Enzymes

5.2

Enzymes

An enzyme is an organic catalyst that is mostly made up of proteins and is produced by living cell organisms.
Substances needed for an enzyme reaction are called substrates.
Substrates will bind with enzymes at a specific site (active site) and form an enzyme-substrate complex.

Enzyme nomenclature:

Name of enzyme is derived by adding '-ase' to the name of the substrate it catalyses.
Example: lactase, protease, and amylase.
A few enzymes that do not follow this naming system.
Example: trypsin, pepsin and renin

General characteristics of enzymes:

Intracellular and extracellular enzymes:

Intracellular enzymes	Extracellular enzymes
Enzymes are synthesized in a cell for their own use. Example: The hexokinase enzyme is used in the glycolysis process during cellular respiration.	Enzymes that are secreted outside the cell. Example: The trypsin enzyme is produced by the pancreatic cells and secreted into the duodenum to break down polypeptides.

Extracellular action:

In the nucleus, the information for the synthesis of enzymes is carried by the DNA in a form of codes.
mRNA is formed to translate the codes into a sequence.
mRNA leaves the nucleus and binds with ribosome for the synthesis of protein to occur.
The synthesized protein is transported enters the lumen of the rough endoplasmic reticulum.
The protein is processed and packaged into a transport vesicle which buds off from the rough endoplasmic reticulum to transports the protein to the Golgi apparatus
In Golgi apparatus, the protein is modified to form an enzyme and is packaged in a secretory vesicle which transports the enzymes to the plasma membrane.
The secretory vesicle will fuse with the plasma membrane to release the enzymes out of the cell.

Mechanism of enzyme action:

The enzyme represented by a 'lock'
The substrate represented by a 'key'
Most reactions inside the cell require high activation energy.
Activation energy is the energy needed to break the bond in the substrate molecule before reaction can occur.
Enzyme function by lowering the activation energy.

Factors affecting the activity of enzymes:

Factor affecting the activity of enzymes

Temperature:

At low temperature, the rate of enzymatic reaction is low.
The rate of enzyme reaction increases as the temperature increases.
This is because of the activation energy of the substrate molecules increases.
Therefore, more collision between the enzymes substrate molecule increases the formation of an enzyme-substrate complex.
The reaction is the maximum at the optimum temperature.
After the optimum temperature, the rate of reaction decreases because the enzyme is denatured in which the bonds that form the structure of the enzyme are changed.
This causes the active site to lose its shape.
Therefore, the enzyme-substrate complex can no longer be formed.

pH:

Optimum pH is the pH at which the rate of reaction is at the maximum.
Small changes in the pH value of a medium will cause the enzyme to be denatured.
The shape of the active site will change.
Therefore, the enzyme-substrate complex cannot be formed again.
Different enzymes have different optimum pH:
- The optimum pH of pepsin is pH 2
- The optimum pH of amylase is pH 7
- The optimum pH of trypsin is ph 8.5

Substrate concentration:

The higher the concentration of substrate, the higher the rate of reaction as more substrate molecules bind to the active site of the enzymes to form the enzyme-substrate complex.
The rate of reaction becomes low when it reaches the maximum point because all of the active sites have been filled up.
At this point, the enzyme concentration is the limiting factor.

Enzyme concentration:

The higher the concentration of enzymes, the higher the rate of reaction as more active sites for substrate molecules to bind to and form the enzyme-substrate complex.
The rate of reaction becomes low when it reaches the maximum point because all substrate molecules have bound to the active sites.
At this point, the substrate concentration is the limiting factor.