Enzymes

Enzymes are PROTEINS folded into a specific 3D shape. Somewhere on that shape is a pocket called the active site, shaped to fit only one particular molecule — the SUBSTRATE.

When substrate enters the active site:

• The enzyme holds it in just the right orientation.
• The reaction proceeds quickly (much faster than without the enzyme).
• The PRODUCTS leave; the enzyme is unchanged and ready to do it again.

This is why enzymes are SPECIFIC: a different substrate (wrong shape) can't fit the active site, so the enzyme won't act on it. Amylase digests starch but not protein. Pepsin digests protein but not fat. Lipase digests fat but nothing else.

Below optimum: molecules move slowly → fewer collisions between enzyme and substrate → slow reaction.

At optimum (~37 °C for human enzymes): reaction is fastest.

Above optimum: the protein's bonds break → 3D shape changes → active site no longer fits the substrate → the enzyme can no longer work. This is called DENATURATION. It is usually IRREVERSIBLE.

Why ~37 °C? Human body temperature. Our enzymes evolved to be best at the temperature our bodies naturally maintain. A fever above ~42 °C is life-threatening because it starts to denature enzymes.

For other organisms the optimum is different:

• Thermophilic bacteria (in hot springs): up to ~80 °C.
• Antarctic fish: ~5 °C.
• Plant enzymes: usually 25-30 °C.

pH has the same shape of curve — there's an optimum, and either side of it the enzyme works less well. Different enzymes prefer different pH:

• Pepsin (stomach): pH ~2 (very acidic).
• Amylase (saliva): pH ~7.
• Trypsin (small intestine): pH ~8 (alkaline).

Industrial uses of enzymes (mostly produced by GM bacteria):

• Biological washing powders. Proteases digest blood/egg stains; lipases digest fat. Work at lower temperatures than non-bio detergents, saving energy.
• Cheese-making. Rennin (from calf stomach or now GM yeast) clots milk → cheese.
• Fruit juice production. Pectinase breaks down fruit cell walls → more juice + clearer liquid.
• Biofuel. Cellulase from fungi turns plant waste into glucose → fermented into ethanol fuel.
• Medical tests. Enzyme-based test strips detect glucose in urine (diabetes screening), etc.

In the human body — every digestive step is enzyme-catalysed:

• Mouth: amylase (saliva) starts starch digestion.
• Stomach: pepsin digests protein.
• Small intestine: amylase, protease (trypsin), and lipase finish the job.