Enzymes as biocatalysts
Life depends on the coordinated, synchronous progress of thousands of chemical reactions inside the cell — digestion of food, absorption of nutrients and ultimately the production of energy. Remarkably, these reactions run at the temperature of the body, about 310 K, and at nearly neutral pH, conditions under which the same reactions would be impossibly slow in a beaker. The agents that make this possible are enzymes: biological catalysts secreted by living plants and animals.
Chemically, almost all enzymes are globular proteins — high-molecular-mass, nitrogenous organic compounds. They are not carbohydrates, not polysaccharides and not lipids. Like the globular proteins insulin and albumin, their polypeptide chains coil into compact, roughly spherical units that are soluble in water, and it is this folded three-dimensional shape that gives an enzyme its catalytic power.
As catalysts, enzymes share the defining features of any catalyst: they are required only in small quantities, they are regenerated unchanged at the end of the reaction, and they speed the reaction up without being consumed. NIOS notes that enzymes can accelerate a biochemical reaction up to ten million times compared with the uncatalysed process, and that enzyme-catalysed reactions rapidly attain equilibrium.
"Enzymes are polysaccharides" — false
A catalyst changes the rate at which equilibrium is reached; it never shifts the position of equilibrium and never alters the yield. An enzyme is no exception. Students also wrongly classify enzymes as carbohydrates because both appear in the Biomolecules unit.
Remember the chemical identity: almost all enzymes are globular proteins. NEET 2022 marked "Enzymes are polysaccharides" as the incorrect statement.
Naming enzymes and the -ase suffix
Enzymes are generally named after the compound, or class of compounds, on which they act, or sometimes after the reaction they catalyse. To this root the suffix -ase is added. Thus the enzyme that hydrolyses maltose into glucose is maltase; the one that catalyses oxidation of one substrate with simultaneous reduction of another is an oxidoreductase; an enzyme that hydrolyses an ester linkage is an esterase. A few enzymes carry older trivial names — pepsin and trypsin — that predate this convention.
| Enzyme | Substrate / reaction | Product(s) |
|---|---|---|
| Maltase | Hydrolysis of maltose | Glucose |
| Invertase (sucrase) | Hydrolysis of sucrose (cane sugar) | Glucose + fructose |
| Zymase | Fermentation of glucose | Ethanol + carbon dioxide |
| Urease | Hydrolysis of urea | Ammonia + carbon dioxide |
| Oxidoreductase | Coupled oxidation–reduction | Oxidised / reduced substrates |
The reaction catalysed by maltase, the textbook example, is written compactly with mhchem as:
$$\ce{C12H22O11 + H2O ->[\text{maltase}] 2\,C6H12O6}$$
where maltose yields two molecules of glucose. The hydrolysis of sucrose by invertase (also called sucrase) gives a one-to-one mixture of glucose and fructose:
$$\ce{C12H22O11 + H2O ->[\text{invertase}] \underset{\text{glucose}}{C6H12O6} + \underset{\text{fructose}}{C6H12O6}}$$
Because the laevorotation of fructose exceeds the dextrorotation of glucose, the product mixture is laevorotatory — the classic "inversion" that gives invertase and invert sugar their names.
Specificity and the active site
The most striking property of an enzyme is its specificity. An enzyme is highly specific and selective both for a particular reaction and for a particular substrate: maltase will hydrolyse maltose but not sucrose, and urease acts on urea alone. There is, in effect, one enzyme tailored to each substrate.
This selectivity arises from a small region of the folded protein called the active site — a pocket or cleft on the enzyme surface whose shape and chemical groups are arranged to fit one kind of substrate. Only a molecule with the complementary shape can settle into the active site; molecules of the wrong geometry simply do not bind. The active site does two jobs at once: it recognises the right substrate, and it holds that substrate in the precise orientation needed for the reaction to proceed.
The lock-and-key model
The enzyme-substrate relationship is classically pictured as a lock-and-key arrangement. The active site of the enzyme is the lock; the substrate is the key. Just as a lock opens only for a key of the exact matching shape, an enzyme accepts only the substrate whose geometry is complementary to its active site. This single image explains both the specificity of enzymes and the formation of the enzyme-substrate complex.
Enzymes are globular proteins, so their folding decides their function. Revise the four levels in Protein Structure — Primary to Quaternary.
Lowering the activation energy
How does an enzyme make a slow reaction fast? In exactly the way any catalyst does — by reducing the magnitude of the activation energy, the energy barrier that reactant molecules must climb before they can be converted into products. By providing an alternative reaction path with a lower barrier, the enzyme allows a much larger fraction of molecules to react at body temperature.
NCERT gives the cleanest quantitative illustration. For the hydrolysis of sucrose:
| Pathway for sucrose hydrolysis | Activation energy |
|---|---|
| Acid hydrolysis (uncatalysed by enzyme) | 6.22 kJ mol−1 |
| Enzymatic hydrolysis by sucrase | 2.15 kJ mol−1 |
The enzyme cuts the activation energy to roughly a third of its value. Crucially, the enzyme lowers the barrier for the forward and reverse directions equally, so it accelerates the approach to equilibrium without altering the equilibrium position itself.
Activation energy, not enthalpy or yield
A frequent error is to claim the enzyme increases the yield of product or releases more energy. It does neither. The enthalpy change of the reaction and the equilibrium constant are fixed by the reactants and products, not by the catalyst.
Enzymes lower activation energy and speed up the reaction; ΔH and equilibrium position stay the same.
The enzyme-substrate complex
The mechanism of enzyme action proceeds in two steps. First, the substrate molecule binds to the active site of the enzyme to form an enzyme-substrate complex. Within this complex the substrate is positioned in exactly the right orientation to facilitate the reaction — the enzyme effectively forces the reacting groups close together and at the correct angle. Second, this complex breaks down, releasing the molecule of product and regenerating the free enzyme, which is then ready to bind the next substrate molecule. The cycle can be summarised as:
$$\ce{E + S <=> E\!\cdot\!S -> E + P}$$
where E is the enzyme, S the substrate, E·S the enzyme-substrate complex and P the product. Because the enzyme is regenerated at the end, a single molecule of enzyme turns over very many substrate molecules — which is precisely why enzymes are needed only in small quantities.
Factors affecting enzyme activity
An enzyme is a folded protein, and its catalytic shape is held together by relatively weak hydrogen bonds, disulphide linkages and electrostatic forces. Anything that disturbs this folding destroys the active site and abolishes activity. Two such factors are central for NEET: temperature and pH.
| Factor | Optimum | Effect of moving away from optimum |
|---|---|---|
| Temperature | Moderate (around 310 K in the body) | Excess heat breaks hydrogen bonds; the globular protein unfolds (denatures) and the active site is lost |
| pH | A specific, narrow pH for each enzyme | A change in pH disturbs the hydrogen bonds and ionic interactions, denaturing the enzyme |
| Concentration / amount | Small amounts suffice | Enzyme is regenerated each cycle, so a little catalyses much |
When a native protein is subjected to a change in temperature or pH, its hydrogen bonds are disturbed, the globules unfold and the helix uncoils; the protein then loses its biological activity. This loss is denaturation. During denaturation the secondary and tertiary structures are destroyed while the primary structure (the amino-acid sequence) remains intact. A denatured enzyme is an inactive enzyme — the coagulation of egg white on boiling and the curdling of milk by lactic acid are everyday examples of the same process.
Heat and pH inactivate enzymes by the same route that boils an egg. See Denaturation of Proteins for the full picture.
Coenzymes and cofactors
In addition to the protein part, most active enzymes are associated with a non-protein component required for their activity, called a coenzyme. A classic example is nicotinamide adenine dinucleotide (NAD), the coenzyme for a number of dehydrogenation enzymes. Many vitamins act as coenzymes, which is one reason vitamin deficiencies impair metabolism. Certain metal ions also activate enzymes: NEET 2020 noted that the potassium ion activates many enzymes and participates in the oxidation of glucose to produce ATP.
Enzyme mechanism in one screen
- Enzymes are biocatalysts; almost all are globular proteins (not polysaccharides).
- Named after substrate or reaction, with the suffix -ase (maltase, invertase, urease, oxidoreductase).
- Highly specific — one enzyme per substrate — via a complementary active site (lock-and-key model).
- Mechanism: $\ce{E + S <=> E\!\cdot\!S -> E + P}$; the enzyme-substrate complex orients the substrate, then releases product and regenerates the enzyme.
- They lower activation energy (sucrose: 6.22 → 2.15 kJ mol−1) without changing ΔH or equilibrium.
- Need only small amounts; function at moderate temperature and a specific pH; denatured (inactivated) outside this range.
- Many require coenzymes (e.g. NAD) or metal-ion activators (e.g. K+).