Metabolism & Enzymes

Enzymes are biological catalysts - they speed up (or slow down) chemical reactions without being consumed in the process.

The human body has nearly 75,000 enzymes controlling the rate of metabolic reactions.

Metabolism = the sum of all chemical reactions occurring in a living organism. It has two branches:

• Anabolism - small molecules building up to form larger ones (e.g. amino acids → proteins, glucose → starch). Requires energy.
• Catabolism - large molecules breaking down into smaller ones (e.g. starch → glucose, proteins → amino acids). Releases energy.

Enzymes are made from proteins - long chains of amino acids held together by peptide bonds. They are manufactured in cells by ribosomes, following instructions coded in DNA.

In humans, chemical reactions occur at body temperature (37°C) and usually near pH 7. Without enzymes, these reactions would be too slow to sustain life.

Each enzyme has a unique 3D shape with an active site - a specific region where the substrate (the molecule the enzyme acts on) binds.

The Induced Fit Model (current accepted model):

1. The substrate approaches the enzyme's active site
2. The active site changes shape slightly to fit the substrate perfectly (like a hand fitting a glove)
3. An enzyme-substrate complex forms
4. The reaction occurs - substrate is converted to products
5. Products are released; the enzyme returns to its original shape, ready for reuse

Enzyme specificity: Each enzyme only acts on one specific substrate. This is because the active site has a precise shape that only one substrate can fit. For example:

• Catalase breaks down hydrogen peroxide → water + oxygen
• Amylase breaks down starch → maltose
• Helicase unwinds DNA during replication
• Trypsin breaks down protein (casein) in milk

Activation energy: The minimum energy needed to start a reaction. Enzymes lower the activation energy, meaning reactions can happen at lower temperatures.

1. Temperature:

• At 0°C - very slow reaction. Molecules have little kinetic energy, few collisions.
• As temperature increases, rate increases - molecules move faster, more collisions with active site.
• Optimum temperature - the temperature at which an enzyme works fastest (37°C for human enzymes).
• Above optimum - enzyme begins to denature. The active site changes shape permanently and substrate can no longer fit.
• At 80°C+ - enzyme is fully denatured.

2. pH:

• Each enzyme has an optimum pH where it works fastest.
• Most enzymes: optimum pH ~7 (neutral).
• Pepsin (stomach): optimum pH 2 (acidic).
• Extreme pH changes cause denaturation - the active site shape is permanently altered.
• NOT all enzymes denature in acidic environments (pepsin thrives in acid).

3. Enzyme concentration:

• More enzyme = faster rate (more active sites available for substrate).
• Rate increases proportionally until substrate becomes the limiting factor.

4. Substrate concentration:

• More substrate = faster rate until all enzyme active sites are occupied.
• At saturation, rate plateaus - enzyme concentration becomes the limiting factor.

Denaturation is a permanent change in the 3D shape of an enzyme, destroying the active site. Caused by extreme heat or pH.

Bioreactors are sterile vessels where enzyme-controlled biochemical reactions are carried out on an industrial scale.

Free vs Immobilised enzymes:

• Free enzymes - dissolved in solution. Used once, difficult to recover.
• Immobilised enzymes - held in place in a gel bead or attached to a surface. Can be reused many times, product is enzyme-free.

Advantages of immobilised enzymes: Reusable, easy to separate from product, more stable (heat/pH tolerant), continuous processing possible.

Disadvantages: Initial setup cost, reduced activity (some active sites blocked by gel), diffusion of substrate may be slower.

Industrial uses:

• Lactase - breaks down lactose in milk → glucose + galactose. Used to make lactose-free dairy products.
• Pectinase - breaks down pectin in fruit → clearer juice with higher yield.
• Biofuels - enzymes convert plant material (cellulose) into fermentable sugars → bioethanol.

Medical uses:

• Enzyme Replacement Therapy - providing missing enzymes to treat genetic conditions like Gaucher's disease and cystic fibrosis.
• Diagnostic enzymes - e.g. lactate dehydrogenase levels in blood tests indicate heart damage.