Nov 18, 2009


What are enzymes?
All enzymes are globular proteins → spherical in shape (Fig 1)
Control biochemical reactions in cells
They have the suffix "-ase"
Intracellular enzymes are found inside the cell
Extracellular enzymes act outside the cell (e.g. digestive enzymes)
Enzymes are catalysts → speed up chemical reactions (Fig 2, Fig 3)
Reduce activation energy required to start a reaction between molecules
Substrates (reactants) are converted into products
Reaction may not take place in absence of enzymes (each enzyme has a specific catalytic action)
Enzymes catalyse a reaction at max. rate at an optimum state

Lock and key theory
Only one substrate (key) can fit into the enzyme's active site (lock)
Both structures have a unique shape
Induced fit theory (Fig 4)
Substrate binds to the enzyme's active site
The shape of the active site changes and moves the substrate closer to the enzyme
Amino acids are moulded into a precise form
Enzyme wraps around substrate to distort it
This lowers the activation energy
An enzyme-substrate complex forms → fast reaction

E + S → ES → P + E

Enzyme is not used up in the reaction (unlike substrates).

Enzyme Activity

Changes in pH
Affect attraction between substrate and enzyme
Ionic bonds can break and change shape → enzyme is denatured
Charges on amino acids can change → ES complex cannot form
Optimum pH (enzymes work best)
pH 7 for intracellular enzymes
Acidic range (pH 1-6) in the stomach for digestive enzymes (pepsin)
Alkaline range (pH 8-14) in oral cavities (amylase)
pH measures the conc. of hydrogen ions → higher conc. will give a lower pH

Increased Temperature
Increases speed of molecular movement → chances of molecular collisions → more ES complexes
At 0-42°C rate of reaction is proportional to temp
Enzymes have optimum temp. for their action (usually 37°C in humans)
Above ≈42°C, enzyme is denatured due to heavy vibration that breaks -H bonds
Shape is changed → active site can't be used anymore

Decreased Temperature
Enzymes become less and less active, due to reductions in speed of molecular movement
Below freezing point
Inactivated, not denatured
Regain their function when returning to normal temperature