Enzymes (Proteins) Flashcards
What are enzymes?
Enzymes are globular proteins that act as catalysts.
They are biological catalysts.
Enzymes speed up reactions.
What do catalysts do?
Catalysts alter the rate of a chemical reaction without undergoing permanent changes themselves.
They can be reused repeatedly and are therefore effective in small amounts.
What is the active site of an enzyme?
The function region of the enzyme is known as the active site.
The active site is made up of small number of amino acids.
The active site forms a small depression within the much larger enzyme molecule.
What are the 2 things that MUST happen for an enzyme to work?
- come into physical contact with its substrate
- have an active site which fits the substrate.
How does an increase in temperature affect enzyme activity?
Increase in temp = Increased KE of molecules.
= the molecules move around more rapidly and collide with each other more often.
In an enzyme-catalysed reaction, this means that the enzyme and substrate molecules come together more often in a given time.
There are more effective collisions resulting in more enzyme-substrate complexes being formed and so the rate of reaction increases.
The pH of a solution is a measure of its hydrogen ion concentration. Each enzyme has an optimum pH, that is a pH at which it works fastest (Figure 5). The pH of a solution is calculated using the formula: pH = -logo[H+]. A hydrogen ion [H+] concentration of 1 x 10-9 therefore has a pH of 9. In a similar way to a change in temperature affecting the rate of enzyme action, a change in pH away from the optimum affects the rate of enzyme action. An increase or decrease in pH reduces the rate of enzyme action. If the change in pH is more extreme then, beyond a certain pH, the enzyme becomes denatured. The pH affects how an enzyme works in the following ways:
* A change in pH alters the charges on the amino acids that make up the active site of the enzyme. As a result, the substrate can no longer become attached to the active site and so the enzyme- substrate complex cannot be formed.
⚫ Depending on how significant the change in pH is, it may cause the bonds maintaining the enzyme’s tertiary structure to break. The active site therefore changes shape.
The arrangement of the active site is partly determined by the hydrogen and ionic bonds between -NH, and -COOH groups of the polypeptides that make up the enzyme. The change in H+ ions affects this bonding, causing the active site to change shape.
It is important to note that pH fluctuations inside organisms are usually small, this means they are far more likely to reduce an enzyme’s activity than to denature it.
Effect of enzyme concentration on the
rate of reaction
Once an active site on an enzyme has acted on its substrate, it is free to repeat the procedure on another substrate molecule. This means that enzymes, being catalysts, are not used up in the reaction and therefore work efficiently at very low concentrations. In some cases, a single enzyme molecule can act on millions of substrate molecules in one minute.
As long as there is an excess of substrate, an increase in the amount of enzyme leads to a proportionate increase in the rate of reaction. A graph of the rate of reaction against enzyme concentration will initially show a proportionate increase. This is because there is more substrate than the enzyme’s active sites can cope with. If you therefore increase the enzyme concentration, some of the excess substrate can now also be acted upon and the rate of reaction will increase. If, however, the substrate is limiting, in other words there is not sufficient to supply all the enzyme’s active sites at one time, then any increase in enzyme concentration will have no effect on the rate of reaction. The rate of reaction will therefore stabilise at a constant level, meaning the graph will level off. This is because the available substrate is already being used as rapidly as it can be by the existing enzyme molecules. These events are summarised in Figure 6.
