Describe enzymes as proteins that function as biological catalysts in all metabolic reactions.

5.1 Enzymes

5.1.1 Objective

Describe enzymes as **biological catalysts** that are proteins, explain how they lower the activation energy of metabolic reactions, and show why they are essential for life‑supporting processes such as muscle contraction, active transport, growth and repair.

5.1.2 What is an Enzyme?

• An enzyme is a **protein catalyst** – it speeds up a chemical reaction without being consumed.
• Each enzyme has a unique three‑dimensional shape that creates a specialised region called the active site.
• By lowering the **activation energy (Eₐ)**, enzymes allow reactions to occur rapidly at the moderate temperatures found inside cells.
• After the reaction the enzyme is unchanged and can be reused many times.

5.1.3 Why Enzymes Are Vital for Metabolism

Most metabolic reactions are thermodynamically favourable but are too slow to sustain life. Enzymes increase the reaction rate so that:

• Muscle fibres can contract quickly.
• Active transport pumps can move ions against concentration gradients.
• Cells can synthesise macromolecules for growth and repair.
• Waste products can be eliminated promptly.

5.1.4 Enzyme Action – Active‑Site Shape & Substrate Fit

• The active site is a pocket whose shape and chemical properties match a particular substrate (or a small group of related substrates).
• Two models describe this fit:
  • Lock‑and‑key model: the substrate fits the active site exactly, like a key into a lock.
  • Induced‑fit model: binding of the substrate induces a slight change in the enzyme’s shape, improving the fit.
• When the substrate binds, an **enzyme–substrate complex** is formed. The enzyme then converts the substrate into product(s) and returns to its original shape, ready to bind another substrate molecule.

5.1.5 Real‑World Examples

• Amylase (saliva) – hydrolyses starch to maltose; the classic “starch‑iodine” test demonstrates its activity.
• Lactase (small intestine) – hydrolyses lactose; deficiency causes lactose intolerance.
• Pepsin (stomach) – requires an acidic pH (≈2) and illustrates the importance of optimum pH for enzyme activity.

5.1.6 Factors Affecting Enzyme Activity

Enzyme activity is modulated by internal and external factors. Understanding these factors satisfies the investigation requirements of the syllabus.

5.1.7 Investigating Temperature & pH

1. Temperature: Place equal volumes of starch solution with a fixed amount of amylase in water baths at 10 °C, 20 °C, 30 °C, 40 °C, 50 °C and 60 °C. After a set time add iodine and record colour intensity (less colour = more starch broken down). Plot temperature against colour intensity to locate the optimum temperature and observe the rapid loss of activity due to denaturation at high temperatures.
2. pH: Prepare buffers covering pH 2–9. Add the same amount of pepsin to each tube, then a small amount of gelatin substrate. After a fixed incubation period measure soluble peptide (e.g., spectrophotometrically). Plot pH against reaction rate to identify the optimum pH and the sharp decline caused by pH‑induced denaturation.

5.1.8 Investigating Substrate, Enzyme & Inhibitor Effects

• Substrate concentration: Keep enzyme amount constant and vary [S]. Measure initial rate (v₀). The graph of v₀ versus [S] is hyperbolic, approaching V_max. This demonstrates “rate increases with substrate concentration until the active sites are saturated”.
• Enzyme concentration: Keep substrate in excess and vary the amount of enzyme. Initial rate rises linearly with enzyme concentration, confirming the direct proportionality.
• Inhibitor investigation (competitive): Use a known competitive inhibitor such as a structural analogue of the substrate (e.g., methotrexate for dihydrofolate reductase). Perform the substrate‑concentration experiment with and without the inhibitor. The presence of the inhibitor shifts the curve to the right (higher [S] needed for the same rate) but the same V_max is reached at high substrate levels.

5.1.9 Substrate & Enzyme Concentration – Michaelis‑Menten Relationship

• The rate (v) of an enzyme‑catalysed reaction follows the Michaelis‑Menten equation:
  v = (V_max × [S]) / (K_m + [S])
• V_max – maximum rate when every active site is occupied (enzyme saturated).
• K_m – substrate concentration at which the rate is half of V_max; a low K_m indicates high affinity between enzyme and substrate.
• Increasing enzyme concentration while keeping [S] in excess raises V_max proportionally but does not change K_m.

5.1.10 Types of Enzyme Inhibition

1. Competitive inhibition – inhibitor competes with the substrate for the active site. Adding more substrate can overcome the inhibition (apparent increase in K_m, V_max unchanged).
2. Non‑competitive inhibition – inhibitor binds to a different site, changing enzyme shape. The inhibition cannot be overcome by substrate (K_m unchanged, V_max reduced).

5.1.11 Cofactors, Co‑enzymes & Vitamins (Extension)

• Many enzymes need non‑protein helpers:
  • Cofactor: usually a metal ion (e.g., Zn²⁺ in carbonic anhydrase).
  • Co‑enzyme: an organic molecule, often derived from a vitamin (e.g., NAD⁺ from vitamin B₃).
• These molecules either assist substrate binding or participate directly in the transfer of chemical groups during the reaction.

5.1.12 Key Terms

• Enzyme: protein biological catalyst that speeds up a reaction.
• Active site: region where the substrate binds.
• Substrate: reactant that fits the active site.
• Product: molecule(s) formed after the reaction.
• Activation energy (Eₐ): minimum energy needed for a reaction to proceed.
• Denaturation: loss of the enzyme’s three‑dimensional structure, rendering it inactive.
• V_max: maximum rate when the enzyme is saturated with substrate.
• K_m: substrate concentration at which the rate is half of V_max.
• Cofactor / Co‑enzyme: non‑protein molecules required by some enzymes.
• Competitive inhibitor: molecule that competes with substrate for the active site.
• Non‑competitive inhibitor: molecule that binds elsewhere and alters enzyme shape.

5.1.13 Summary

Enzymes are protein biological catalysts that lower activation energy, allowing the myriad metabolic reactions required for muscle contraction, active transport, growth and other life processes to occur rapidly and efficiently. Their specificity derives from the precise shape of the active site, and their activity is modulated by temperature, pH, substrate and enzyme concentrations, and inhibitors. Because enzymes are not altered by the reactions they catalyse, they can be reused repeatedly. Mastery of these concepts satisfies the core and supplementary requirements of the Cambridge IGCSE Biology syllabus for 5.1 Enzymes.