Bio-AQA Enzymes

Enzymes

Tertiary structure proteins that catalyse reactions by lowering the activation energy

Structure

Enzymes are large molecules but only a small part of the enzymes that attaches to a substrate to catalyse a reaction. This site is known as actice site

Active site

It’s a specific and unique in shape due to the specific folding and bonding in the tertiary structure of the protein

Due to this specific active site, enzymes can only attach to substrates that are complementary in shape.

Lock and key model

• The enzyme is like a lock and that the substrate is like a key that fits into it due to their complementarity in shape.

• The enzyme active site is a fixed shape and that due to random collisions the substrate can collide and attach to the enzyme. This forms an enzyme-substrate complex.

Once the enzyme-substrate complex has formed the charged groups within the active site are thought to distort the substrate and therefore lower the activation energy. The products are then released, and the enzyme active site is empty and ready to be reused

Induced fit model

• The enzyme is like a glove and the substrate is like your hand; the empty glove is not exactly complementary in shape to your hand, but when you hand enters it enables the glove to mould around your hand to become completely complementary

• The enzyme active site is induced or slightly changes shape, to mould around the substrate. When the enzyme-substrate complex occurs, due to the enzyme moulding around the substrate it puts strain on the bonds and therefore lowers the activation energy. The products are then removed, and the enzymes active site returns to its original shape

The induced fit model is the accepted model for how enzymes function

Factors affecting enzymes

1. Temp

2. pH

3. Substrate concentration

4. Enzyme concentration

5. Inhibitors

   
   

Temperature

If the temp is to low there is not enough kinetic energy for successful collisions between the enzyme and substrate

If the temp is to high, the enzymes denature, the active site changes shape because tertiary structure bonds break, and enzymes-substrate complexes cannot form.

pH

Too high or too low a pH will interfere with the charges if amino acids in the active site. This can break the bonds holding the tertiary structure in place and therefore the active site changes shape.

So the enzyme denatures and fewer enzyme-substrate complexes form.

Diff enzymes have diff optimal pH

Substrate and enzyme concentration

If there is insufficient substrate, then the reactions will be slower as there will be fewer collisions between the enzymes and substrate

If there are insufficient enzymes, then the enzyme active sites will become saturated with substrate and unable to work any faster

Enzyme inhibitors

Competitive inhibitors are similar in shape to the substrate and can bind to the active site. This prevents the substrate from binding and the reaction occurring. If you add more substances this will flood/out-compete the inhibitors, knocking them out of the active site

Enzyme inhibitors

Non-competitive bind to the enzyme away from the active site. This causes a change in the active site. And therefore the substrate can no longer complementary in shape and can no longer bind to the active site, regardless of how much substrate is added.