG Resident

GGMC & JJH, Mumbai

AGING: No clear cut definition, final stage of life , signaled by a resurgence of physical and physiological change.

Senescence: “The decline of fitness components of an individual with increasing age, owing to internal deterioration” -Michael Rose

Hutchison- Gilford, Werner’s and Down syndrome are three human genetic disease whose pathologies include an acceleration of many of the physiological events associated with aging.

Life expectancy vs. Longevity LIFE EXPECTANCY: Calculated by

averaging over all births. Dramatically influenced by infant mortality rate.

LONGEVITY: Expected lifespan only for those persons who survived infancy.

Aging and death are, in all likelihood, multifactorial processes, some nondeterminant and others programmed.

THEORIES OF AGING WEAR & TEAR THEORY- Hydrolytic damage to proteins and

nucleotides- Generation of ROS on respiration- Multiplication of destructiveness of ROS by

chain reaction- Free radicals and mitochondrial theory of

aging- Damage by UV radiation- Damaging cross-links by protein glycation

THEORIES OF AGING cont… MOLECULAR REPAIR MECHANISMS

COMBAT WEAR & TEAR- Enzymatic and chemical reactions intercept

damaging ROS- DNA integrity maintained by proofreading and

repair mechanism- Repair of damaged proteins AGING AS PROGRAMMED PROCES- Metabolic theory of aging- Telomere: A molecular countdown clock- Discovery of aging genes

WEAR & TEAR THEORY Hydrolytic damage to proteins and

nucleotides- Water, a weak nucleophile- Ubiquitous presence and high conc. React with

susceptible targets

- The amide bond most frequently targeted and found on the side chains of asparagine and glutamine as they are more exposed.

- These replaces neutral amide group to acidic carboxylic group leading to introduction of both a negative charge and of a potential proton donor

Amino groups projecting from the heterocyclic aromatic rings of the nucleotide bases cytosine, adenine and guanine are each susceptible to hydrolytic attack in which amino group is replaced by a carbonyl to form uracil , hypoxanthine and xanthine respectively.

The bond between nucleotide base and de-oxy ribose moiety in DNA also vulnerable to hydrolysis.

Leave a gap in the sequence

If not repaired lead to FRAMESHIFT mutation

ROS generation by respiration Enzyme catalyzed oxidation of biomolecules by

molecular oxygen required in many processes

- Hydroxylation of proline and lysine in collagen side chain.

- Detoxification of xenobiotics by Cyt P450.

- Generation of chemiosmotic gradient in mitochondria by ETC.

LEAKAGE from ETC is by far and away the major source of ROS in human cells

Among different ROS hydroxyl free radical are most reactive and destructive .

It is unfortunate that two pathways exist in living mammalian cells by which highly toxic hydroxyl radicals can be generated from less destructive ROS

ROS display a strong tendency to form ADDUCTS, direct addition of two (or more) compounds, with nucleotide bases, polyunsaturated fatty acids or other biomolecules possessing multiple double bonds.

Lead to formation of cross-linked lipid-lipid, lipid- protein adducts and a loss of membrane fluidity and integrity.

Triggers APOPTOSIS or programmed cell death.

FREE RADICALS & MITOCHONDRIAL THEORY OF AGING

Denham Harmon (1956)

“A free radical is any species capable of independent existence (hence the term ‘free’) that contains one or more unpaired electron”

Barry Halliwell & John Gutteridge

Harmon’s observation – lifespan was inversely proportional to metabolic rate, and by extrapolation, respiration

Damage to mitochondria adversely affect the efficiency with which it performs its most important function, synthesis of ATP.

………………..wholesale decline in physiological function of cell that occur in aging.

Several components of ETC encoded by mitochondrion’s indigenous genome

……………….mitochondrial genome is reduced, vestigial remnant of ancient bacterium …………………….ENDOSYMBIOSIS

Mitochondrial genome lacks surveillance and repair mechanism that help to maintain integrity of nuclear DNA

Mitochondria plays central role in sensor-response pathways that trigger APOPTOSIS

PROTEIN GLYCATION: FORMATION OF DAMAGING CROSSLINKS

Amino group found in side chain of lysine and some of the nucleotide forms reversible ADDUCTS with reducing sugars through SCHIFF’S base formation.

Undergoes AMADORI rearrangement to have conjugated C-C double bond which react with amino group of neighboring proteins (AGEs)

Physiological impact of AGEs more pronounced when long-lived protein COLLAGEN or β- crystallins are involved.

Provides the opportunity for multiple glycation and crosslinking events to occur.

Progressive crosslinking of collagen network in vascular endothelium

Progressive loss of elasticity and thickening of

blood vessels

Promotion of PLAQUE formation

Vulnerability of S-containing amino acids -SH group of cystein

(reduced) crucial in catalysis, regulation or structure

Both –SH and thioether group of CYS and MET extremely vulnerable to oxidation giving rise to cystein-disulfides, sulfenic acid, sulfonic acid and sulfoxide of CYS and MET.

Unfortunately, reduction potential of GSH and NADPH is only sufficient to reduce lowest oxidation state of these S atoms: CYS- disulfide, CYS-sulfenic acid and MET-sulfoxide

AGING- A PROGRAMED PROCESSED

“The brighter the candle, the Quicker it burns”……………..ancient Chinese quote

……METABOLIC or RATE OF LIVING HYPOTHESIS……Raymond Pearl (1928)

“the duration of life varies inversely as the rate of energy expenditure (BMR) “

‘Metabolic theory of aging’ cont…. Animals differ markedly in size, longevity and

heart rate; over their lifetime each expends a similar amount of total metabolic energy per unit body mass, 7 x 105 J/g.

Over time continued generation of energy and related consumption of O2leads to accumualtion of ROS-induced damage to protein, lipids until a tipping point.

Calorie restricted diet

Fewer burning calories

Fewer production of damaging ROS

TELOMERES: A Molecular Countdown Clock

Provides some DISPOSABLE DNA to accommodate wastage that occurs during DNA replication.

Telomeres cont… Wastage is a consequence of unidirectional ( 3’ to 5’) work of DNA polymerases.

Replication of 5’ end of a linear double stranded DNA via discontinuous 3’ to 5’ synthesis and ligation of small OKAZAKI fragments do not provide enough space to accommodate RNA primer and DNA polymerases.

Synthesis of 5’ end of each strand generally fall 100 bp or more short.

Telomeres cont…. Provides an innocuous source of DNA whose

decreasing length is of little consequence to cell

When exhausted, roughly after 100 cell division in human, mitosis ceases and somatic cell enters REPLICATIVE SENESCENCE.

TELOMERASE (a ribonucleoprotein), expressed in stem cells and most cancer cells restore their telomeres to full length.

The ability to prevent replicative senescence using this enzyme that maintains telomere at full-length represents MOST COMPELLING EVIDENCE of operation of telomere clock.

NEW HORIZON- AGING GENE Work by Cynthia

Kenyon and colleagues

Code for small set of transcription factors e.g. PHA-4, DAF-16 that presumably control expression of aging critical gene

Drosophila melanogaster