Q4 LONG ANSWER 10M ★ High-yield

Define free radicals. Describe the reactive oxygen species, the mechanisms of tissue damage, and the antioxidant defences of the body.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Free radical = any atom or molecule containing one or more unpaired electrons in its outer orbital, making it highly reactive and short-lived.

They react by stealing an electron from a neighbouring molecule, which itself becomes a radical — a self-propagating chain reaction

Reactive Oxygen Species

Species	Formula	Origin	Reactivity
Superoxide anion	O2•−	Leak from complexes I and III; NADPH oxidase; xanthine oxidase	Moderate
Hydrogen peroxide	H2O2	From superoxide, by SOD	Not a radical, but crosses membranes freely and is the precursor of the worst one
Hydroxyl radical	OH•	Fenton and Haber–Weiss reactions	The most damaging — reacts with almost anything it meets
Singlet oxygen	1O2	Photosensitisation	High
Peroxynitrite	ONOO−	Superoxide + nitric oxide	Very high
Hypochlorous acid	HOCl	Myeloperoxidase in neutrophils	High — bactericidal

The Fenton reaction

Fe2+ + H2O2 → Fe3+ + OH• + OH−

This is why free iron and copper are dangerous, and why the body binds them so carefully to transferrin, ferritin and caeruloplasmin

Sources

Endogenous	Exogenous
Respiratory chain leak — 1–3% of oxygen used	Ionising and ultraviolet radiation
NADPH oxidase — the respiratory burst of phagocytes	Cigarette smoke, air pollution
Xanthine oxidase — especially on reperfusion	Drugs — paracetamol, doxorubicin, bleomycin, paraquat
Cytochrome P450	Heavy metals, pesticides
Peroxisomal oxidases; autoxidation of haemoglobin	Hyperoxia — oxygen therapy

Mechanisms of Damage

Target	Damage	Marker
Lipids	Lipid peroxidation of membrane polyunsaturated fatty acids → loss of fluidity and integrity	Malondialdehyde (MDA); 4-hydroxynonenal
Proteins	Oxidation of –SH groups, fragmentation, cross-linking → loss of enzyme activity	Protein carbonyls
DNA	Base modification, strand breaks → mutation and carcinogenesis	8-hydroxydeoxyguanosine
Carbohydrates	Depolymerisation of hyaluronic acid	Loss of joint lubrication

Lipid peroxidation is a chain reaction — one radical can destroy many fatty acid molecules until an antioxidant terminates it

Antioxidant Defences

A. Enzymatic

Enzyme	Reaction	Cofactor
Superoxide dismutase (SOD)	2 O2•− → H2O2 + O2	Cu and Zn (cytosol); Mn (mitochondria)
Catalase	2 H2O2 → 2 H2O + O2	Haem; in peroxisomes
Glutathione peroxidase	H2O2 + 2 GSH → 2 H2O + GSSG	Selenium
Glutathione reductase	GSSG → 2 GSH	NADPH from the HMP shunt

B. Non-enzymatic

Vitamin E — the chief lipid-phase antioxidant; breaks the lipid peroxidation chain
Vitamin C — the chief aqueous-phase antioxidant; regenerates vitamin E
Glutathione — the most abundant intracellular thiol
β-Carotene — quenches singlet oxygen
Uric acid — a major plasma antioxidant, which is one argument for not lowering it excessively
Bilirubin — an effective antioxidant, possibly explaining a benefit of mild unconjugated hyperbilirubinaemia
Trace elements — selenium, zinc, copper, manganese as enzyme cofactors
Metal-binding proteins — transferrin, ferritin, caeruloplasmin, albumin; they prevent the Fenton reaction

Free Radicals in Disease

Condition	Role of free radicals
Atherosclerosis	Oxidation of LDL → foam cell formation — the key step
Cancer	DNA damage and mutation
Ageing	Cumulative oxidative damage — the free radical theory
Reperfusion injury	Xanthine oxidase burst when oxygen returns to ischaemic tissue
Neurodegeneration	Parkinson and Alzheimer disease; SOD1 mutation causes familial ALS
Retinopathy of prematurity, bronchopulmonary dysplasia	Oxygen therapy in an infant with immature defences
Rheumatoid arthritis	Neutrophil-derived radicals in the joint
Cataract	Lens protein oxidation and cross-linking
Paracetamol and CCl4 toxicity	Reactive metabolites deplete glutathione

Applied Aspects

N-acetylcysteine replenishes glutathione — the antidote in paracetamol poisoning; the clearest therapeutic success of antioxidant reasoning
Large trials of antioxidant vitamin supplements have been disappointing, and β-carotene increased lung cancer in smokers. A convincing mechanism does not guarantee that supplementation helps
Dietary antioxidants from whole foods behave differently from isolated supplements — the practical advice remains fruit and vegetables, not pills
Free radicals are not simply harmful — the respiratory burst kills bacteria, and nitric oxide is a signalling molecule. Its absence causes chronic granulomatous disease

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