Enzymes

Enzymes are biological catalysts — almost always globular proteins — that speed up metabolic reactions by lowering activation energy without being consumed. The PMDC MDCAT 2026 syllabus expects you to explain the mechanism of enzyme action, interpret the effects of temperature, pH and substrate concentration, distinguish competitive from non-competitive inhibition, and identify cofactors and coenzymes. Expect 2–4 MCQs from this chapter.

Where the marks are. Two areas produce most enzyme MCQs: the graph questions (temperature, pH, substrate concentration) and the inhibition comparison (what happens to K_m and V_max). Learn those two cold and the rest of the chapter follows.

Nature and Mechanism of Enzyme Action

An enzyme provides an alternative reaction route with a lower energy barrier. It does not supply energy, does not change the free energy difference between reactants and products, and does not shift the position of equilibrium — it only lets the system reach that equilibrium sooner.

Activation energy (E_a)

The minimum energy colliding molecules must possess to react. Enzymes lower E_a by binding and stabilising the strained transition state, so a far larger fraction of collisions becomes productive.

Active site

A small pocket built from just a few amino acid residues. Those residues may sit far apart in the primary sequence but are brought together by the tertiary folding of the protein — which is exactly why denaturation destroys activity.

Lock-and-key vs induced fit

Feature	Lock and key (Fischer, 1894)	Induced fit (Koshland, 1958)
Active site	Rigid, pre-formed shape	Flexible, moulds around substrate
Substrate	Must match exactly	Induces the final shape on binding
Explains	Basic specificity	Specificity and allosteric regulation
Status	Historical, simplified	Currently accepted model

Sequence to remember. E + S → ES complex → EP complex → E + P. The enzyme emerges chemically unchanged, which is why a trace of enzyme converts a huge quantity of substrate.

Factors Affecting Enzyme Activity

Temperature

Below the optimum, each 10 °C rise roughly doubles the rate (more frequent, more energetic collisions). Above the optimum, hydrogen and ionic bonds break, the active site loses shape and the enzyme is denatured — normally irreversibly. Human enzymes peak near 37 °C; thermophilic bacterial enzymes work near 80 °C because extra ionic bonds, disulphide bridges and a tightly packed hydrophobic core resist thermal disruption.

Classic trap. Low temperature does not denature an enzyme — activity is low simply because collisions are infrequent, and it returns fully on warming. Only high temperature denatures.

pH

Each enzyme has a narrow optimum. Moving away from it alters the ionisation of acidic and basic R groups, disturbing the ionic bonds that hold the active site in shape; the effect is reversible for moderate shifts and irreversible at extremes.

Enzyme	Site of action	Optimum pH
Pepsin	Stomach	~2.0
Salivary amylase	Mouth	~6.8
Trypsin	Small intestine	~8.0
Arginase	Liver	~10.0

Substrate and enzyme concentration

With enzyme fixed, raising substrate increases the rate until every active site is occupied; the curve then plateaus at V_max. Once saturated, the only way to go faster is to add more enzyme — so with excess substrate the rate is directly proportional to enzyme concentration.

Enzyme Kinetics

V_max

The maximum velocity, reached when the enzyme is fully saturated with substrate.

K_m (Michaelis constant)

The substrate concentration that gives half of V_max. It is an inverse measure of affinity: low K_m = high affinity (half-maximal rate achieved with very little substrate).

Turnover number

Substrate molecules converted per enzyme molecule per second. Catalase has one of the highest known values, which is why traces of it clear hydrogen peroxide so fast.

The Michaelis–Menten treatment assumes a steady state in which the ES complex forms and breaks down at equal rates, so its concentration stays effectively constant while initial velocity is measured.

Enzyme Inhibition

Feature	Competitive	Non-competitive
Binding site	Active site	A site elsewhere on the enzyme
Resembles substrate?	Yes	No
Effect of adding more substrate	Inhibition is overcome	Inhibition is not overcome
Apparent K_m	Increases	Unchanged
V_max	Unchanged	Decreases
Example	Malonate on succinate dehydrogenase	Cyanide on cytochrome oxidase

Irreversible inhibitors form covalent bonds and cannot be displaced by substrate or dilution — for example organophosphates on acetylcholinesterase. Heavy metal ions (Hg²⁺, Ag⁺, Pb²⁺) bind –SH groups of cysteine and denature the protein.

Medical link. Sulphonamide antibiotics mimic para-aminobenzoic acid and competitively inhibit a bacterial enzyme of folate synthesis. Humans obtain folate from the diet, so the drug harms the bacterium and not the patient.

Cofactors and Coenzymes

Many enzymes are inactive on their own. The protein part is the apoenzyme; apoenzyme + cofactor = the active holoenzyme.

Coenzyme: Organic, loosely and reversibly bound; often vitamin-derived. NAD (from niacin), FAD (from riboflavin), coenzyme A (from pantothenic acid).
Prosthetic group: Organic but firmly and permanently attached — for example the iron-containing haem of catalase.
Activator / metal ion: Inorganic ion required for activity: Zn²⁺ in carbonic anhydrase, Mg²⁺ for kinases, Cl⁻ for salivary amylase.

This is why a deficiency of B-group vitamins cripples many pathways at once: the vitamins are precursors of the coenzymes that dehydrogenases and transferases depend on.

Regulation of Enzyme Activity

Allosteric regulation

An effector binds a regulatory site distinct from the active site, changing the conformation of the whole molecule. Multi-subunit allosteric enzymes show cooperativity, giving a sigmoid (S-shaped) velocity–substrate curve instead of a hyperbola.

Feedback (end-product) inhibition

The final product of a pathway inhibits an earlier enzyme — usually the first — preventing wasteful over-production. Phosphofructokinase inhibited by ATP is the standard example: glycolysis slows when the cell is energy-rich.

Zymogens (proenzymes)

Secreted inactive to protect the secreting tissue. Pepsinogen is activated by gastric HCl; trypsinogen by enterokinase. Pepsin then activates further pepsinogen autocatalytically.

Classification and Examples

Enzymes are usually named by adding –ase to the substrate or reaction type (maltase, dehydrogenase, lipase); pepsin and trypsin are older exceptions. Isoenzymes are different molecular forms catalysing the same reaction in different tissues — the lactate dehydrogenase family is the classic case.

Catalase — breaks hydrogen peroxide into water and oxygen.
Lysozyme — in tears and saliva; hydrolyses peptidoglycan of bacterial walls.
Rennin (chymosin) — clots milk casein in cheese making.
Immobilised enzymes — bound to an inert support so they can be recovered and reused, and are more heat-stable; widely used industrially.

Worked MCQs

Five MCQs covering the high-yield testing patterns for enzymes. Read every explanation carefully.

Q1. In the presence of a competitive inhibitor, the apparent K_m and V_max respectively:

Both decrease
Increase and remain unchanged
Remain unchanged and decrease
Both increase

Competition means more substrate is needed to reach half-maximal velocity, so apparent K_m rises. Because a large excess of substrate can out-compete the inhibitor for the active site, V_max is still eventually reached. Non-competitive inhibition is the mirror image: V_max falls, K_m unchanged.

Q2. An enzyme with a low K_m for its substrate has:

A low affinity for the substrate
A high maximum velocity
A high affinity for the substrate
No requirement for a cofactor

K_m is the substrate concentration giving half of V_max. If only a little substrate is needed to get halfway to maximum, binding must be tight. K_m and V_max are independent quantities, so a low K_m says nothing about how fast the enzyme is.

Q3. An enzyme cannot drive a thermodynamically unfavourable reaction because it does not alter the:

Activation energy
Rate of the forward reaction
Rate of the reverse reaction
Overall free energy change

A catalyst lowers the energy barrier for both directions by the same amount, so it accelerates the approach to equilibrium without moving it. The free energy difference between reactants and products is fixed by their chemistry, not by the catalyst.

Q4. The coenzyme NAD is derived from which vitamin?

Riboflavin (B₂)
Thiamine (B₁)
Pyridoxine (B₆)
Niacin (B₃)

NAD comes from niacin and FAD from riboflavin — the pair most often confused in the exam. Coenzyme A comes from pantothenic acid (B₅). A niacin deficiency therefore impairs every dehydrogenase that needs NAD.

Q5. A plot of reaction velocity against substrate concentration for an allosteric enzyme is:

A rectangular hyperbola
A straight line through the origin
A sigmoid curve
A curve that falls continuously

Cooperativity between subunits means the first substrate molecule bound improves binding at the remaining sites, producing an S-shaped curve. A single-subunit enzyme following Michaelis–Menten kinetics gives the hyperbola instead.

Quick Recap

Enzymes lower activation energy; they never change ΔG or the position of equilibrium.
Active site = a few residues brought together by tertiary folding → denaturation destroys it.
Fischer = lock and key (rigid); Koshland = induced fit (flexible, accepted).
+10 °C roughly doubles the rate below the optimum; above it, denaturation. Cold slows but does not denature.
Optimum pH: pepsin ~2, salivary amylase ~6.8, trypsin ~8.
K_m = substrate concentration at ½V_max; low K_m = high affinity.
Competitive → K_m up, V_max same. Non-competitive → V_max down, K_m same.
Apoenzyme + cofactor = holoenzyme. Coenzyme = loose (NAD, FAD, CoA); prosthetic group = permanent (haem).
Allosteric enzymes give a sigmoid curve; feedback inhibition is by the end product.
Zymogens (pepsinogen, trypsinogen) are secreted inactive to protect the tissue.

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.