Enzymes are globular proteins (biological catalysts). They speed up catalyse) chemical reactions in all living things, and allow them to occur more easily.
They are too small to be seen either when they are inside cells or after they have been released from them, for example in the digestive system.
Each particular enzyme has a unique, 3-dimensional shape shared by all its molecules. Within this shape there is an area called the active site where the chemical reactions occur.
The active site makes an enzyme specific as it fits only one type of substrate.
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How do enzymes work?
Enzymes work by 2 mechanisms:
1. The Lock and Key Model
-->The enzyme is like a lock with a specific shape into which the key (substrate) fits.
-->Enzymes are usually larger than the substrates that they act on.
-->Once formed, the products cannot fit into the active site and are thus released, leaving the site free.
Click the link to see: the lock and key model in action
2. Induce Fit Model
-->The active site is not rigid and there is no exact fit, instead it can be modified as the substrate interacts with it.
-->The active site is moulded into the shape of the substrate on contact, improving the fit (makes a tighter fit).
Click the link to see: the induce fit model in action
What do enzymes do?
Enzymes lowers the amount of energy required for a chemical reaction to take place. (This energy is called the activation energy.) This causes a reaction involving enzymes to speed up in other words it takes a shorter time for this reaction to form products.
Some enzymes help to break down large molecules. Others build up large molecules from small ones. While many others help turn one molecule into another.
Properties of Enzymes ?
- Enzymes are catalysts → speed up chemical reactions
- 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 remain unchanged at the end of a reaction. [E + S → ES → P + E]
- Enzymes are specific
Enzyme activity is how fast an enzyme is working and is also called the "Rate of Reaction". It is affected by the following factors Temperature, pH, substrate concentration and enzyme concentration.
Although they can do fantastic things they are sensitive and work best under specific conditions.
Each type of enzyme has its own specific optimum conditions under which it works best.
Enzymes work best when they have a high enough substrate concentration for the reaction they catalyse. If too little substrate is available the rate of the reaction is slowed and cannot increase any further.
Sometimes, if too much product accumulates, the reaction can also be slowed down. So it is important that the product is removed.
The pH must be correct for each enzyme. If the conditions are too alkaline or acidic then the activity of the enzyme is affected (it slows down). This happens because the enzyme's shape, especially the active site, is changed. It is denatured, and cannot hold the substrate molecule.
Graph of enzyme activity verses temperature
As the temperature rises, reacting molecules have more and more kinetic energy. This increases the chances of a successful collision and so the rate increases. There is a certain temperature at which an enzyme's catalytic activity is at its greatest (see graph above). This optimal temperature is usually around human body temperature (37.5 oC) for the enzymes in human cells.
Above this temperature the enzyme structure begins to break down (denature) since at higher temperatures intra- and intermolecular bonds are broken as the enzyme molecules gain even more kinetic energy. At very low temperature enzymes are inactive.
Concentration of enzyme and substrate
Graph of enzyme activity verses enzyme concentration Graph of enzyme activity verses substrate concentration
The rate of an enzyme-catalysed reaction depends on the concentrations of enzyme and substrate. As the concentration of either is increased the rate of reaction increases (see graphs).
For a given enzyme concentration, the rate of reaction increases with increasing substrate concentration up to a point, above which any further increase in substrate concentration produces no significant change in reaction rate. This is because the active sites of the enzyme molecules at any given moment are virtually saturated (occupied) with substrate. The enzyme/substrate complex has to dissociate before the active sites are free to accommodate more substrate. (See graph above on the right)
Provided that the substrate concentration is high and that temperature and pH are kept constant, the rate of reaction is proportional to the enzyme concentration. (See graph above on the left).
Inhibition of Enzyme Activity
Some substances reduce or even stop the catalytic activity of enzymes in biochemical reactions. They block or distort the active site. These chemicals are called inhibitors, because they inhibit reaction.Inhibitors that occupy the active site and prevent a substrate molecule from binding to the enzyme are said to be active site-directed (or competitive, as they 'compete' with the substrate for the active site).
Inhibitors that attach to other parts of the enzyme molecule, perhaps distorting its shape, are said to be non-active site-directed (or non competitive).
Enzymes in digestion- must know!
